Subject: [lm 7/27/95] Frequently asked earthquake references, part III
From: eugene@sally.nas.nasa.gov (Eugene N. Miya)
Date: 22 Jan 1997 13:56:11 GMT
Archive-name: ca-earthquakes, Part III
Cathy Smither (PhD, Caltech) asked that this post be dedicated to
Alfred Wegener.
The purpose of this FAQ is NOT to circumvent conventional media.
If it did, the FAQ would easily be as long as library text books.
If you are unwilling to look up information in a book,
this FAQ could easily be just as long as any book.
A source for most general questions about earthquakes is
THE classic reference work on the subject:
%A Charles F. Richter
%Z Caltech
%T Elementary Seismology
%I W. H. Freeman Publishers (try a library)
%C New York
%C San Francisco (it appears this office is now closed)
%D 1958
Why do we suggest THIS book?
Because most readers are familiar with Richter's name. Other people will
question other authors (either authorities [whom the cranks question]
or cranks [who the authorities question], you are welcome to read anyone).
Although this text was published before the theory of plate tectonics,
it is quite readable by the general public (the extensive math being
in the Appendix). It covers earthquake dynamics, questions like animals,
weather, etc. And he is THE authority. Also it's harder to question the dead.
For instance page 133:
"Earthquake weather," as commonly described, is merely a popular fable.
. . .can be traced to classical writing. The Greeks . . .
Visit a library to find this book to answer your questions before thinking
about posting to the net. Do you really think an 100+ line net posting can
answer everything? Richter's 1.5 inches thick and packed with information.
There are numerous other good books. Bruce Bolt (UC Berkeley),
Norris (UCSB), and others. Just check a library out. Community
college classes are fun and useful (field trips!). Additionally,
the Bay Area is especially fortunate to have the Menlo Park offices
of the US Geological Survey. The USGS does work on cartography
(map making), seismology, volcanology, land slides, hydrology, etc.
They have tours, open house, and have speakers available on a wide
variety of subjects. Just visit them. They have maps, reports, etc.
IF YOU receive a prediction: earthquakes happen practically everyday
around the world. Any GOOD prediction will have, epicenter:
latitude
longitude
depth
date (with time)
magnitude
Insist on knowing these things and you will get ride of 80% of the cranks.
Simplying saying "A quake will happen" is like dowsing for water.
Richter specifically wrote:
Amateur predictors are legion, and will continue to be, so long as claiming
to predict earthquakes is an easy way to get one's name into the newspaper.
Many of them are honestly self-deceived; they usually have
(1) no concept of the frequency of small earthquakes (100,000 a year is
a good figure to think of, although it needs definition in terms of the
lower limit of the magnitude included),
(2) no means of knowing what earthquakes have occured or how large they are,
beyond mention in the press and the space allotted there, and
(3) no effective training in scientific thinking.
Some "predictors" select a large number of quakes through the year and then
claim as predicted anything which happens within a few days of any such
date -- so that the earthquakes of half the year or even more are called
in as "verification."
...Predictions based on positions of the sun and moon have to be regarded
a trifle more seriously, since there is evidence that tidal forces may
occasionally act as triggers for earthquakes otherwise on the point of
taking place; in this way the date and hours of occurence may show a
slight statistical correlation with the tides.
Page 386
Chapter 24, Earthquake Risk and Protective Measures
Part I, Earthquake Nature and Observation
Charles Richter
The front of your California phone book, the Red Cross, Sunset Magazine,
your state and county offices of emergency services are
all good sources of information concerning earthquake preparedness.
Topics include:
+Preparing your home to prevent property damage and injury
from falling objects (e.g. bolting houses to foundations,
securing water heaters and bookcases, NOT having your bed under a window).
+Stocking a few days of food and water (albeit it could do you little good
if you are not near your cache).
+Knowing what to do during (get under a doorway or desk, DO NOT run outside)
and after an earthquake (Do you know how to turn off your utilities?).
+Try to stay off of the telephone except in EXTEREME emergencies.
Emergency services personnel need the lines for disaster assistance.
Temporarily disable UUCP connections as well. If you need to make a
call, wait patiently for a dial tone. Clicking the receiver will
prevent you from ever getting one. To contact family, set up a contact
person out of state whom everyone can call. Calls out are given
preference over calls into the damaged region. Pay telephones (at
least the ones run by Pacific Bell) are on priority service and should
provide reliable service.
+LEARN FIRST AID. SF 1989 showed that volunteer rescuers are critical.
If you want to learn something useful, get a Red-Cross certification
card, and even then, that's not a guarantee, it's First-Aid. Any smart
class will prepare you with pre-cautions.
Remember STOP!
Think
Observe
Plan
+One previous net.thread involves fire-arms. Guns have no use in this
natural disaster. That silly discussion belongs in other newsgroups.
Lastly, on a frequently mentioned topic peripheral to earthquakes:
The following message comes from a former Vice-President of Caltech:
As for "Caltech" vs. "Cal Tech," it is DEFINITELY supposed to be CALTECH,
one word, not Cal Tech. Linda is a stickler on this, and woe unto him
who makes this mistake! She frequently finds the error in written stuff,
often stuff coming from Caltech itself. She blasts the miscreant.
Best wishes -----Barclay
As far as I know, it was always one word. Supposedly, the powers that
be (or were) -- Millikan, Hale, and Noyes -- decided at the beginning that
Caltech would be different from Georgia Tech etc. -----Barclay
This is useful first order check of outgoing Tech material.
END BOILER PLATE.
From: greg@perry.berkeley.edu (Greg Anderson)
Subject: Quake magnitudes
Summary: arbitrary, arbitrary, arbitrary
In article <24eo56INNjlf@darkstar.UCSC.EDU> andy@cse.ucsc.edu
(Andy John) writes:
>Is the magnitude the magnitude of the total energy released, or the amount of
>shaking at it's worst point?
In short, Richter magnitude is none of the above.
In long, I offer the following:
Richter's definition of the local magnitude scale came about because
he was looking for a quantitative way to compare earthquakes, based on
instrumental recordings rather than statements of 'the quake felt like XX to
me here'. Basically, he wanted to be able to say, based on instrumental
recordings, `This earthquake here on this date was larger (or smaller)
than this earthquake over there on this other date.'
Richter borrowed the idea of a magnitude scale from astronomers, who
use such to classify the brightness (either apparent or absolute) of stars.
What he did was to define a magnitude scale based on the distance to the quake
and maximum amplitude of that quake as recorded on the photographic Wood-
Anderson instrument.
Here's how it works:
You have an earthquake and you have a Wood-Anderson instrument at some
distance from that earthquake. You locate the quake and calculate the distance
in kilometers from your instrument to the quake's location. Next, you get out
your Wood-Anderson record and your ruler and measure the maximum amplitude of
the earthquake's recording on that instrument, like so:
^
/ \
/ \
---------- ---- \ /--------- <-
\ / |
record w/o quake record \ / max amplitude
with \ / |
quake \/ <-
The formula for determining the Richter magnitude of an earthquake is:
ML = log10(max. amplitude) - log10(A0)
The term -log10(A0) is a correction for the distance from the quake to the
instrument, and is designed so that an earthquake 100 kilometers from your
station will have a maximum amplitude of 1 millimeter if the quake's magnitude
is 3.0. The important thing to note here is that that number, ML = 3.0, is
completely arbitrary. Richter could have defined it to be anything he wanted
it to be -- 0.0, 1.0, 10.0, whatever.
You have now read the maximum amplitude (in mm) from your record and
have determined the distance from the quake to your location, so now you look up
the distance correction factor, take the log (base 10) of the max amplitude,
add those two numbers together, and you have your Richter magnitude.
Now, several important things about Richter magnitude:
1) Richter magnitude is physically meaningless.
The Richter magnitude is based solely on how a specific
type of instrument responds to the motion of a quake, and
tells you nothing about the physical quake itself.
But this is OK, because, and this is critical to keep in
mind, the ONLY reason for having the Richter Scale is to
have an objective means of comparing two quakes. It is
NOT an inherent thing of quakes -- it is an arbitrarily
designed scale created by Richter for his convenience.
There have been several papers written which attempted to
find an equation linking energy release and Richter magnitude
or any of several other physical parameters to Richter
magnitude, but these are all EMPIRICAL relationships, not
something inherent to the scale itself.
2) Richter magnitude is inaccurate, particularly for large quakes.
This is due to the fact that, as quakes get larger, they
release more and more energy into the ground, and this will
eventually overwhelm the recording device's capabilities.
For the Wood-Andersons, this happens at about ML=6.5 or so.
3) Richter magnitude does NOT depend on how people felt the quake.
Posts on ca.earthquakes often contain the statement, 'I am
in city XX, and it felt like a Y.Y. What was it in city
ZZ?' These people are NOT describing magnitude. They are
describing intensity -- in other words, how the quake
affected people or objects near them. Intensity varies
widely with distance, what the person was doing at the time,
and what kind of soil were they on. Other factors enter into
it, such as what kind of building they are in and how sensitive
they are.
Magnitude, on the other hand, uses only distance and amplitude.
From station A at a distance of, say, 20 km from the quake,
we might get an ML=4.2, while at station B at a distance of,
say, 100 kilometers in the other direction from the quake, we
might get an ML=4.4, and at a third station at 50 km and
along a third azimuth, we might get an ML=4.1. The magnitude
for the quake would be given as a 4.2, and that would be it.
The intensity might vary drastically from place to place and
distance to distance, but the magnitude would not.
4) Little differences in magnitude are meaningless.
These little differences, such as those between UC Berkeley
and the USGS in Menlo Park and Caltech, are caused by two
major factors. First, slight differences in the methods
and instruments used to measure Richter magnitude, and second,
variations in the earthquake's energy release (i.e. more energy
might go north than would go south, which would cause the
northern stations to give higher magnitudes than average, while
southern stations might give lower than average magnitudes) as
well as variations in the earth's crust. They are also due to
slight differences in reading techniques from one person to the
next (for example, Suzanna Loper at UCB and I have found that
the average difference in magnitude between her mags and mine
is about 0.1 magnitude units) and other, less important factors.
But all of this is really meaningless, especially considering
that Richter himself recommended only using magnitudes to about
0.5 magnitude units. Basically, magnitude differences from
one group to another of +/- 0.2 magnitude units are very
common, and essentially meaningless.
5) The magnitude scale is logarithmic in amplitude.
A very pompous way of saying that, for each whole unit
increase (or decrease) in magnitude, the recorded maximum
amplitude goes up (or down) by a factor of 10. So a quake
with ML=4.0 has 10 times higher recorded amplitude than a
quake with ML=3.0 and 10 times lower than a quake with ML=5.0.
An EMPIRICAL relationship between ML and energy release has
been found and basically states that, for each increase (or
decrease) in magnitude of a 1.0 unit, the energy released
by the quake goes up (or down) by a factor of about 32. But
keep in mind that this is something found by calculating the
energy and magnitude individually, and then trying to connect
them, not something inherent in the magnitude scale itself.
Whew!
Now, to complicate the issue further, there are many different types of
magnitudes in use by seismologists, each with its own purpose. The Richter
magnitude is the most familiar to the public because it is the one that gets
reported to the press for local quakes. And all of the scales, except one,
give you no physical information about the quake, and are only designed to
help compare one quake to the next.
That one exception is moment magnitude, which is based not on the instrumental
recordings of a quake, but on the area of the fault that ruptured in the quake.
This means that the moment magnitude does tell you something physical about
a single quake. But that's another post, and there are others better suited
than me to do that one.
Hope that helps and doesn't confuse anybody.
Greg Anderson
_____
From: anderson@mahi.ucsd.edu
Subject: Rates of Earthquakes
Recently, someone asked about the rate of occurrence for earthquakes
of a given size in California. Dr. Robert Uhrhammer at UC Berkeley has
examined the UC Berkeley catalogs for the past forty years, and has
come up with some statistics on rate and probabilities for earthquakes
of a given size or larger in certain areas of California and for California
as a whole. Below, I have summarized the results in a few
tables. Thanks go to Bob for the data, and any and all mistakes (either
typos or, more importantly, errors in interpretation) are mine.
Below, I present the tables for the Central Coast Ranges of California,
a region that generally extends from Santa Rosa in the north to San Luis
Obispo in the south, Northern and Central California (north of the transverse
ranges), and California overall. These tables give the rate of earthquakes
per year and the probability of earthquakes occurring in one hour, day, week,
month, and year, for a range of magnitudes.
If you want all the details on how these tables are generated, they are at
the bottom of this section.
CENTRAL COAST RANGES, CALIFORNIA
Having looked at all the earthquakes with M >= 2.5 that were recorded by the
UC Berkeley network between 1949 and 1988, and declustered them into about
3000 groups, the following equation was found:
log N = 3.97 - 0.832*M
This can be translated into the following table:
Probability in one
ML eqs/yr hour day week month year
_____________________________________________________________________
2.5 77.625 0.8816 19.1465 77.5254 99.8449 100.0000
3.0 29.785 0.3392 7.8313 43.6051 91.6432 100.0000
3.5 11.429 0.1303 3.0807 19.7308 61.4186 99.9989
4.0 4.385 0.0500 1.1935 8.0875 30.6110 98.7541
4.5 1.683 0.0192 0.4596 3.1841 13.0835 81.4124
5.0 0.646 0.0074 0.1766 1.2340 5.2383 47.5681
5.5 0.248 0.0028 0.0678 0.4753 2.0434 21.9439
6.0 0.095 0.0011 0.0260 0.1826 0.7890 9.0682
6.5 0.036 0.0004 0.0100 0.0701 0.3035 3.5818
7.0 0.014 0.0002 0.0038 0.0269 0.1166 1.3898
NORTHERN AND CENTRAL CALIFORNIA
Having looked at all the earthquakes with M >= 3.0 that were recorded by
the UC Berkeley network between 1949 and 1988, and declustered them into
about 2900 sequences, the following equation was found:
log N = 4.44 - 0.857*M
which translates to the following table:
Probability in one
ML eqs/yr hour day week month year
___________________________________________________________________
3.0 73.961 0.8402 18.3312 75.8846 99.7895 100.0000
3.5 27.574 0.3141 7.2716 41.1554 89.9524 100.0000
4.0 10.280 0.1172 2.7754 17.9380 57.5431 99.9966
4.5 3.833 0.0437 1.0439 7.1054 27.3407 97.8348
5.0 1.429 0.0163 0.3905 2.7105 11.2258 76.0426
5.5 0.533 0.0061 0.1457 1.0192 4.3422 41.2995
6.0 0.199 0.0023 0.0544 0.3812 1.6415 18.0130
6.5 0.074 0.0008 0.0203 0.1423 0.6151 7.1371
7.0 0.028 0.0003 0.0076 0.0531 0.2298 2.7228
CALIFORNIA
This table is likely to be somewhat of an underestimation, because of
the fact that some magnitude 3.5 or smaller earthquakes may have been missed.
But, assuming the data are complete, the equation
log N = 4.816-0.82*M
is found, which translates to the following table:
Probability in one
ML eqs/yr hour day week month year
___________________________________________________________________
3.0 226.986 2.5562 46.2847 98.7287 100.0000 100.0000
3.5 88.308 1.0024 21.4772 81.6993 99.9363 100.0000
4.0 34.356 0.3912 8.9775 48.3504 94.2902 100.0000
4.5 13.366 0.1524 3.5933 22.6661 67.1701 99.9998
5.0 5.200 0.0593 1.4136 9.5162 35.1653 99.4483
5.5 2.023 0.0231 0.5524 3.8157 15.5140 86.7744
6.0 0.787 0.0090 0.2153 1.5022 6.3483 54.4812
6.5 0.306 0.0035 0.0838 0.5871 2.5194 26.3758
7.0 0.119 0.0014 0.0326 0.2288 0.9878 11.2302
7.5 0.046 0.0005 0.0127 0.0891 0.3855 4.5287
Now, armed with these tables, you can ask yourself a very important
question regarding earthquake 'predictions'. How good is the prediction?
How probable is it that the predicted quake will happen just by random
chance in the given interval of time?
A good earthquake prediction will have the following components:
(1) a specific geographic location
(2) a specific time window
(3) a specific magnitude
(4) a probability estimate, and how it was determined
A specific geographic location means, for example, 'in the area within
20 km of Cape Mendocino' or 'within 15 km of San Jose'. Saying something
like, 'somewhere between San Francisco and Mexico' or 'on the eastern
side of the Sierra Nevada' won't cut it, because it's by far too large
an area.
A specific time window means, for example, 'between 20 July and 22 July,
1993', not 'sometime within the next three months'.
A specific magnitude range means, for example, 'between 4.0 and 4.5 on the
Richter scale' or 'larger than 6.5 on the Richter scale'.
And the probability estimate means, 'There is a 65% chance of this happening.'
And it's important that the person doing the 'predicting' explain how he or
she arrived at the probability estimate. Simply guessing that there is, oh,
about a 65% chance is not good enough for people to judge the prediction.
So a good earthquake prediction will read something like this:
There is a 65% probability that there will be an earthquake of
magnitude approximately 5 on the Richter scale within 20 kilometers
of Cape Mendocino between 20 July and 22 July, 1993. The probability
estimate is based on my historic success rate, as shown by the
published predictions I have made.
This kind of a prediction gives you enough specific information to (a) decide
if you believe the person *before* the quake, (b) decide if it's worth worrying
about or making some sort of change in your life, and (c) test it when it
either does or does not happen.
A bad earthquake prediction reads something like this:
On 24 April 1992, there was a magnitude 6 earthquake near Cape
Mendocino. I predicted this quake, saying that there would be
a magnitude 6 earthquake somewhere between Japan and Washington
State.
The reasons this one is bad should be obvious, but in case they aren't, here
are some of them:
(1) the 'prediction' is announced after the fact
(2) the geographic area is much too large to be of any use,
and in fact is so large as to almost guarantee the success
of the prediction by chance.
(3) there is no time estimate. Magnitude 6 earthquakes happen
with regularity in this zone, and not giving a time window
practically guarantees success (unless the quakes simply
stop happening).
(4) there is no probability estimate.
Anyway, sermon over. With the above tables, when someone 'predicts' a quake in
California, you can take the time and magnitude window given and estimate the
chances that such an earthquake would happen simply by chance. So, for
argument's sake, let's say that I make the prediction I did above for Cape
Mendocino between the 20th and 22nd of July, 1993. Look in the table for
Northern California, with magnitude equal to 5. The table says that on an
average day, there is a 0.39% chance of such an earthquake occurring somewhere
in Northern and Central California. Given that we are talking about a much
smaller area, a 400 square kilometer area, there is a much smaller chance that
such an earthquake will happen at random. So, if I make this prediction, and
the earthquake does come, it is a strong indication that my 'prediction' method
works. If I do this a number of times, and succeed a large percentage of the
time, it looks good for my method.
Whew! This turned out to be considerably longer than I had expected. Once
again, I should thank Dr. Robert Uhrhammer for his data and help.
I should also state (although I would hope it is obvious) that the opinions
in here are mine, and not UC Berkeley's, and not those of the Scripps
Institution of Oceanography, or of my mother, of of your mother, or of Mother
Teresa. At least, not that I know of. ;-)
****
Now, here are the details on how the tables are generated:
Simply munging a catalog together and counting earthquakes of a given size in
a given length of time will give you a false idea of the average rate of
seismicity. The reason is that simply counting quakes doesn't take into
account the fact that many of the earthquakes will be aftershocks of some other
principal earthquake. What you really need to count are the principal
earthquakes, because doing otherwise will bias your counting. This process of
deciding whether a given earthquake is a principal earthquake or part of an
aftershock sequence is called 'declustering'.
Once the data are declustered, you make a graph with the logarithm (base 10)
of the cumulative number of earthquakes of a given size or larger in a given
period of time on the y-axis, and magnitude on the x-axis. Since the number
of quakes goes up rapidly with decreasing magnitude, your chart will look
something like this:
|
|
| *
| *
log N | *
| *
| *
| *
|_____________________
M
Then you attempt to fit a straight line through the data, with the form:
log N = a - b*M
This leads to
rate = 10^(a - b*M),
where 'rate' is defined as the number of earthquake sequences of a given
magnitude or larger in a given period of time, usually one year.
(Of course, all this is really done using a computer, using a number of
different line-fitting methods.)
OK. Now, we have a way of calculating the rate at which earthquakes of a given
size or larger occur, on average, in the given area. For a concrete example,
take a look at the "Northern and Central California Seismicity" table. From
this table, you can see that there are, on average, 1.44 principal earthquakes
with magnitude 5.0 or larger in Northern and Central California in a given year.
Or that there are, on average, about 10.3 principal earthquakes with magnitude
of 4.0 or larger in Northern and Central California in a given year.
Now, you are ready to answer the following question:
What is the probability that an earthquake of a given size or larger
will occur in my area of interest in a given amount of time?
To answer that, you have to make one final assumption, and that this that the
earthquakes are somehow distributed in time. Usually, it is assumed that
earthquake occurrences in time follow the Poisson distribution (for an
explanation of the Poisson distribution, I refer you to the nearest probability
text, since I'm highly likely to get the explanation wrong, anyway.) Suffice
to say that, because of the structure of the Poisson distribution used in the
tables above, with a high rate of seismicity for a given magnitude, the
probability rapidly approaches 100%, while for a lower rate, it takes much
longer. An equivalent way of saying that is that, the lower the magnitude
you look at, the faster the probability of an earthquake of that magnitude
happening in a given time rises to nearly 100%.
Greg Anderson
_____
From: tcsmith@netcom.com (Ted Smith)
Subject: CDMG BBS
Robert Stroh wrote:
Not only do I have it, but here are all the other GeoInfo Net BBS
numbers, too. We'll be posting an updated network list very shortly.
Hubs:
GeoNet BBS 316-265-6457 2400b Wichita, Kansas
and 316-265-1994 14.4K bps
GeoFuel GeoScience BBS 416-829-4097 16.8K bps Oakville, Ontario
CDMG ONLINE 916-327-1208 14.4K bps Sacramento, California
Computer Solutions BBS 504-542-9600 14.4K bps Hammond, Louisiana
GISnet BBS 303-447-0927 14.4K bps Boulder, Colorado
Dark Matter 604-534-7667 14.4K bps Langley, British Columbia
Nodes:
PreCambrian BBS 602-881-5836 14.4K bps Tucson, Arizona
Computer Plumber BBS 319-337-6723 14.4K bps Iowa City, Iowa
PSN San Jose 408-226-0675 2400b San Jose, California
PSN Pasadena 818-797-0536 2400b Pasadena, California
PSN Memphis 901-360-0302 2400b Memphis, Tennessee
COGSNET BBS 303-526-1617 14.4K bps Golden, Colorado
SurveyNet BBS 207-549-3213 14.4K bps Whitefield, Maine
Snowshoe BBS 304-572-2531 14.4K bps Snowshoe, West Virginia
_____
From: Richard Stead
> >: The traditional Richter measurement is based on the amplitude of
> >: surface movement.
> >
> >Officially, it's based on the movement _on_paper_ of a particular
> >class of seismograph.
> >
>
> This is semantics...
> All scientific measurements are based on instruments which were designed
> to report "natural" events.
No.
The Richter magnitude has a very specific definition. There is a particular
instrument called a Wood-Anderson torsion seismometer that the measurement
must be made on. This instrument only functions as a horizontal seismometer.
It has a wire stretched vertically to which a small cylindrical weight is
welded (against the wire, not with the wire through the middle). There
is also a small mirror mounted. A beam of light is directed at the mirror
and reflects onto a rotating drum over which a sheet of paper film is mounted.
When the earth moves horizontally, the inertia of the weight creates torsion
in the wire, deflecting the beam of light from the mirror. The weight and
wire are tuned such that the instrument has a natural period of 0.8 seconds.
(The 0.8 version must be used for Richter magnitude, but other periods
have been used, particularly 6.0 seconds). The computation of Richter magnitude
begins by measuring the peak amplitude of the deflection on the paper film.
Today, of course, real Wood-Anderson seismometers are rarely used. Instead,
we use instruments that are much more capable. To compute a Richter magnitude,
however, we first convert the records of these instruments to exactly what
an 0.8 second Wood-Anderson torsion would have recorded on its film. We then
measure the peak amplitude and procede from there. We can do this only because
modern instruments include all the information the old Wood-Anderson would
have recorded, but more in addition. The best are called broadband wide
dynamic range seismometers. Kinda sounds like the promotions you see on
certain records or CDs. A modern instrument will respond in a band from
microHertz to 100 Hertz and have a dynamic range of over 150 dB. The
Wood-Anderson isn't even close to that. It has a useful range of about
0.01 to 10 Hz and a useful dynamic range of about 30 dB.
The limits of the Wood-Anderson cause the Richter magnitude to "saturate"
at about M8.5-8.7. It can never get bigger no matter how big the quake
really is. The reason for this is something called the "corner frequency".
As quakes get bigger, a given site will no longer receive more and more
of the higher frequencies. The lowest frequency that is constant in this
respect is the "corner". Frequencies below that continue to increase as
quakes get bigger. Really large quakes have to be distinguished at frequencies
in the range of 0.01 to 0.001 Hz (and even lower - the energy at DC is
what is most useful). These frequencies still cause ground motion,
but it is not perceptable and not damaging except at very close range to
the fault. The Kanamori-Richter or open-ended Richter scale addresses
this problem, and basically eliminates the physical Wood-Anderson
instrument from the computation.
Modern seismology prefers Mw as a magnitude measure for the large and
great earthquakes. It is computed from the estimate of energy released
by the quake. This is the measure that makes Chile, 1960 the largest
quake ever at Mw=9.6. All magnitude scales are adjusted to be in good
agreement to Richter over the range that Richter is most applicable,
so Mw=9.6 is like saying the quake would be a Richter 9.6, if Richter
could measure that high.
_____
From: stead@seismo.CSS.GOV (Richard Stead)
Subject: Re: Pacific Northwest questions
In article <27t0p9INNpk5@darkstar.UCSC.EDU>, andy@cse.ucsc.edu
(Andy John) writes:
> My parents live in Seattle, and have recently heard about the subduction
> zone etc... They said that the entire area is bowed upwards about a meter
> or so, due to the plate tectonics. I think it was the plate coming in
> from the ocean, hitting a mountain range, and being
That's not the best way to picture it. The plate is going down because it
is unstable on top. Unlike ice, which is lighter than water, solidified
mantle material (the ocean crust) should not float above the viscous mantle.
This is complicated by the fact that the mantle is also solid on short time
scales, and that pressure from the miles of rock above it make the mantle
more dense than the ocean crust. When you can move ocean crust into the
mantle, the pressure also makes the crust more dense, and then it is denser
than the mantle at the same pressure. Thus, it will sink if it starts going
down. (But sink slowly, the timescale over which the mantle is fluid is
millenia). Thus, no mountain is necessary to push the plate down. In fact,
they can just start going down on their own - there are several subduction
zones that are out in the middle of the ocean. However, subduction can
create mountains two ways. First, the force of collision, if a plate wants
to move over a subduction zone, can cause the over-riding plate to buckle
and thus produce mountains. Second, as the plate "de-waters" (the ocean
crust under the ocean undergoes chemical reactions in which water is
incorporated in the chemical composition of the minerals, for example,
in the formation of hydrates and hydroxides - heat and pressure release
the water), the water lowers the melting point of the surrounding rock, and
the now molten rock moves to the surface. It breaks through in the form
of strato-volcanoes, characterized by explosive eruptions. This always
happens as the down-going slab reaches a particular depth. The slab rarely
goes down straight, so the volcanoes form in back of the subduction zone on
the over-riding plate. The Cascades are an example of this kind of volcanic
activity.
> pushed under. They said there could be a magnitude 9 earthquake, and the
> area could settle about a meter!!!!!!!! Is this true!!!!!! Is it even
> possible to survive a 9?
Actually, I would expect more than 1 meter maximum subsidence in a magnitude 9.
Certainly, areas of Chile in 1960 and Alaska in 1964 had more than 1 meter.
It is certainly possible to survive a 9. As far as what you can feel or most
damage it would cause, a 9 is no different than an 8. It just covers more area
and lasts a lot longer. Of course, subsidence is larger in a 9, and thus more
land can get flooded. Also, there is more damage, because some structures
can outlast the shaking of an 8, but when asked to survive the same shaking
for a few more minutes, just aren't up to it. Finally, and more significantly,
tsunamis are likely to be much worse in a 9. That's the big trouble.
If you're up on a nice high, flat plateau in a 9, just lie back and enjoy
the experience. You will be in no danger, and you will get to experience
one of the rarest natural wonders. I won't vouch for you safety in a building,
under a cliff or even among trees. Trees snap off, cliffs fall and buildings
may collapse under the shaking. Trees and cliffs can't be helped, but there
is no reason buildings should not withstand a 9, it is only bad design that
causes them to collapse.
_____
Article 2687 of ca.earthquakes:
From: ho@helen.CS.Berkeley.EDU (Kinson Ho)
Subject: Re: Sliver Creek Fault in San Jose
[I submitted the following info to the former/current maintainer of
the earthquake FAQ ages ago, but apparently it was decided that
the info is not relevant. I was never told why.]
If you are planning to buy a house in the Bay Area,
I would strongly recommend a visit to the ABAG (Assoc of Bay Area
Governments) building in Oakland, near Chinatown. There are
two places of interest:
a. BAREPP (Bay Area Regional Earthquake Preparedness Project)
101 8th (510-540-2713)
They have lots of handouts, a huge technical library, and perhaps
lots of experts right there. They will also send you a "home pack"
if you call them up.
b. ABAG library (in the same building)
They have maps of fault lines, maps of regions with different
magnitude of ground shaking in case of a quake, and earthquake
planning scenario documents (i.e. what the experts think will
probably fail in the BIG ONE) etc. You may be able to buy some
of these in one of the offices there. Ask the librarian for help.
Plan to spend a LOT of time there. BTW, they get a lot of visits
from people who are planning to buy a house.
Welcome to earthquake country...
Kinson Ho
_____
Article 2947 in ca.earthquakes:
From: Ted Smith
Subject: Lone Pine Scarp and Special Studies Zones
K>Many years ago, on a geology field trip, I went to see the scarp
>of the 1872 quake near Lone Pine. I'd like to go there again, but can't
>remember exactly where we went. I looked in my copy of Earthquake Country,
>and it says "Take the Whiney Portal highway west out of town. After crossing
>the Los Angeles aqueduct, walk (or drive on jeep roads) about a quarter-
>mile north to where a very obvious 23-foot east-facing scarp cuts
>across an alluvial deposit of a former channel of Lone Pine Creek".
>Unfortunately, this book is 30 years old. Are these directions useful, or
>does someone have more up-to-date or accurate information? Any help
>would be appreciated. Thanks.
The fault has been zoned as a fault-rupture hazard by the California
Department of Conservation's Division of Mines and Geology.
1:24,000-scale maps are available showing the zones and the faults. An
index to the maps appears in DMG Special Publication 42 ("Fault-Rupture
Hazard Zones in California"). Copies of the maps are available at
county planning offices and from Blue Print Service Company, 1157
Mission Street, San Francisco, CA 94105; phone 415-495-8700, ext. 550.
_____
Article 2933 of ca.earthquakes:
From: stead@seismo.CSS.GOV (Richard Stead)
Subject: Re: Circular quake diagrams???
In article <2c8jr2$8ni@darkstar.UCSC.EDU> andy@cse.ucsc.edu (Andy John) writes:
>Is there a easy to explain relationship between those circular
>diagrams that always come with quakes? Is the plane the figure is
>in supposed to be the surface of the ground? From the figure can
>you tell where shaking does and doesn't happen?
I haven't been reading for a few days, but it looks like no one answered this.
The diagram is informally named a "beachball". The plane is the ground (though
I have seen them used in scientific papers with the plane oriented differently).
The diagram, given that the plane is the ground, represents the lower
hemisphere of the focal sphere. The focal sphere is an imaginary sphere
large enough to completely contain the fault rupture. The diagram represents
the permanent distortion of that sphere that the quake created. Normally,
colored areas represent zones that now protrude outside the original sphere,
and the other area has intruded toward the center of the sphere. The
boundaries between these are called the nodal lines. They still intersect
the original sphere surface.
So you can see, since it is just a hemisphere, the plane could be other
than the ground - for example, it might be useful to see it from the side,
or oriented at another angle, like the surface of a subducting slab.
The sphere is not determined by actually measuring displacements of a
spherical surface. Instead, the relative motion of this surface is
projected from the observation of the motion of waves as recorded at
sesimometers. Many simplifying assumptions are made:
1) at the very least, the "moment tensor" is assumed to be symmetric.
This allows a hemisphere to represent the entire sphere.
The asymetric information must be carefully extracted and evaluated,
but it is valuable for identifying the fault plane, directivity, etc.;
but it is difficult to do and rarely used.
2) normally, the earthquake is assumed to have a point source. This means
that all the rupture happened at a single point. This never happens,
but is reasonable to first order. Some seismologists study how the
mechanism changes as a function of time and position on the fault -
in that case they really do break the fault and the quake process
into individual points in space and time.
3) Often, the mechanism is examined in terms of the best-fitting "double
couple". The double couple is the theoretical exact motion of a
point source fault. This makes the nodal lines run precisely along
lines that divide the sphere into hemispheres ("great circles") and
that represent planes that intersect each other at exactly 90 degrees.
On an ordinary fault, one plane would then be the fault plane and the
other is called the "auxillary plane". In most faulting environments,
the stress producing the faulting is equally satisfied by slip on
either of the two planes. This is why so many faults are at right
angles to each other in CA. The most noticeable is the San Andreas
and the Garlock, but Landers/Big Bear counts and the Superstition
Hills/Elmore Ranch quakes also did this.
You will notice that Doug's mechanism for the recent Parkfield quake
did not show the double couple, but showed the full moment tensor.
However, you can easily see where the double couple lines would go,
the mechanism is mostly double couple. The other component is
"CLVD" - compensated linear vector dipole. This is a pure compression
or extension motion. Explosion/implosion could be part of a moment
tensor, but it is normally factored out, particularly when drawing
focal spheres.
Up to this point, I have described only one kind of focal sphere plot.
This is that for radial distortion of the sphere, and is appropriate
for P-waves. The shear distortion may also be represented in the same
way, except the different areas become twists in opposite directions.
Since shear is two-dimensional, while radial distrotion is only
one-dimensional, the shear motion must be separated into to focal sphere
plots. This is normally done for shear parallel to the ground (SH for
shear horizontal), and normal to the ground (SV). These are much more
difficult to describe in netnews. The moment tensor plot for radial
distortion contains sufficient data to extract the SH and SV spheres,
assuming an elastic solid, so the mechanisms are generally used by
seismologists only to visualize the shear wave patterns, rather
than represent any information the P-wave one can't. (The focal sphere
is often also called a "radiation pattern" - by analogy to radiation
patterns of antennae. In fact, a map of the world can be projected
onto the focal sphere, given a propagation model, to determine first
motion anywhere in the world).
You also ask if you can tell where shaking happens. No. First, shaking
happens everywhere. However, even if we were imbedded in a perfectly
uniform, infinite elastic solid, and the quake was a point source, there
would only be a single infinite line along which there was no motion -
that line is the intersection of the two planes of the double couple.
The nodal line for the P-wave is the peak for the S-wave and vice-versa.
(The case for a full moment tensor is tougher).
Beyond purely theoretical considerations, the earth is not infinite or
uniform. This causes energy to refract, reflect and diffract around such
that, while a point may miss the first motion, it will certainly get
plenty of shaking after that. However, since the S waves are stronger
than the P, it is better to be nodal to S - shaking will be a small bit
less.
What the figure really tells you is what the first motion is expected to
be everywhere, but things get a lot less certain after that first swing.
_____
Article 3049 of ca.earthquakes:
From: jtchew@csa3.lbl.gov (Ad absurdum per aspera)
Subject: PHYSICS NEWS UPDATE #156, 24 Dec 93
[Written by the American Institute of Physics and posted by us.
Respond to or other references below. Always
posted here on sci.research; sometimes crossposted to other
interested groups with followups directed here. Back issues,
along with FYI and the American Physical Society news/opinion
column WHAT'S NEW, are archived on NIC.HEP.NET for your anonymous
FTP'ing pleasure.
I won't be logging back on until early January, so this might be
your last PNUp for a while. Have a merry Christmas, happy Hannukah,
rootsy Kwanzaa, enchanted solstice, or poignant seasonal feast day
appropriate to your religion, ethnicity, or other identity group
(this *is* coming to you from Berkeley, you know! :) -jc]
GEOELECTRIC SIGNALS: DO THEY PRECEDE
EARTHQUAKES? Speaking at the recent meeting of the American
Geophysical Union, Anthony Fraser Smith of Stanford (415-723-
3687) reviewed his data from four years ago which showed that local
measurements of Earth's magnetic field fluctuated much more
vigorously than usual in the days and hours before the 7.1-magnitude
Loma Prieta earthquake. Many scientists hesitate to infer any
correction between the signals and the quake, particularly on the basis
of only one such data set. To study the matter further, Fraser Smith
has set up several detectors around California near faults. Simon
Klemperer, also of Stanford (415-723-7344), attempts to model Fraser
Smith's signals by suggesting that in the buildup to a quake, a flexing
fault system might squeeze pockets of water together (which are
sparse at these depths--18 km), altering the electrical conductivity of
the fault, which in turn can act like an antenna to modulate the
measured magnetic field at the surface. Other types of geoelectric
signals possibly related to quakes were reported at the AGU meeting.
Seiya Uyeda of Tokai University in Japan and Texas A&M; cited data
linking four recent earthquakes in Japan with anomalies in the static
voltage differences between various measurement stations. Jean Chu
of MIT presented a small portion of an extensive Chinese study (over
20 years) of earthquakes and possible precursors in the form of
changes in the conductivity of the Earth.
_____
From: shirriff@cs.berkeley.edu (Ken Shirriff)
Subject: magnitude & power
This posting is a long explanation of scales for earthquake magnitudes.
In short, the magnitude is the base 10 log of the ground movement
amplitude, with a bunch of fudging to make results come out the
same at different measuring sites and to make the results comparable
between different earthquakes.
In general, the magnitude is determined by a formula of the form
mag = log(a/T) + f(delta,h) + Cs + Cr
where mag is the magnitude, a is the ground amplitude in microns,
T is wave period in seconds, f is a function to correct for the
effects of distance and focal depth, delta is the epicentral distance
in degrees, h is the earthquake focal depth in km,
Cs is a correction for the local structures at the station and
Cr is a regional correction.
The original Richter scale was designed in 1935 for comparing local
earthquakes in Southern California and cannot be used directly for
comparing earthquakes in other areas. It is defined as M sub L =
the logarithm of the maximum recorded trace amplitude in microns
of a Wood-Anderson torsion seismograph with specified constants
(free period=0.8s, magnification=2800, damping=0.8) at an epicentral
distance of 100 km. (Note: all logarithms are base 10).
The magnitude M was designed in 1945 by Gutenberg based on surface
waves. Considering Rayleigh surface waves in a period range of
20+-2 sec for earthquakes of normal depth, the equation becomes M
= log a + c1 log delta + c2, where a is the horizontal ground
amplitude in microns, delta is the epicentral distance in degrees,
and c1 and c2 are constants. (As best as I can tell, this is the
magnitude normally reported.)
There is a third magnitude m, similar to M, based on body waves.
These magnitudes are related by:
m = 1.7 + 0.8 ML - 0.01 ML^2 (where ML is M sub L)
m = 0.56 M + 2.9
The International Geophysical Assembly in Zurich in 1967 adopted the
following recommendations for magnitude determinations of distant events:
1. Magnitudes should be determined from (a/T)max for all waves for
which calibrating functions f(delta,h) are avaliable: PZ, PH, PPZ, PPH,
SH, LH, (LZ). (Z=vertical component, H=horizontal component,
L=surface wave).
2. Amplitudes and periods used ought to be published. Two magnitudes
(m=body-wave magnitude, M=surface-wave magnitude) should be used.
For statistical studies M is favoured. The conversion formula m =
0.56M+2.9 is recommended.
3. For body waves the f(delta,h) values of Gutenberg and Richter are
used. For surface waves, the Moscow-Prague formula:
M = log(a/T) + 1.66 log(delta) + 3.3 is used.
(a is the horizontal componente of Rayleigh surface waves; T should
be in the period range of 10-30 sec.)
4. If short period records are used exclusively, too low magnitudes
result. In order to eliminate this error, it is strongly recommended
that for short-period readings either a/T or f(delta,h) be
adjusted such that the agreement with long-period instruments is
achieved.
The energy in ergs released by the earthquake is given by:
log E = 12.24 + 1.44 M, where M is the magnitude > 5.
This is an empirical equation, derived by integrating over the whole wave
train in time and space. From this equation, each increase of 1 in
magnitude increases the energy released by a factor of 27.5.
The maximum acceleration a0 (in cm/sec^2) is related to magnitude by
M = 2 + 2 log(a0).
The information in this posting came from "Introduction to Seismology",
Markus Bath, 1973, John Wiley&Sons;, New York. This book explains
most of what I wanted to know, with lots of formulas. Check it out.
_____
Article 4101 of ca.earthquakes:
From: gnelson@megatest.com (Glenn Nelson)
Subject: Speed of Seismic Waves
Here's a brief tutorial about estimating the distance to an earthquake.
You can figure the distance to a lightning strike by timing the difference
between the flash and the thunder, then calculating based on the speed
of sound, approximately 1 mile in 5 seconds. Similar for earthquakes
except both P and S waves travel slow, whereas light is virtually instantaneous.
Here's how you figure how far away the quake is. The total travel time
for S is Ts = dist / Vs and for P is Tp = dist / Vp. The S wave arrives
after P, P wave feels like a hammer blow, mostly vertical, S is stronger
and longer and often has sideways components. Time the difference of the
two arrivals, then
(Ts - Tp) = dist/Vs - dist/Vp ==> dist = (Ts - Tp) / (1/Vs - 1/Vp).
If you are within 80-100 km, this is a reasonable formula:
dist = 8 * (Ts-Tp) kilometers.
The P wave travels fastest, the S wave slower. P stands for primary and
S for secondary, or maybe P is for pressure (the type of wave) and S for
shear. In normal matter shear waves always travel slower. In the earth's
crust, 25 to 50 km thick on the continents, P velocity is close to sqrt(3)
faster than S (that's the year George Washington was born). In California
we often use 1.75 or even 1.78. In California, for distances up to
about 40 km, the P waves travel about 6 km/sec, after that, maybe 7 km/sec.
In the formula above you substitute Vs=Vp/1.75, then choose some number
for Vp, I usually use 6. Substitute above to get the simplified formula.
>From chucko@rahul.net Thu Mar 3 22:16:39 1994
I tried plowing through all three sections of the FAQ on my little 24x80
home terminal. It seemed to me there was a great deal of repetition
about the basics (e.g. intensity vs. magnitude, why the Richter scale is
not particularly accurate for measuring large quakes, etc.).
I think some of this material would be best made available via FTP, and
the FAQ should contain a pointer to that site and a very brief summary
of all that, but then I have high-bandwidth Internet access at work and
am spoiled.
In any event, thanks for organizing this.
-- Chuck
>From hannah@ai.sri.com Thu Mar 3 10:16:43 1994
I am posting this to ask for opinions on the structure of the
FAQ for ca.earthquakes, which is cross-posted to sci.geo.geology.
The FAQ has grown to a fairly large size, now consisting of three
individual parts, the last of which contains the new sections added
this month. I am soliciting opinions as to the idea of splitting
the FAQ a little differently into N (currently three) sections,
which will contain the current FAQ, and posting the new sections
in their own individual post. This would allow readers of the FAQ
who are solely interested in the newest chunks to read those
sections fairly quickly, and skip the sections they may have already
read. I was thinking I would label this "new" section something
like
[lm mm/dd/yy] Earthquake FAQ New Sections
to allow easy identification.
Greg Anderson
I agree that the current version of the ca.earthquake FAQ is a little
cumbersome. There's good info in there, but the FAQ has "just
growed", and I suspect that the info in it could be presented in a
more usable fashion. Currently, topics are all mixed together, so a
newbie might have to wade thru a lot of incomprehensible (to him)
technical stuff to find the answer to a common "civilian" question.
Topics are in chronological order, so long-time readers have to wade
thru a lot of old info to find what (if anything) has been added.
I've seen this handled in a couple of different ways on other
newsgroups.
The FAQs for rec.pets.dogs (and .cats) are maintained by a single
individual, who saves interesting information from net discussions and
"weaves" it into the text of the FAQ (which reads more like a pamphlet
than a compendium of messages). The first section of the dog-FAQ
(which is up to something like 10 or 15 files, now) gives an outline
of what is in the FAQs, as well as their names and how to access them
via FTP or the e-mail request version thereof. Each FAQ subsection is
pretty much a self-contained document on a single topic (such as "Your
New Dog"), with some references to the other FAQs. Each begins with a
statement of when it was updated last, with changes marked by "|"s in
the margin. Thus, when the dog-FAQs are posted, I can glance at the
beginning of each one and decide whether it has anything new, then use
my text editor to search for "|", and read only the new sections. I
believe that the FAQs are posted monthly, with a weekly posting that
summarizes what they are, where they are, and how to get them. The
keeper of the FAQ has an abbreviated version of the instructions in
her .sig lines, so every time she posts, folks get a clue about the
FAQ.
The FAQs for rec.equestrian are rarely posted. What is posted
semi-regularly is a FAQ-FAQ, which tells what topics have FAQs, and
how to access them. Each of these topics is a file, more like the
current ca.earthquakes FAQ, that mainly quotes someone's posting(s),
with only minimal cleanup.
I'm mainly a consumer of information on ca.earthquakes, from the
perspective of a scientist in a different field, with the usual
"civilian" experiences with Bay Area quakes over the last 24 (yikes,
how time flies!) years. Consequently, I'm not in a good position to
tell you how best to subdivide the technical information. However, I
think that there should be a shorter, less technical document written
just to answer the obvious questions (and calm the emotions) about how
quakes happen, aftershock sequences, Richter vs Mercalli (apologies
for botched spellings), suggested reading (my personal favorite is
Yanev's "Peace of Mind..."), etc. Perhaps the very capable ladies at
Caltech have some "canned" verbage they could donate?
Certainly, a FAQ on preparedness is in order---things like what to
have on hand, things to do in advance to minimize structural damage
and injury, etc. Is there a possibility of getting an on-line version
of the widely-distributed pamphlet that USGS prepared after Loma
Prieta? (I realize it'll lose something in the translation to ASCII.)
The technical discussions are interesting, but probably should go in a
separate file(s), which will probably be skipped by the scientifically
challenged, and those in a panic over an all-too-recent temblor.
Well those are my thoughts. Just what you needed---lots of "advice",
and no volunteers.... ;-)
>From ho@helen.CS.Berkeley.EDU Wed Mar 2 14:55:50 1994
I would suggest splitting the FAQ into different posts, by subject.
e.g. Magnitudes, why earthquakes happen, preparedness etc.
There does not seem to be too much of the "car kit" or "home kit"
lists included. (I've seem them posted by individual readers
from time to time --- perhaps we should integrate them.)
I would also suggest that each FAQ posting includes some form
of Index/table of contents, right at the beginning.
Thanks for maintaining the FAQ.
>From iva@monty.rand.org Thu Mar 3 09:56:38 1994
I like the idea of "FAQ new sections", perhaps with periodic (every
three or six months?) reworking of the basic FAQ.
>From simutis@ingres.com Thu Mar 3 08:50:00 1994
Yes, breaking it up seems the right thing to do.
Possibly a 'major changes' post, with the info getting grouped
with other similar topics; I'm afraid this will mean developing
an overall and per-post table of contents...
FAQ could also use the Mercalli Scale someone posted, and Richard
Stead's explanation of the 'beachball' diagram; I have both of those
saved - except I don't have an actual 'beachball' diagram!
It's amazing how much work a 'labor of love' turns out to be - thank
you for doing the FAQ.
From: e_gs18@va.nmh.ac.uk
Subject: Re: quake-l mailing list?
Sender: news@c1.nkw.ac.uk (Ed Marchewicz)
Date: Fri, 1 Apr 1994 18:56:09 GMT
In article <1994Mar31.075406.503@amoco.com>, awrobinson@amoco.com (Andrew W. Robinson) writes:
> Elsewhere I saw a reference to an email mailing list, quake-l. Can
> anyone pass along any information about this mailing list? Is it a
> general discussion of earthquakes? How does one subscribe?
QUAKE-L was started by folks interested in the social consequences of
earthquakes, so is not technically oriented. It carries a mix of
first-hand accounts, occasional reposts from SEISM-L, basic enquiries
and general chit-chat. In recent months, a few other seismologists
(all of them, I think, also s.g.g participants) have been active there,
so I feel less alone :-) . The tone has been popular and the volume
moderate (maybe 3/4 messages a day). Subscribe by sending the message :
SUBSCRIBE QUAKE-L
to LISTSERV@VM1.NODAK.EDU In the traditional LISTSERV fashion,
subscriber lists, back issues &c; are available from that address.
The LISTSERV program will send you details when you subscribe.
There are quite a few other list- and mail-servers covering the
seismological community. Most are rather specialised, but the following
might be of interest to a wider community. Note: *all* are much more
technically focussed than s.g.g or QUAKE-L!
SEISM-L redistributes data messages from the US National Earthquake
Information Service. It's not a discussion list, but new subscribers
all too regularly post "What's this stuff mean?" messages instead of looking
through the archives :-(. About 3/4 *data* messages a day, so requires
serious interest! Subscribe by sending the message :
SUBSCRIBE SEISM-L
to LISTSERV@BINGVMB.BITNET (or possibly LISTSERV@bingvmb.cc.binghamton.edu)
SEISM-L has a parallel discussion list, SEISMD-L, which appears to be
moribund.
There is also a VOLCANO list from the LISTSERV@ASUACAD.BITNET. This is
again fairly technical, and I am under the impression that list membership
as well as posts are screened by the moderator. The HAZARDS list can be
obtained from the LISTSERV@LISTS.COLORADO.EDU -- this takes the form of an
edited newsletter; also, the server appears to have been a bit shaky of late.
Russ Evans
British Geological Survey, Edinburgh e_gs18@va.nmh.ac.uk
From: ted.smith@mtnswest.com (TED SMITH)
Subject: Re: quake-l mailing list?
Date: Fri, 1 Apr 1994 22:23:00 GMT
In <1994Mar31.075406.503@amoco.com>, awrobinson@amoco.com (Andrew W.
Robinson) wrote:
A>Elsewhere I saw a reference to an email mailing list, quake-l. Can
>anyone pass along any information about this mailing list? Is it a
>general discussion of earthquakes? How does one subscribe?
The following excerpts are from ftp.nisc.sri.com
netinfo/interest-groups [dated 14 June 1993]:
QUAKE-L@VM1.NODAK.EDU
QUAKE-L@NDSUVM1.BITNET
QUAKE-L%NDSUVM1.BITNET@VM1.NODAK.EDU
Mailing list for discussion of the ways various national and
international computer networks can help in the event of an
earthquake, or the help can be enhanced. One of the basic problems
discussed might be network reconfigurations which would be temporarily
required; others might be in actually putting various groups in
electronic contact with each other.
Public notebooks for the list will be available from LISTSERV, can be
searched with the LISTSERV database facility (send LISTSERV the
command info database for details), and are available via anonymous
FTP from VM1.NODAK.EDU (134.129.111.1) after entering CD LISTARCH (use
DIR QUAKE-L.* to see any notebooks/archives).
BitNet or Internet users may subscribe to the list by sending a
message or e-mail to LISTSERV@NDSUVM1 or LISTSERV@VM1.NODAK.EDU,
respectively. On the first line of the text or body of the message
enter the command; SUB QUAKE-L your_full_name where your_full_name is
your real name, not your login Id.
Coordinator: Marty Hoag
SEISM-L%BINGVMA.BITNET@MITVMA.MIT.EDU
Seismological topics of general interest.
To subscribe send the following command to LISTSERV@BINGVMB
(non-BitNet users send mail to LISTSERV%BINGVMB.BITNET@MITVMA.MIT.EDU)
SUBSCRIBE SEISM-L your_full_name To unsubscribe, send UNSUBSCRIBE
SEISM-L
Coordinator: Jim Blake
* QMPro 1.52 * Why experiment on animals with so many lawyers out there?
From: Joe Dellinger
Subject: FAQ
I've collected some stuff I found interesting; it's in
sepftp.stanford.edu
under
pub/geology/information
I just collected stuff I happened to read that I thought was
interesting, or stuff that I thought would be useful and happened to get
a chance to get.
I've been doing my small part to help out NASA the last couple of
weeks; I installed an e-mail daemon to produce SEDS-2 visual pass predictions
for those who needed individually tailored info but couldn't calculate it
themselves. (Try it out, if you want; send e-mail to
seds@montebello.soest.hawaii.edu
)
The people who are researching the tether thought this was a good
idea but evidently couldn't do it themselves...
From: cochrane@netcom.com (Larry Cochrane)
Subject: Re: Personal Seismic Network? FAQ?
Date: Thu, 21 Apr 1994 01:40:06 GMT
First it's the Public Seismic Network. PSN has 4 bulletin boards around the
country, the numbers are:
San Jose, Ca 1(408)226-0675
Menlo Park USGS, Ca 1(415)327-1517
Pasadena, Ca 1(818)797-0536
Memphis, Tenn 1(901)360-0302
There is also a gopher site at gopher.ceri.memst.edu Port 70
I also have some of the PSN files in my ftp directory at ftp.netcom.com
in /pub/cochrane.
>If you are part of the PS network, would a 1 axis seismometer be helpful?
Most PSN stations only have one sensor, it would be nice to have three, two
horizontal, one pointed north-south and the other west-east, and one vertical
sensor. My Lehman (a horizontal seismometer) is pointed north-south since most
of the local quakes I receive here in Redwood City (between San Jose and
San Francisco) Ca. originate north or south of me. This sensor can also
receive quakes from all over the world. I was able to get a 7.3 in
Indonesia a few months ago. The plans to build a Lehamn are in file
lehmansei.zip on any of the PSN systems and my ftp directory.
>How about a Endevco accelerometer (also 1 axis).
I think that accelerometers are only good for strong motion detection.
To receive magnitude 3's 100km-200km away, or large distant quakes,
you need a sensor that is very sensitive to very small ground motions.
I don't know if an accelerometer would be sensitive enough. One of the
problems is that large quakes can saturate the system very easily.
The 6.7 LA quake in January saturated my system and LA is 500km (300m) away.
The ideal station would have a set of high sensitivity sensors and a set
to strong motion sensors. Maybe someday...
>If I can monitor, I would want to digitize the signal(s) from the sensor
>to 8 or 10 bits depending on micro controller, is this good enuf?
You really need 12 or more bits, 8 or 10 bits will also cause saturation
problems. There are plenty of 12 bit A to D cards out there that work
with the software on the PSN BBS's that most stations run to collect the
data.
>What is the typical sampling rate?
Anywhere from 30 to 100 samples per second. I run at 36 sps and have
a hi-gain low noise 10 Hz low pass amp/filter card between the sensor
and my A to D card. I sample at the low end because I save all info to disk.
This creates a 4.7Meg file each day. I save everything to disk because
I live in a very noisy location, a freeway about 500 yards from me and
train tracks about 2/3 of a mile from me. The software on the PSN BBS's
only saves data when an event happens and I could not come up with
a good trigger point. I ended up writing my own data collecting
software to get around this problem. I can then go back and see what
I got after I hear about a quake from either Andy Michael (USGS) finger
service or his weekly quake report. I then create a PSN compatible
quake file from this logged data. My system does have an alarm on it so
I do know about larger quakes right away.
BTW: I have just released a new version of my Windows PSN quake file viewer
called WinQuake, in file winqk15.zip. This new release is in my ftp
directory or any of the PSN systems. It is also available on
Ted Smith's CDMG ONLINE BBS system at 1(916)327-1208 (thanks Ted). I plan
to make a more formal announcement about WinQuake in a few days to
ca.earthquakes and sci.geology.
Hope this helps.
Larry Cochrane
San Jose, Ca PSN
cochrane@netcom.com
From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Subject: Re: Earthquake Predications
Date: 27 Apr 1994 19:49:50 GMT
In article <2pkhcd$47m@search01.news.aol.com> gigi55@aol.com (GIGI 55) writes:
>A couple of years ago I heard
>something about the ability of seismologists to predict (scientifically of
>course) the possibility of an upcoming earthquake. The story, if I'm not
>mistaken, said that they (the Official Seismological Institutes here in CA)
>would start issuing some kind of warning. Something like the weathermen do. X%
>of probability of an earthquake in the next 3 days. I hope this doesn't sound
>too off the wall but although I don't remember the exact details, the story
>really stuck in my mind. Any comments?
The short term probabilities are based on the clustering of earthquakes
into foreshock-mainshock-aftershock sequences. One of the primary features
of earthquake catalogs is that earthquakes cluster like this. Of course,
aftershocks are much more common than foreshocks, and it is hard to tell
foreshocks from the more common background (or unclustered) earthquakes. But
we have enough historical experience to be able to estimate the odds that
a given earthquake is a foreshock which would mean that it would be followed
by something larger. I will resist explaining any of the math because this
is Lucy Jones' specialty. Such forecasts are also made for the odds of
large aftershocks occurring, and these have been discussed here recently.
>Also, what does Jack Cole do for a living? Is he/was he a Seismologist? What
Jack worked in a electronics store (actually I'm pretty sure it was "The
Good Guys" which is a stereo/TV store chain that has branched out into general
consumer electronics now.
>methods do you use as an investigator to check these predictions out. And how
>do you decide what predictions are even worth checking out?
His predictions were checked out because they were being reported on in the
press and were starting to create public concern. It was the effect of his
predictions on the public that got us interested.
To investigate the methods I got together a team that consisted of one
of our electronics people that has followed electrical methods and
earthquake prediction for many years, Tony Fraser-Smith from Stanford
who has been involved with ultra-low frequency electromagnetic waves
and earthquake prediction, and at Tony's suggestion another radio
frequency expert from Stanford. We then visited Jack to look at his
equipment, get him to describe his techniques, and attempted (and
completely failed) to get a documented list (or actually any list) of
past predictions. The makeup of the team was designed to be able to
verify if there was any link between what Jack was doing and what Tony
Fraser-Smith was doing (such a link had been claimed and did not
exist), to be able to analyze the probable source of any signals he
could show us (those that he could show us on his signal analyzer were
clearly manmade noise, however his predictions were made by listening
to noise bursts on AM radios. He, at least then, was unable to record
these noise bursts so we couldn't do much with them), and to analyze
his record of success and failures (my part of the team, but of course
there wasn't a record).
I should add that Jack fully cooperated with our visit, although he
did grandstand a bit by inviting a TV news crew to show up near the
end of it. So, I got stuck doing an interview at his home before we
could finish our report. They also filmed footage of us getting into
our car complete with close-ups of the US Government plates (oh, the
drama of it all :-) ).
We then wrote a short report that was then used by the state and the USGS
to issue joint statements refuting his prediction. That report had the
effect of largely getting people to ignore his predictions (e.g. no
schools or large businesses closed and there were no runs on supermarkets
for supplies. These things have happenned in other cases and some schools
were considering closing before our findings were released). I consider
this one of the more direct contributions I have made to society, although
I'd rather have had the time to do my own research.
Andy
From: cochrane@netcom.com (Larry Cochrane)
Subject: Re: Is there a description of the format ...
Date: Wed, 27 Apr 1994 03:23:57 GMT
In article Pete Carah wrote:
>Is there a description of the format or at least file-read source around so
>those of us with unix (or clone) workstations can view them too?
>(e.g. parse the file into a simple list and use gnuplot, or better...)
>I have looked at the PSN (Pasadena) file list and can't see anything
>like this at first glance.
I got the format for the Public Seismic Network quake data files from the
documentation for SDAS the DOS program that most PSN stations use to record
quake data. Here is the part of the doc file that explains the format:
User's Guide for Seismic Data Acquisition System
APPENDIX A - SEISMIC DATA FILE INTERNAL FORMAT
The seismic data file is saved in BASIC BSAVE format. The
BSAVE statement creates an unencoded, binary file, which is
an exact image of memory contents. Seven bytes of control
information are written at the beginning of the file, and
these bytes are followed by the data bytes copied from
memory. The file length shown in the DOS directory entry
will be the BSAVE-specified length plus 7, rounded up to a
multiple of the BASIC buffer size. The format of the file
is as follows:
Offset Length Contents
0 1 X'FD' (Constant BSAVE file format identifier)
1 2 BASIC DS segment value at BSAVE execution time
3 2 Offset in the DS segment specified in the
BSAVE statement
5 2 Data length specifed in the BSAVE statement
7 * Memory image data
The segment value, offset, and length values are all stored
in Intel low-high-order format.
If a BLOAD statement is executed with an offset term, then
that offset value and the BASIC DS segment value in effect
at BLOAD execution time will be used to place the loaded
data; if a BLOAD statement is executed without an offset
term, then the stored segment and offset values will be
used. In both cases, the stored length value determines the
amount of data loaded. The memory image data portion of the
file is described below. These 2-byte integer fields are
also stored in Intel low-high-order format. Offsets as
listed should be used as shown to index into the array after
it has been BLOAD'ed into memory, i.e. ARRAY%(1) contains
the year of the start of data collection. This assumes you
have not changed the BASIC default array origin of zero.
The file format is shown below:
OFFSET NAME CONTENTS
0 Format Flag describing format of data.
Earlier versions of SDAS saved data
in a different format. If this field
contains the value 2, then the following
format description applies to the file.
1 SYEAR Year of start of data collection
2 SMON Start month of data collection
3 SDAY Day of start of data collection
4 SHOUR Hour of start of data collection
5 SMIN Minute of start of data collection
6 SSEC Second of start of data collection
7 STENTH Tenths of second at start
8 FHOUR Hour of finish of data collection
9 FMIN Minute of finish of data collection
10 FSEC Second of finish of data collection
11 FTENTH Tenths of seconds at finish
12 COUNT Count of valid elements in file
including the 100-byte header
13 BASE BASELINE value used for this file
14 MIN Min. sampled value in this file
15 MAX Max. sampled value in this file
16 ORIENTATION CHARACTER N = North-South, E = East-West,
Z = Vertical
The next 4 values are dependent on the location of the station, and
are copied from values supplied in the SDAS.PRO file.
17 LATINT Station latitude N of equator (neg for S)
Integer portion only
18 LATDEC Decimal portion of latitude multiplied by
100 and rounded to nearest integer
Also negative if South Latitude
19 LONGINT Longitude E of Greenwich (neg. for W)
Integer portion only
20 LONGDEC Decimal portion of longitude multiplied by
100 and rounded to nearest integer
Also negative if West Longitude
21 NEIC hour of quake origin
Set to -1 by this program, modified by the
user via the SDASCOM program later after
checking with the National Earthquake
Information Center or other authority.
22 NEIC minute of quake origin
23 NEIC second of quake origin
24 NEIC tenths of second of origin
25-39 RECLOC Name of recording location (15 chars max)
40-99 COMMENT Description of quake. Added to quake file
with the SDASCOM program at a later time.
100-25099 Two-byte-integer digitized seismometer
data values.
WinQuake was written in C++ so I don't use the first 7 bytes. The program
skips the 7 bytes in the file, then reads in the next 200 bytes in to a
structure, I get the samples count in COUNT (offset 12), subtract 100,
then read in that many samples.
Hope this help. If anyone has any questions about this format or WinQuake
feel free to e-mail me. I'll also put a copy of the format in file
format.psn in my ftp directory.
BTW: We could also use a good Mac program. Any Mac programmers out there...
From michael@garlock.wr.usgs.gov Thu Apr 28 17:56:16 1994
Date: Thu, 28 Apr 94 18:04:16 PDT
From: michael@garlock.wr.usgs.gov (Andy Michael)
Message-Id: <9404290104.AA14139@garlock. wr.usgs.gov>
To: eugene
Subject: Re: Earthquake Predications
Jack Coles, who previously worked in a consumer electronics store, has
made earthquake predictions for several years. His methods were
investigated by the U.S.G.S. in 1991 because one of his predictions
had made the press and was starting to create public concern.
To investigate his methods, I put together a team that consisted of
another geophysicist from the USGS that has a lot of electronics
expertise and who has followed electrical methods and earthquake
prediction for many years, Tony Fraser-Smith from Stanford who has been
involved with ultra-low frequency electromagnetic waves and earthquake
prediction, and at Tony's suggestion another radio frequency expert
from Stanford. We then visited Jack to look at his equipment, get him
to describe his techniques, and attempted (and completely failed) to
get a documented list (or actually any list) of past predictions. The
makeup of the team was designed to do the following:
To verify if there was any link between what Jack was doing
and what Tony Fraser-Smith was doing (such a link had been
claimed and did not exist).
To analyze the probable source of any signals he could show
us. Those that he could show us on his signal analyzer were
clearly manmade noise, however his predictions were made by
listening to noise bursts on AM radios. He, at least then,
was unable to record these noise bursts so we couldn't do
anything with them.
To analyze his record of predictions to see if he was doing
better than random chance, but there was no record.
After this visit I wrote a short report and our joint efforts led to a
press release by the State of California's Office of Emergency
Services. The key part of it is, "In the opinion of Dr. Davis [State
Geologist], of the State Department of Conservation/Division of Mines
and Geology, as well as USGS and Stanford University scientists who
have discussed his efforts with him, Mr. Coles' work does not provide
systematic evidence that radio signals were precursory to the
[aforementioned] earthquakes. In comments to OES, Dr. Davis stated
that 'the present status of this work does not warrant its use in
public policies by local or state government and would not justify
special efforts in preparedness on the dates specified in his
forecasts.'"
The report and press release had the effect of largely getting people
to ignore his predictions. E.g. no schools or large businesses closed
and there were no runs on supermarkets for supplies. These things have
happenned in other cases and some schools were considering closing
before our findings were released. I consider this one of the more
direct contributions I have made to society.
Article: 8007 of ca.earthquakes
Newsgroups: ca.earthquakes
From: flowers@lanai.cs.ucla.edu (Margot Flowers)
Subject: Re: WHY MORNING EARTHQUAKES?
After pointing out differences in characteristics between the moon and
its quakes, and the earth and earthquake, such as mass differences,
differing rock characteristics, Bruce Bolt observes that the deep
quakes on the moon:
... commonly occur within an interval of a few days during
perigee, ... About equal numbers of deep moonquakes occur at
these centers at opposite phases of this tidal pull, so that the
most active periods are 14 days apart. These periodic properties
at least suggest that the tidal pull of the Earth on the moon
triggers the occurrence of the deep seismic-energy releases.
[Bolt, _Earthquakes_ 1993, p. 178]
>From what I understand, these kinds of patterns have been sought on
the earth but not confirmed. Is anyone aware of any successes?
Article: 8067 of ca.earthquakes
From: kjn@netcom.com (Ken Navarre)
Subject: Re: Plate Tectonic FAQ?
One of the best books that I have found is EARTHQUAKES AND GEOLOGICAL
DISCOVERY by Bruce Bolt, 1993, Scientific American Library, distributed by
W.H. Freeman & Co., available thru most bookstores. Bruce bolt is a
Professor of Seismology and former Director of the UC Berkely Seismographic
Stations. In his book he discusses seismology, the various waves and
their propagation, plate tectonics, how and why scientist study
earthquakes, and how earthquakes affect buildings. Quite an undertaking
in a mere 225 pages! It has some excellent photos and illistrations and
is an easy read.
Article: 8070 of ca.earthquakes
From: hough@seismo.gps.caltech.edu (Susan Hough)
Newsgroups: ca.earthquakes
Subject: Re: Plate Tectonic FAQ?
Date: 13 May 1994 18:32:19 GMT
In article isis@netcom.com (Mike Cohen) writes:
>What are the causes of earthquakes inland away from the plate boundaries &
>how big/common are they?
Inland ('intraplate') quakes can be caused by a number of different
kinds of stress--large scale 'glacial rebound' (the slow flexure of
the crust back up after a large sheet of ice is removed), for example,
or the broad compressional stress caused within eastern North America
by the 'push' forces from the mid-Atlantic spreading center. The New
Madrid region is classified as a 'failed rift'--a place where the
continental crust started to rift apart, but then stopped. Mary Lou
Zoback (USGS, Menlo Park) has shown theoretically how the presence of
this rift will translate into the occurence of (infrequent) large
earthquakes. The rift extends northward from the New Madrid region,
and may imply a seismic hazard for the Wabash Valley region (Illinois/
Indiana)--there is paleoseismic evidence that large prehistoric
earthquakes have occurred in this region as well.
Overall, the rate of earthquakes in the stable part of the continent
is something like 1/10-1/100 of the rate in California; much more
infrequent, but still present. Arch Johnston (Memphis State) has
tried to classify world wide intraplate earthquakes and argues that
all of the very large intraplate events may occur at failed rifts,
with some large events at continental margins (the 1888 Charleston
earthquake being an example of the latter). Moderate-sized events,
magnitude 6 or so, are considered more 'fair game' for other areas,
as was illustrated by last year's deadly earthquake in India.
I confess to very little direct knowledge of Wyoming's seismotectonics.
It is not linked to the New Madrid zone. A map of historic seismicity
reveals a spattering of fairly small-moderate size earthquake across
the state, more than, say, North Dakota, but still a very low
overall level. There has been a concentration of events at the
_very_ NW corner of the state, part of a zone that continues
southward into Idaho (Snake River areas) and Utah. Probably somebody
out there knows more about this zone than I do(?)
Sue
From: geomagic@seismo.do.usbr.gov (Dan O-Connell)
Subject: Re: Richter scale
Date: Mon, 23 May 1994 23:18:46 GMT
In article <1994May23.161537.10493@neutron.nacamar.de> jui@neutron.nacamar.de (Uwe Harmening) writes:
> maybe this is a bit of a silly question, but we had a discussion today about
> the Richter scale. How is an earthquake measured, is the Richte scale still
> used and if yes, will it be replaced?
> Thanx in advance!
The Richter scale is no longer used to report most earthquake magnitudes,
although the much of the press continues to incorrectly call some
magnitudes "Richter magnitudes." The Richter scales was developed in
southern California by studying the decay of of maximum seismic amplitudes
observed on Wood-Anderson short-period torsional seismographcs with
distance. He developed distance (attenuation) corrections so seismic
amplitudes could be corrected to zero distance and thereby characterize
the "size" of the earthquake. Since the attenuation relations were
developed from southern California earthquakes and seismographic
stations, the application of the Richter scale elsewhere
is suspect unless area specific attenuation curves are developed.
Even then, it does say much about the physical size of the earthquake.
The moment-magnitude, Mw, is now the most popular magnitude scale. Seismic
moment is defined as
Mo = udA, where u is average rigidity, d is average fault slip, and
A is fault area. Mo is most commonly derived from moment tensor
inversions of long-period seismograms from worldwide seismic networks.
The Hanks and Kanamori (1979) relation,
Mw = 2/3 *log(Mo) - 10.7
is used to calculate the moment magnitude. Through Mo, Mw is directly
proportional to the actual "size" of the earthquake. The methods
used to derive Mo essentially correct for the radiation pattern and
attenuation effects. The Richter magnitude scale begins to "saturate"
between 6.5 and 7.0. (it does not increase with increasing earthquake
size above these magnitudes due to the frequency band it samples).
Hanks and Kanamori (1979) developed Mw so it corresponded as well
as possible to various existing magnitude scale (within the
appropriate frequency bandwidths of the various magnitude scales).
Let's use the 1992 Landers earthquake for an example. The fault
length was ~80 km, fault width about ~10 km, average slip about 4.5m,
and average rigidity about 3E11 dyne/cm^2. You could infer these
parameters from field observations, aftershock seismicity, geodetic
deformation measurements, long-period moment tensor inversions, etc.
The moment, Mo, is 1.08e27 dyne-cm and Mw = 7.3.
This scratches the surface, but hopefully, it helps.
Dan O'Connell
geomagic@seismo.do.usbr.gov
Seismotectonics Group, U.S. Bureau of Reclamation
Denver Federal Center, P.O. Box 25007 D-3611, Denver, CO 80225
"We do custom earthquakes (for food)"
or
"Just more roadkill on the information superhighway"
/\
/ \
/ \ /\ /\
/\ / \ / \ / \ /\ /\/\ /\/\
___/ \ /\/\/\/ \ / \ /\ / \ / \/ \/ \ /\_______
\/ \ / \ / \/ \/ \/
\/ \/
--
Dan O'Connell
geomagic@seismo.do.usbr.gov
Seismotectonics Group, U.S. Bureau of Reclamation
From: geomagic@seismo.do.usbr.gov (Dan O-Connell)
Subject: Re: Richter scale
In article <2rrd4j$gcp@quartz.ucs.ualberta.ca> buri@probe2.phys.ualberta.ca (Michael Burianyk) writes:
In article
geomagic@seismo.do.usbr.gov (Dan O-Connell) writes:
> The moment-magnitude, Mw, is now the most popular magnitude scale. Seismic
> moment is defined as
>
> Mo = udA, ......
>
> Mw = 2/3 *log(Mo) - 10.7
>
> is used to calculate the moment magnitude.
> Next question ... what is the relationship between moment-magnitude,
> or seismic moment, and the energy released during an earthquake?
The radiated seismic energy could be calculated by integrating
the square of the absolute value of the slip velocity radiated by
all portions of the fault or
/+x /max(z)
| | |v(x,z)|^2 dz dx (1)
/-x /min(z)
where v(x,z) is the slip velocity on the fault. Using the Parseval-Rayleigh
theorem, (1) can be calculated from the integral of the velocity
spectra of broadband seismic recording (if propagation, attenuation,
and radiation effects can be estimated and removed). To first order,
slip velocity is proportional to stress drop. So you could have two
earthquakes on the same size fault that release different amounts
of energy if the stress drops are significantly different (assuming
equivalent slip durations). The problem with (1) is that slip velocity
on the fault is one of the "mysteries of the universe" parameters
that has not been directly measured. Fault slips are inferred from
inversions of geodetic and/or seismic recordings, but there are lots
of different slip velocity-slip duration-stress drop combinations
that can produce the same slip distribution on a fault.
So to provide an "easy" answer to the question, you can using
the empirically derived Gutenberg-Richter energy-surface wave
magnitude relation
log E = 11.8 + 1.5*M (2)
10 S S
where M is surface wave magnitude and each unit increase in M corresponds
S S
to a 32-fold increase in energy. Substituting Mw for M gives a quick
S
and dirty estimate in ergs.
or (From: shirriff@cs.berkeley.edu (Ken Shirriff))
Markus Bath's log E = 12.24 + 1.44 M (3)
where M is the magnitude > 5. This is an empirical equation, derived by
integrating over the whole wave train in time and space. From this
equation, each increase of 1 in magnitude increases the energy released
by a factor of 27.5.
Both (2) and (3) represent attempts to approximate the integral (1)
by accounting for the loss of amplitude with distance of propagation
of various seismic phases (surface waves for (2) and ensembles of
all wavetypes for (3)).
Anyway, seismic energy increases proportionately with moment, as each
represents integrals over the fault. For seismic moment, its the integral
of slip over the fault. For energy, its the integral of squared slip
velocity over the fault. You can infer slip distribution from various
data sets. Slip velocity is a transient that is difficult to
measure, but it is an extremely important factor in determining
the behavior (time and space distributions) of strong motions
(accelerations).
Dan O'Connell
geomagic@seismo.do.usbr.gov
Seismotectonics Group, U.S. Bureau of Reclamation
Denver Federal Center, P.O. Box 25007 D-3611, Denver, CO 80225
"We do custom earthquakes (for food)"
or
"Just more roadkill on the information superhighway"
/\
/ \
/ \ /\ /\
/\ / \ / \ / \ /\ /\/\ /\/\
___/ \ /\/\/\/ \ / \ /\ / \ / \/ \/ \ /\_______
\/ \ / \ / \/ \/ \/
\/ \/
--
Dan O'Connell
geomagic@seismo.do.usbr.gov
Seismotectonics Group, U.S. Bureau of Reclamation
From: stgprao@st.unocal.COM (Richard Ottolini)
Subject: Re: World-wide earthquake frequencies?
>From Bolt "Earthquakes" Appendix A:
M(s) #>
8 2
7 20
6 100
5 3000
4 15000
3 100000
Seismicity size-rates tend to follow power laws, with each
increase in magnitude N times more common.
N varies from region to region, and in southern CA, from
decade to decade..
(The numbers above 7 look too high and the jump from 5 to 6
high too. Are Bolt's numbers correct?)
From: John Gunn
Subject: Re: World-wide earthquake frequencies?
Date: Sat, 11 Jun 94 17:13:21 -0500
Telnet neis.cr.usgs.gov and login as qed. This BBS is run by the USGS
and has many interesting facts about earthquakes worldwide in the last
century.
From: emeth@beretta.ramp.com (David F Jones)
Subject: Re: Northern California quake info?
World Wide = finger quake@gldfs.cr.usgs.gov
Hawaii Volcano Obs = finger quake@tako.wr.usgs.gov
IRIS Teleseisms = spyder@dmc.iris.washingtion.edu ( non-finger )
finger quake@gldfs.cr.usgs.gov (Worldwide Earthquakes)
finger quake@andreas.wr.usgs.gov (Northern California)
finger quake@scec.gps.caltech.edu (Southern California)
finger quake@fm.gi.alaska.edu (Alaska)
finger quake@slueas.slu.edu (Central U.S.)
finger quake@seismo.unr.edu (Nevada)
finger quake@eqinfo.seis.utah.edu (Utah and Yellowstone)
finger quake@geophys.washington.edu (Washington & Oregon)
file://garlock.wr.usgs.gov/pub/earthquake.html is one place to start
Photo URLs:
http://nisee.ce.berkeley.edu/eqiis.html
http://geology.usgs.gov/program/earthquake/gallery/index.html
From: kjn@netcom.com (Ken Navarre)
Subject: Re: Scientific America home seismograph
The article was published in Scientific American, 1979, v. 241, No.1,
pg 152-161. It described the construction of a horizontal seismic sensor
and preamp circuit designed by James D. Lehman. At the time, Mr. Lehman
was with the Physics Dept., James Madision University, Harrisonburg, Va., 22807.
Another source of information that describes modification to the Lehman
sensor was published in the:
Journal of Geological Education, 1987, v.35, pg. 124 by
Richard Lawrence Koll
Dept. of Geology and Meteorology
Kean College of New Jersey
Union, NJ, 07083
The modifications include the use of common pipe fittings to construct
the frame and support for the sensor. Construction is simple and
straightforward. The amplifier, however, was prone to significant noise
and oscillation. Newer circuits and components are available that eliminate
this problem.
=====
Web (WWW) Pages:
http://vulcan.wr.usgs.gov/home.html
Article 11338 of ca.earthquakes:
From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Newsgroups: ca.earthquakes
Subject: Re: usgs bbs number?
Date: 4 Jan 1995 01:55:36 GMT
The bbs in Menlo Park is (415)327-1517 and from within the Bay Area
1-800-328-1517.
A list of available services is can be gotten by sending email to
help@quake.wr.usgs.gov as long as your return path is set correctly.
Article 246 of sci.geo.earthquakes:
From: gibson@polycarp.NoSubdomain.NoDomain (Rick Gibson)
Newsgroups: sci.geo.earthquakes
Subject: Re: Who was the first seismologist?
Date: 9 Jan 1995 22:17:43 GMT
Organization: Massachvsetts Institvte of Technology
In article , rmwm@va.nmh.ac.uk (Roger Musson) writes:
|> Who was the first seismologist?
|>
|> Of course, it rather depends on your definition of seismologist. Perhaps the
|> question should be who was the first person to be referred to as a
|> seismologist? First use of the word "seismometer" I can date precisely, but
|> "seismologist" I can't.
|>
|> Any views, opinions?
|>
|> Roger Musson
|> British Geological Survey
|> e_rmwm@va.nmh.ac.uk
For one perspective, the online Oxford English DIctionary gives the following as its
early references to the word:
1858 Mallet in Rep. Brit. Assoc. i. 1 The few physicists who are engaged in
Seismology.
1879 Rutley Stud. Rocks iii. 9 The branches of physical geology known as
Vulcanicity and Seismology.
For the word seismometer, they cite the following as the earliest reference:
1841 J. D. Forbes in Edin. Phil. Trans. XV. i. 220 The self-registering part of
the apparatus, which Mr. David Milne has termed a Seismometer.
How do these correspond with your results?
Rick Gibson
MIT
gibson@erl.mit.edu
From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Subject: Re: Who was the first seismologist?
Date: 10 Jan 1995 01:55:44 GMT
In article <3escm7$sc0@senator-bedfellow.MIT.EDU> gibson@polycarp.NoSubdomain.NoDomain (Rick Gibson) writes:
>In article , rmwm@va.nmh.ac.uk (Roger Musson) writes:
>|> Who was the first seismologist?
>|>
>|> Of course, it rather depends on your definition of seismologist. Perhaps the
>|> question should be who was the first person to be referred to as a
>|> seismologist? First use of the word "seismometer" I can date precisely, but
>|> "seismologist" I can't.
>
>For one perspective, the online Oxford English DIctionary gives the following as its
>early references to the word:
>
> 1858 Mallet in Rep. Brit. Assoc. i. 1 The few physicists who are engaged in
> Seismology.
....
To proceed down this route for a bit...
That's for seismology, for seismologist the OED gives the very similar:
1859: R. Mallet in Rep. Brit. Assoc. 1858 133 "The subject appears to me worthy
of more examination at the hands of Vulcanologists and Seismologists."
(Apparently this issue came out late.)
Richter, in his textbook, cites a few studies earlier than Mallet's work on the
1857 Naples event although that one gets a lot of credit because of its very
systematic nature and the attempt to use physics to learn something about the
event. It is also the first one in Richter's list with a clear author.
In any case it seems clear that the words seismology and seismologist
are both credited to Mallet. He is also a good candidate to have been
called a seismologist first.
The problem with an OED search is that it looks at the usage of the word
not how it could have been applied in retrospect.
Probably all civilizations made studies of earthquakes and their
effects, but the Chinese seismoscopes were in use at least as early as
132 A.D. Richter credits these to Chang Heng, so perhaps he is the
first recorded seismologist. At least he lived long before Mallet and
invented a device we now call a seismoscope and therefore is arguably
a seismologist.
As noted above, it also depends on how broadly you want to apply the
term seismologist. The original usage was for the study of earthquakes
and their causes and effects. Now, we tend to apply it more to those
who use waveforms to study either the earth structure or earthquakes.
The people who study effects without waveforms are often earthquake
geologists or earthquake engineers. For the more limited modern
definition it might go to Gray, Milne, and Ewing for developing the
first good seismographs (motion versus time). For the broader
definition you can probably go back as far as the written record
allows, although I would restrict the search to those that display some
of what we would currently call scientific method to their studies.
That may allow you to skip Chang Heng depending on how he interpreted
the results of his device. It also may allow you to skip whoever wrote
the Old Testament. However, Amos Nur has argued that some of the
descriptions from the Old Testament fit the actual motion on faults in
that area. So, do you depend on good observations or good
interpretation. Yes, I am having fun clouding the issue.
That said, Mallet does deserve a lot of credit. If you can ever get
your hands on a copy of his 1862 book "Great Neapolitan Earthquake of
1857" its worth a look. I sometimes wonder just how much I would be
willing to pay for a copy. Anyone got one they want to sell?
Andy
Article 217 of sci.geo.earthquakes:
Newsgroups: sci.geo.earthquakes
From: zjad49@trc.amoco.com (Joe Dellinger)
Subject: Re: Increasing Frequency and Magnitude
Organization: Amoco Production Company, Tulsa Research
Here is an article on the subject of increasing quake death rates that's
worth dusting off again:
ftp://sepftp.stanford.edu/pub/geology/information/from_ca.earthquakes/quake_death_rates
-------------------------------------------------------------------------
From: alanf@tekig6.PEN.TEK.COM (Alan M Feuerbacher)
Subject: Re: Natural Disasters in the 20th Century
Date: 6 Nov 93 01:42:07 GMT
Organization: Tektronix, Inc., Beaverton, OR.
In article <2b8fn6$qgg@seismo.CSS.GOV> stead@seismo.CSS.GOV (Richard Stead) writes:
>In article <12269@tekig7.PEN.TEK.COM> alanf@tekig6.PEN.TEK.COM (Alan M Feuerbacher) writes:
>>A religious article entitled "Natural Disasters -- A Sign
>>of the Times?" said the following:
>> _New Scientist_ warns that "the world can expect more
>> disasters in the 1990s than in past decades."....
>
>Without question, there is no evidence that the rate or intensity of natural
>processes has changed at all, not this century, not this millenium, not
>in millions of years. All the evidence available points to a very constant
>rate for these things.....
>Quakes are more likely to have been
>constant with time, though it is difficult to measure. There is a
>possibility that there have been times of increased tectonic activity
>in the past - probably causing more quakes. There is no evidence however
>for any time having less seismicity than we do now.
>
>I sure hope you are asking these questions because you really want to know
>the answers. Too often, we get fruitcakes who are trying to push some
>bizarre theory or religious doctrine and trying to get things out of the
>data that aren't there. They come here with their minds made up and think they
>are going to educate the dumb, stubborn scientists. They refuse to listen
>to reason or accept the evidence. A lot of people here will react
>negatively if they get the impression that someone is doing this, and I
>wouldn't really blame them.
You hope right! I do not agree with the writer of the article I quoted.
I was hoping to get a number of answers like yours, which will help me
show certain people that their confidence in sources like this is quite
misplaced.
To all who responded, many thanks!
I've done a bit of my own research on earthquake frequency and casualty
rates. Below is presented some data I found through a variety of sources.
What do you professionals think?
___________________________________________________________________________
A Comparison of Earthquake Victims
1715-1783: 1915-1983:
Year Location Deaths Year Location Deaths
1715 Algeria 20,000 1915 Italy 29,970
1717 Algeria 20,000 1920 China 180,000
1718 China 43,000 1923 Japan 143,000
1719 Asia Minor 1,000 1927 China 200,000
1721 Iran 100,000 1932 China 70,000
1724 Peru (tsunami) 18,000 1933 USA 115
1725 Peru 1,500 1935 India (Pakistan) 60,000
1725 China 556 1939 Chile 30,000
1726 Italy 6,000 1939 Turkey 23,000
1727 Iran 77,000 1946 Turkey 1,300
1730 Italy 200 1946 Japan 2,000
1730 China 100,000 1948 Japan 5,131
1730 Japan 137,000 1949 Ecuador 6,000
1731 China 100,000 1950 India 1,500
1732 Italy 1,940 1953 Turkey 1,200
1736 China 260 1953 Greece 424
1737 India 300,000 1954 Algeria 1,657
1739 China 50,000 1956 Afghanistan 2,000
1746 Peru 4,800 1957 Iran (Northern) 2,500
1749 Spain 5,000 1957 Iran (Western) 2,000
1750 Greece 2,000 1960 Chile 5,700
1751 Japan 2,000 1960 Morocco 12,000
1751 China 900 1962 Iran 10,000
1752 Syria 20,000 1963 Yugoslavia 1,100
1754 Egypt 40,000 1964 Alaska 131
1755 China 270 1966 Turkey 2,529
1755 Iran 1,200 1968 Iran 11,588
1755 Portugal 60,000 1970 Turkey 1,086
1755 Morocco 12,000 1970 Peru 66,794
1757 Italy 10,000 1971 USA 65
1759 Syria 30,000 1972 Iran 5,057
1763 China 1,000 1972 Nicaragua 6,000
1765 China 1,189 1973 Mexico (Western) 52
1766 Japan 1,335 1973 Mexico (Central) 700
1771 Japan (tsunami) 11,700 1974 Pakistan 5,200
1773 Guatemala 20,000 1975 China 200
1774 Newfoundland 300 1975 Turkey 2,312
1778 Iran (Kashan) 8,000 1976 Guatemala 23,000
1780 Iran (Tabriz) 100,000 1976 Italy 900
1780 Iran (Khurasan) 3,000 1976 Bali 600
1783 Italy (Calabria) 60,000 1976 China 242,000
1783 Italy (Palmi) 1,504 1976 Philipines 3,373
1783 Italy (Monteleone) 1,191 1976 Turkey 3,790
1977-1983 addition: 44,623
_________ _________
Total 1715-1783: 1,373,845 Total 1915-1983: 1,210,597
Annual average: 19,911 Annual average: 17,545
___________________________________________________________________________
Alan Feuerbacher
alanf@atlas.pen.tek.com
-------------------------------------------------------------------------
--
/\ /\ /\/\/\/\/\/\/\.-.-.-.-.......___________
/ \ / \ /Amoco Production Research Tulsa Oklahoma\/\/\.-.-....__
___/ \/ \/Joe A. Dellinger Internet: joe@sep.stanford.edu \/\.-.__
-------------- Uh oh, Toto, I think we're back in Kansas! ----------------------
From: jgk@netcom.com (Joe Keane)
Subject: Human Involvement
Summary: We need reasonable precautions.
Keywords: acceleration
Date: Fri, 20 Jan 1995 09:38:46 GMT
This is from _Geology of California_ by Norris and Webb:
In an unstable region with large population and continued growth, maximum
safety precautions to offset earthquake hazards are extremely important.
In California, about 75 percent of the population resides in the areas of
greatest instability. It is difficult to implement earthquake precautions,
however. They are expensive, and because earthquakes may never affect
the majority, appropriate precautions are often ignored. Building for
earthquake resistance may increase costs by 10 percent or more. In the past,
incorporating earthquake safety into structures was hampered because geologic
science could not provide adequate data for structural engineers, architects,
and construction firms. Today better information is available.
An interesting example of this problem relates to the development and
deployment of strong-motion seismographs. For many years, seismologists had
been most interested in developing extremely sensitive instruments that would
allow scientists working at places like Berkeley or Pasadena to record and
analyze in detail the nature of distant earthquakes. Nearby strong shocks
were expected either to disable or deactivate these sensitive instruments.
Structural engineers, on the other hand, were much less concerned with the
nature of a quake in Kamchatka, for example, and instead wanted to know what
forces local earthquakes imposed on buildings and other structures. Beginning
about 1965, strong-motion instruments were perfected and placed in buildings,
on dams, bridges, and other structures. These began to yield data that
surprised nearly everyone. During the moderately strong 1971 San Fernando
earthquake (magnitude 6.4), accelerations equal to or slightly greater than
gravity were measured. This meant that objects not tied down tended to
float when the acceleration exceeded gravity, just as they do in spacecraft;
rocks tossed in the air often landed upside down, for example. These
high accelerations forced considerable review of the adequacy of existing
construction standards and design. Such large accelerations were thought
to have been extremely rare, but it is now clear that they are not.
Establishing safety standards assumes willingness to recognize earthquake
hazards, expenditures of large sums of money required, and counteracting the
apathy that arises from the ``it can't happen to me'' philosophy. This last
obstacle might be overcome quickly if many continuing, sufficiently strong but
not disastrous earthquakes were to occur. Instead, infrequent, localized, but
unfortunately sometimes severe tremors affect comparatively few people at a
time. For example, the 1987 Whittier Narrows earthquake (magnitude 5.9, $400
million damage) was termed a ``wake-up call'' to the Los Angeles metropolitan
area which, at present, is bracing for ``the big one.''
*Engineering*
It is now feasible to construct modern earthquake-resistant (not
earthquake-proof) buildings. Since the locally disastrous 1933 Long Beach
earthquake (magnitude 6.3), the Field Act has required that all California
public school buildings be earthquake resistant. Moreover, the uniform
building codes adopted in the 1950s have applied the Field Act standards to
virtually all buildings. Yet damage sustained in the 1971 San Fernando quake
showed that even these standards were not completely adequate. Nonetheless,
dams, waterworks, highways, and utility structures may all be built with
reasonable safety provided certain precautions are followed and provided
that they are not built directly on faults or on unstable ground subject
to liquefaction or sliding.
Public lack of awareness and failure to demand reasonable precautions are
the main impediments to earthquake safety, although some hazards will always
remain. In the 1971 San Fernando earthquake, damage to newly constructed
(built with reasonable safeguards) was quite extensive. The quake's
surprisingly large forces led some geologists to assert, however, that the
San Fernando case can reasonably be expected to occur every 5,000 years. Most
seismologists are now much less sure that these high accelerations are rare,
and on the basis of what is known about the setting and history of the San
Fernando fault system it is now estimated that the recurrence interval is
between 100 and 300 years. Should huge expenditures be allocated to safeguard
structures, roads, and bridges that have an estimated life of 50 years or so?
Reasonable precautions are required, but some degree of risk must be accepted
when people choose to live in an unstable, earthquake-prone region.
*Psychological Factors*
The psychological discord experienced when an earthquake strikes can be
severe for a few people. There is often a tendency to accept rumors about
earthquakes without regard for the facts. The psychological consequences of
other destructive natural phenomena such as tornados, hurricanes, and floods
are also extremely traumatic, but they do not seem to be accompanied by
false rumors to the same degree as earthquakes. Perhaps other natural
disaster-producing phenomena are less damaging to the psyche because they can
be seen--people see them arrive and see them leave--something rarely possible
with earthquakes. Storms, floods, landslides, windstorms, and fires take far
larger tolls of life and property in the United States than do earthquakes.
Furthermore, safeguards against earthquakes are no more costly than those
required to prevent damage from other natural phenomena.
Date: Fri, 27 Jan 95 10:30:49 MST
From: "Craig Jones"
Subject: magnitudes
Hi!--the following note sent to me from Greg Anderson will explain the rest
of this message...
Craig --
As one of the others who finally wore out to the point of no longer
posting *anything* to either sci.geo.geology or sci.geo.earthquakes
(not to mention ca.earthquakes), I want to say that your post did a
very great job of explaining things. I seriously think you should
consider posting it (or auto-posting it) every couple weeks, or
perhaps months.
You might well also want to send a copy to Eugene Miya at NASA Ames,
as he took over from me (after I took over from him) the duties of
posting the ca.earthquakes FAQ...
In short, your post was great, and I can now rest with a clear
conscience ( ;-) )
Greg Anderson
anderson@mahi.ucsd.edu
PS Eugene Miya's e-mail address is eugene@wilber.nas.nasa.gov
----
Anyway, though I doubt there is anything really all that special about the
post he referred to, I am including a copy here for your perusal.
Craig Jones, cjones@mantle.colorado.edu
Research Associate, CIRES, Univ. of Colorado
---------
In Article <3g1ggfINNpt0@titan.oit.umass.edu>, cyndal@titan.oit.umass.edu
(Cyndal) wrote:
> Hi all,
>
> I happened to see a post last week where the sender was a bit
>out of sorts (a polite term) because the media was taking the
>magnitude of the Kobe quake and confusing it with a Richter scale
>measurement. I've seen 3(and maybe there are 1-2 more) different ways
>recently that the Kobe quake was measured: Magnitude, x.y Richter, and one
>other one.
> For those of us who aren't seismologists/geologists/etc, could someone
>explain(in layman's terms) what the differences are? I've puttered around
>a few places that list daily/weekly quakes and have seen things like
>"Mag 4.5" and always assumed it was 4.5 on the Richter scale. Now I see
>that this is probably a very wrong assumption. So, please, what ARE
>the differences, and why are quakes given different measurements?
> Thanks!
> Michele
>
This qualifies as the most often asked question on this newsgroup and
sci.geo.geology; there should be a permanent post on what magnitudes mean,
how they are defined, etc. The reason this question usually goes unanswered
in these newsgroups is that all of us who have answered it a few times are
just getting worn out.
So let's go ahead and see what's there....
First, there are two fundamental ways of describing the "size" of an
earthquake that often make it to newspapers and the public: magnitude and
intensity. We'll cover magnitude in a minute, but it is equivalent to the
wattage on a light bulb--it to some degree should be proportional to the
total energy output of the earthquake regardless of where the earthquake is.
Intensity is how strongly the earthquake is felt at a particular spot, which
depends on distance from the earthquake, local geology, the presence of
observers (often), etc. It is equivalent to how bright the light from a
bulb is at different places in a room. Intensity in the U.S. is generally
reported as Modified Mercalli Intensity and is usually assigned a roman
numeral (in part to distinguish it from magnitude, in part tradition, and in
part because these observations are no more precise than an integral value);
when reported in the media, usually the peak value is reported. This
occasionally is confused with magnitude. In some countries other intensity
scales are used (and I seem to recall that Japan is one of those countries).
OK, so now magnitude. The fundamental to all magnitude measures is that for
a 1 point increase in any magnitude for fixed location of earthquake and
seismometer, the amount of ground motion measured for that magnitude scale
will increase by a factor of 10. It happens that this works out to mean
that an increase of 1 in magnitude will also represent an increase in the
total energy of an earthquake of a factor of about 28. To calculate the
magnitude, corrections are made for the distance from the epicenter, depth
of the earthquake, model of seismometer, and Earth structure; thus the
magnitude of an earthquake should be identical regardless of where it is
measured if using a common scale (discussed below). In reality, the
corrections for Earth structure are complex and are affected as well by the
radiation pattern of seismic waves from an earthquake. As a result,
individual measurements on an earthquake will vary quite a bit. Most of the
regional seismic networks in the U.S. will average readings from several
stations to make their magnitude estimate. Again, because of some
difficulties in correction and the way seismic energy might radiate from an
earthquake somewhat differently to two different networks, the estimates of
magnitude might vary by a few fractions of a magnitude. Although this
occasionally seems like incompetence, it just reflects the uncertainty of
the whole proceedure. If you return to the light bulb analogy, if two
observers tried to estimate the wattage of the light bulb at two spots, they
would in fact measure the intensity and then try and correct for the
distance to the bulb. But if observer A had, say, a moth between her and
the bulb, her wattage (magnitude) might be a little low, and observer B
lacked the moth but caught a reflection from a window as well, his estimate
might be a little too high.
Why different magnitudes? It boils down to the way the Earth transmits
seismic energy. First, there are body waves and surface waves. Surface
waves only travel (create motion) within the upper few kilometers to a few
hundred km of the Earth; body waves can travel within the entire Earth. An
analogy in water are ocean waves and sound. Scuba divers are probably
familiar with the rapid decrease of motion from ocean swells (waves) with
depth--a ship can bob about a lot at the surface while a diver 100 feet down
might not move much at all. That is a surface wave. Sound, however,
travels equally well throughout the ocean, as SONAR demonstrates (we'll omit
all the peculiarities of SOFAR channels and the like, thank you very much
ex-submariners)--it is a body wave. Second, the Earth has different flavors
of seismic waves. For instance, body waves come in P and S waves. P waves
are compressional waves and S waves are shear waves; P waves travel faster
than S waves. If you want to see P and S waves, get a Slinky, lay it on a
nice, slick table, stretch it out some, and then push and pull one end
towards the other end quickly--you will see a wave travel down the Slinky of
coils close together and further apart than the Slinky at rest--this is a P
wave. Now rapidly move one end of the Slinky at right angles to the length
of the Slinky--you'll see a wave move down where the Slinky slides over to
one side and then back again. This is an S wave (more or less). Surface
waves also have different flavors, but for the most part this isn't
exploited in magnitude determinations.
OK, so what magnitude scales exist? There are more than I will go over, but
these are the most common: duration magnitude (Md), local (Richter)
magnitude (ML), body wave magnitude (Mb), surfave wave magnitude (Ms),
moment magnitude (Mw). The reason for using the different scales is that
each type of seismic wave is easily observed only over certain magnitude or
distance ranges.
Duration magnitude is used for small earthquakes or in areas with generally
poorly calibrated seismometers. It relies on the observation that the time
it takes for the motion from an earthquake to fall into the background noise
is proportional to other, more physically defensible measures of magnitude.
Local (Richter) magnitudes are measured for most earthquakes up to somewhere
around M 7; beyond that the measurement fails to increase with earthquake
size. This is basically Richter's technique--you measure the maximum
amplitude of ground motion (often restricted to a certain part of the
seismogram but not always), take that and the distance from the earthquake
and you can get the Richter magnitude. While the original definition was
based on a specific instrument at a specific distance, corrections for
distance and other seismometers have been worked out over the years. Large
earthquakes generally swamp the instruments used to make these measurements
and the Earth does an increasingly inefficient job of transmitting these
particular waves for very large events.
Body wave magnitude is a very similar measure to local magnitude except it
can be applied to the body waves from distant earthquakes and it has quite a
number of corrections for which body wave (P or S, for instance), the
Earth's structure, and the period of the wave being examined. It is
proportional to the log of the amplitude of ground motion divided by the
period of the wave being measured (A/T).
Surface wave magnitudes also use the log of the amplitude divided by period,
but surface waves are only generated by relatively shallow earthquakes.
Again corrections exist for distance, depth, and seismometer type. Surface
waves tend to saturate in the M 8+ range.
Moment magnitude has confused many people because it isn't really a direct
magnitude measure (log of an amplitude of a seismic wave) as are the others.
Instead it is derived from seismic moment, which is simply the product of
the shear modulus (a property of the rock), the fault area, and the slip on
the fault. Seismic moment is usually derived from fits made to entire
seismograms or large parts of seismograms using certain physical models;
this has been automated for several years by a group at Harvard and has been
applied in near-real time by several other groups including the USGS.
Unlike other magnitude estimates, you can also derive an estimate of seismic
moment from geodetic observations (how far points have moved on the surface
of the Earth due to an earthquake) or from geologic observations because you
only need fault area and displacement and some estimate of shear modulus.
In practice you will not see such estimates in the public media because they
take time to measure and are not necessarily measuring the same exact
phenomena (for instance, if sizable slip occurs without generating seismic
energy, a geodetic or geologic moment will exceed a seismic moment). The
measure "moment magnitude" is somewhat like the duration magnitude in that
it has been made to correlate with the other scales, though there are good
physical arguments for this relation. In practice, moment magnitudes are
the best measure because they can include all the observations that go into
the other magnitudes plus the duration of the arrivals plus additional
very-long period oscillations of the Earth.
In general these magnitudes have all adjusted been to agree where they
overlap (the worst case is usually Mb and Ms, which often disagree a fair
bit--mostly due to the way surface waves are generated). Because of this,
you could say that the ground motion would be 100,000 times greater in a
magnitude 7 than a magnitude 2 earthquake at a given spot despite the
different measures probably used (though the reality is that the ground
motion would have probably saturated). A statement less affected by the way
the Earth transmits energy would be that a magnitude 7 earthquake generates
about 16,000,000 times more energy that a magnitude 2.
Hopefully this goes some distance in explaining magnitudes.
Craig Jones
CIRES, University of Colorado, Boulder
cjones@mantle.colorado.edu
From: ted.smith@cdmg.uucp.netcom.com (Ted Smith)
Subject: [lm 5/1/95] Frequently as
Date: Mon, 22 May 1995 23:53:00 GMT
Organization: California Division of Mines and Geology
TO: eugene@amelia.nas.nasa.gov (Eugene N. Miya)
>Archive-name: ca-earthquakes, Part III
1. Here is an updated list of the GeoInfo BBS Network member BBSes:
Information as of May 22, 1995--
BBS NAME PHONE NUMBER INFO* LOCATION
----------------------- ------------ ------- ------------------------
Netmail Hubs:
GeoNet BBS 316-265-1994 2,I Wichita, Kansas
CDMG ONLINE 916-327-1208 4,I Sacramento, California
GeoFuel GeoScience BBS 905-829-4097 2,I Oakville, Ontario
GISnet BBS 303-447-0927 2,I Boulder, Colorado
Dark Matter 604-534-7667 M,F Langley, British Columbia
NASA MLP BBS 206-871-3965 2,F Port Orchard, Washington
Pacific PowerNet BBS (02) 959-9533 2,I Sydney, NSW, Australia
Megalopolis 801-489-7910 8,F Springville, Utah
Mountains West BBS 916-726-2771 1,I Citrus Heights, California
Net Nodes:
Computer Plumber BBS 319-337-6723 1 Iowa City, Iowa
PSN San Jose 408-226-0675 1 San Jose, California
PSN Pasadena 818-797-0536 1 Pasadena, California
COGSNET BBS 303-526-1617 1 Golden, Colorado
SurveyNet BBS 207-549-3213 1 Whitefield, Maine
U.S. Geological Survey 415-327-1517 1 Menlo Park, California
PSN BBS
[Also accessible at 1-800-328-1517 from some parts of
Northern California.]
RMAG BBS 303-473-0048 1,I Denver, Colorado
GeoTechnica BBS 303-933-0712 1 Littleton, Colorado
Lynn's Live Wire 408-746-0370 1 Sunnyvale, California
The Scientist's BBS 216-639-9508 10,I Concord, Ohio
Genius Loci BBS 904-383-0472 1,F Mt. Dora, Florida
Ideas and Innovations 904-742-1295 3 Mt. Dora, Florida
Earth Sciences BBS 801-487-9703 1,F Salt Lake City, Utah
Wild Man BBS 916-642-8832 3 El Dorado, California
MININGnet BBS 303-415-1182 2,I Littleton, Colorado
Newberry BBS 210-233-4877 2 Los Fresnos, Texas
The following system is operating but the connection with GeoInfo Net
has been disrupted for some time:
PSN Memphis 901-360-0302 1 Memphis, Tennessee
* Key to Info: All systems are equipped with 14.4kbps or faster modems,
unless otherwise noted. # = number of incoming lines, if known;
M = multi-line bbs with unknown number of lines; I = Internet e-mail
and/or USENET news available; F = FTS (FidoNet) available.
2. An index of World-Wide Web resources devoted to earthquakes and
earthquake seismology has been assembled and posted as part of Online
Resources for Earth Scientists (ORES). Point to
http://www.gisnet.com/gis/ores to access the main ORES gateway. The
earthquake resources are a subset of the Geology section of ORES.
-- Ted Smith
================================================================
California | Ted Smith
Division | -------------------------------------------------
of Mines & | Sysop, CDMG ONLINE (BBS modem line: 916-327-1208)
Geology | GeoInfo * MLPnet * SurveyNet * & Selected USENET
---
* QMPro 1.52 * Make a joyful noise (but deny it when smelled by others)!
A frequently asked question is "What is the seismic risk of living
in my state?" Bruce Bolt's 1993 book "Earthquakes" has a USA map
(figure 12.2) made by the Applied Technology Council in 1977.
The map is contoured in peak acceleration levels of fractions
of gravity for a 50 year period. A value of 0.10 gravities
can start causing damage. I compiled a list by state
of the maximum level in that state and location in that state.
Some interpretative comments follow the list.
STATE ACCEL LOCATION
Alabama 0.05 N
Arizona 0.20 W & E
Arkansas 0.20 W & NE
California 0.40 much of the state
Colorado 0.10 S & central
Connecticut 0.10 all over
Florida 0.00
Georgia 0.10 N
Idaho 0.40 E
Illinois 0.10 S
Indiana 0.10 SW
Iowa 0.10 SW
Kansas 0.10 E
Kentucky 0.20 SW & SE
Louisiana 0.00
Maine 0.10 much of the state
Maryland 0.00
Massachusetts 0.10 all over
Michigan 0.05 SW
Minnesota 0.00
Mississippi 0.20 NW
Missouri 0.20 SE
Montana 0.40 S & W
Nebraska 0.10 NW & SE
Nevada 0.40 much of the state
New Hampshire 0.10 all over
New Jersey 0.10 N
New Mexico 0.10 central
New York 0.10 N & E
North Carolina 0.10 W & S
North Dakota 0.00
Ohio 0.05 N
Oklahoma 0.10 E
Oregon 0.05 N (recent numbers are higher)
Pennsylvania 0.10 E
Rhode Island 0.10 all over
South Carolina 0.10 all over
South Dakota 0.05 SW
Tennesee 0.20 W & E
Texas 0.05 W & NE
Utah 0.20 much of the state
Vermont 0.10 all over
Virgina 0.10 W
Washington 0.20 N
West Virgina 0.10 S
Wisconson 0.00
Wyoming 0.40 NW
Comments:
- The map was made in 1977. Some places are better studied now.
For example, western Oregon is now known to experience
large quakes.
- The map appears to be based on historical large quakes.
Appendix B of Bolt's book lists these quakes.
- 42 of the 48 states have some risk.
- Landers (1992) and Northridge (1994) had accelerations up to
1.0+ gravities in the hardest hit areas.
