From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Newsgroups: ca.earthquakes
Subject: Re: AGU Annual Meeting Brief in Science Magazine
Date: 8 Jan 1993 21:06:16 GMT
Organization: Stanford Univ. Earth Sciences
Distribution: na
NNTP-Posting-Host: pangea.stanford.edu
Keywords: Landers, Andy Michael

In article <1ihrgiINNff2@gap.caltech.edu> sdg@wag.caltech.edu (Siddharth Dasgupta) writes:
>Since I have seen no mention of this yet, I decided to post it.
>In the last issue of Science magazine, there is a two page
>meeting report from the AGU annual meeting in SF recently. One
>item in particular caught my attention because of where I live
>and also because it mentions Andy Michael's work. Other nettors
>might find it interesting.
>
>LANDERS QUAKE'S LONG REACH IS SHAKING UP SEISMOLOGISTS
>
lots of stuff deleted

>Those distant shocks have startled seismologists as well as ordinary
>residents. Conventional thinking, at least among U.S. researchers,
>holds that the stress generated when a fault slips in an earthquake
>peters out within a distance equal to a couple of times the length of
>the ruptured fault. For Landers, where about 70 kilometers of fault
>ruptured, that would amount to only about one-tenth of the observed
>reach. Researchers are now inventing mechanisms to explain the
>disparity, ranging from a domino effect of slipping faults to geologic
>equivalent of shaking up a can of soda. And they are beginning to shed
>some of their traditional skepticism about such long-distance
>connections (Science, 15 March 1991, p. 1315) as they look for
>possible long-range links among earlier quakes.
The two types of stress transfer considered are the static stress changes
which die off quite quickly and the dynamic stresses from the waves.
These can travel much further, but only act for a small amount of time.
It is how to get those dynamic stresses to cause a sequence of events
that lasts for days that is the interesting question at this point (IMHO).

>After hearing about the Landers triggering, Minster followed a hunch
>and found the algorithm's prediction of Loma Prieta depended heavily
>on the earlier seismicity at four of the same volcanic areas sensitive
>to Landers -- areas that lie hundreds of kilometers from Loma Prieta
>as well. "At this point it's only a suggestion" of another
>long-distance connection, says Minster, but "if true, the algorithm
>will have pointed us toward a [new] physical phenomenon. That is
>useful even if it doesn't predict earthquakes."

I'll be honest, I didn't hear Minster's talk but have talked to others
about it.

>[Would Andy Michael care to comment on how accurate the reporting is?

The reporting is very accurate.  Dick Kerr is probably the best science
writer covering earth sciences today.  Also he did give us the chance to
correct any blunders when he had a draft of the article.  So any real
errors would be our fault and not his anyway.

>Also I have some questions that this article raised. This long
>distance connection - has it not been observed before - say the Sierra
>Madre quake recently or the Whittier or the two large Northern
>California ones also recently? Or is this connection only now being
>sought for these earlier events? If this type of connection was not in
>the list of things to investigate earlier, what tripped Andy and
>others to do it this time?

The connection is being sought for some earlier events, but one has to
remember that Landers is really a substantially larger event than the
other ones mentioned.  And its rupture direction directed a lot of energy
toward the east side of the Sierra intensifying its effects at many of
these areas.  The Cape Mendocino mainshock ruptured out toward the ocean
and so did not transfer as much energy to these regions.  Basically what
happenned this time is that the increase was quickly noted at Mammoth
due to Dave Hill's intensive monitoring efforts there, and then when it
became clear that this change was real he and Paul Reasenberg started 
searching for it other places.  And the Little Skull Mt event was hard
to miss.  The problem is that one linkage can always be dismissed as
coincidence, but with enough of them the pattern pretty much hit people
over the head.  I got involved after a few days when I got back from
a meeting.  It seemed important to pin down the initiation of these
events in time and space, but this hadn't been done because the Lander's
mainshock was obscuring any possible local earthquakes.  So I started
trying some signal processing techniques to enhance them.  Turned out
a simple bandpass filter was all that was necessary.

>What exactly is microseismicity? Also is the computer program from the

Small earthquakes.  Its a fairly loosely used term for anything under
M4 or so.

>Russians available to most seismic researchers in US? How accurate

Yes.  Although their code is almost impossible to understand which is
why a few people including Minster were funded to study it.

>have its predictions been and what is predicted for the future? I

Its predictions are for very broad areas and time windows and their
accuracy is hotly debated.  Most people seem to have written it off
as data fitting and not actual prediction (I tend to agree with this
view).  I'm not sure what predictions have currently been made, but
then no one would expect them to be considered for policy actions
given the high level of controversy surrounding this technique.

>understand it has to correlate increased seismic activity over a wide
>area before it can predict such an event, but what time window does it
>take into account when it correlates? Comments welcome.]

I think the time windows are on the order of 6 months to a year.  It
varies depending on the size of the event you think you are trying to
predict.

Andy