From: sdg@wag.caltech.edu (Siddharth Dasgupta)
Newsgroups: ca.earthquakes
Subject: AGU Annual Meeting Brief in Science Magazine
Date: 7 Jan 1993 18:07:14 GMT
Organization: California Institute of Technology, Pasadena, CA
Distribution: na
NNTP-Posting-Host: eastie.wag.caltech.edu
Summary: Landers Quake's Long Reach, Andy Michael
Keywords: Landers, Andy Michael

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

As if a big earthqauke weren't unsettling enough, it is inevitably
followed by lots of smaller ones -- just the thing to give victims the
jitters for months after the big shake. True to form, the magnitude
7.5 quake that shook the Southern California town of Landers last June
has been followed by a long, rumbling train of aftershocks. But this
time, much to researchers' astonishment, the aftermath wasn't limited
to the vicinity of the main quake -- the Mojave Desert east of Los
Angeles. Instead, they reported at the meeting, the tremblor triggered
surges of seismic activity up to 1250 kilometers away, in areas as
distant as Northern California and Yellowstone National Park.

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.

In the case of Landers, it didn't take a lot of looking. "If I showed
my mother" some of the data, Paul Reasenberg of the U.S. Geological
Survey (USGS) in Menlo Park told an AGU session, "she'd have little
trouble seeing" the connections. In records from Long Valley, east of
Yosemite National Park and 450 kilometers north of the epicenter, for
example, Andrew Michael of the USGS in Menlo Park found surges of
magnitude 2 and smaller quakes beginning just 30 seconds after the
first seismic shear waves arrived. One such coincidence would have
been ignored, Reasenberg said. But other seismologists had little
trouble finding abrupt increases of seismic activity at 11 other sites
strung out north of Landers (150 kilometers east of Los Angeles) along
the eastern California border and into the northern part of the state,
Idaho, and Wyoming. The most dramatic activity by far struck Little
Skull Mountain in southern Nevada, where an immdeiate surge in
microseismicity culminated in a magnitude 5.6 quake 22 hours later.

In the standard picture of how the crust transmits stress from a
rupturing fault, there's no place for such distant effects. The crust,
assumed to act like a uniform, perfectly elastic layer, should gobble
up stress like a big block of foam rubber. But Joan Gomberg of the
USGS in Denver and her colleagues suggested at the meeting that the 
real-world crust, which is riddled with innumerable faults, might
act more like a mosaic of rigid blocks. When one fault slips in an
earthquake, it might trigger silent slip on a cascade of increasingly
distant faults culimnating in a second, distant earthquake. Listeners
were generally skeptical of this scenario, but they couldn't rule it
out in the case of Little Skull Mountain, at 280 kilometers away among
the nearest of the distant aftershocks. However, most researchers
agree with Reasenberg's gut feeling that the most distant sites are
beyond the reach of interconnected, silent faults.

What most seismologists are falling back on is not long-range stress
transmission but shaking: the seismic waves that spread away from
Landers. The most powerful prolonged shaking comes from surface waves,
which are ocean-wave like undulations of the uppermost crust; their
arrival at several sites coincided within seconds with the surge in
seismicity. By working the crust back and forth, Reasenberg says,
surface waves might generate enough local stress to trigger ruptures
on faults already strained to near the breaking point -- much like
banging on a wall to free a stuck window.

Shaking alone, however, can't explain the duration of the seismic
surges. The seismic wave from Landers was over in a few minutes, but
the distant aftershocks kept rumbling away for hours or days. Some
seismologists see a clue to what might have stretched out the activity:
Many, but not all, of the triggered sites lie in volcanic regions,
with magma or geothermal fluids beneath the surface. No one knows
exactly how deep fluids would help, but it might have been as
straight-forward as shaking a soda bottle. The seismic waves might
have driven dissolved gases out of the magma, which would have
pressurized the rock around the magma and triggered quakes until the
pressure leaked away.

Those same regions may be sensitive to other kinds of distant
influences as well. At the meeting, seismologist Bernard Minster of
Scripps Institution of Oceanography told how he had come across hints
of a connection between some of the sites sensitive to Landers and
1989 Loma Prieta earthquake -- but in this case the distant rumbling
preceded rather than followed the main event. Minster has been
dissecting a computer program developed by Volodya Keilis-Borok of the
Institute of Earthquake Prediction Theory and Mathematical Geophysics
in Moscow and his colleagues that monitors huge areas for possible
precursors of a large earthquake, such as swarms of small earthquakes
triggered by deep-seated crustal strain. The algorithm predicted the
Loma Prieta quake, but it is so complex that no one was sure whether
its success wasn't just good luck (Science, 15 March 1991, p. 1314).

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."

[Would Andy Michael care to comment on how accurate the reporting is?
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?
What exactly is microseismicity? Also is the computer program from the
Russians available to most seismic researchers in US? How accurate
have its predictions been and what is predicted for the future? I
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.]


-- 
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Siddharth Dasgupta (sdg@wag.caltech.edu) (818) 397-2725
Materials & Molecular Simulation Center, Beckman Institute, CALTECH,Pasadena
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Newsgroups: ca.earthquakes