From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Subject: Re: Questions raised by SoCal Activity
Keywords: usgs refs, landers
Organization: Stanford Univ. Earth Sciences
Date: Sun, 16 Aug 1992 09:10:11 GMT

In article <jester.713904311@crash> lee@bnf.com writes:
>1)  Refer to book _Earthquake_Hazards_in_the_Los_Angeles_Region,
>    USGS, 1985, map pages 34 and 35 showing "...1970 to 1981 record of
>    well-located epicenters of M>=3 earthquakes...".  Note the number
>    of epicenters along the Desert Hot Springs-Yucca Valley-Landers
>    alignment, so well known now since June 28.  Also note the cluster
>    of epicenters around Lat N 34 35' Long W 116 18' compared to
>    recent activity.  What would a similar map for 1981 to 1991 show?
I'm making a first answer to this from home as I won't be in my office
until Tuesday.  As my copy of that book is there I can't really answer
that question.  However, the fact that there were small earthquakes
in that area is true and I'm pretty sure it continued at some level.
>    If the 1970-1981 level of activity was maintained through the
>    80's, should such areas of high seismic activity be placed on some
>    alert basis?
It depends on how you define such areas as I will discuss below.
>    It appears that the June 28 earthquake was
>    telegraphed for several decades.  Are there other areas in
>    California where similar long-term clusters of epicenters might
>    indicate future large-scale earthquakes (other than Long Valley
>    Caldera)?  Are such observations a part of the development of a
>    method of seismic prediction?  Why is a relatively quiet portion
>    of a fault considered to be a risk when the Landers experience
>    shows a great deal of activity may instead be the precursor of a
>    major quake?
The basic question here is what do small earthquakes tell us.
Certainly one of the major achievments of the installation of local
earthquake networks in the 1960's has been the ability to define
the structure of many active faults.  However, just what these
earthquakes mean for earthquake prediction is hard to say.  The
most obvious dense activity along the San Andreas fault runs from
the Loma Prieta segment to the Parkfield segment through the area
called the creeping segment.  In this area geodetic measurements
show that the fault is slipping at, or close to, the geologic
slip rate.  Thus no strain is being stored and the fault is moving
primarily through aseismic slip mixed with M<=5 earthquakes that
are responsible for about 5% of the motion.  No historic great
earthquakes are known in this area.  On the other hand the two
areas that are known to have slipped in M>8 events in 1857 and
1906 are almost without small earthquakes now.  Geodetic measurements
in these areas show that the fault is locked and storing strain.
The difference between the creeping section and the 1857 and 1906
sections are the basic observation that drive the idea that the
most dangerous parts of the faults are quiet.  Also at Loma
Prieta the fault that actually slipped had no background seismicity
on it, although the slip patch was outlined by activity on a
neighboring fault.  Again the mainshock came from an area that
was quiet beforehand.  The same was true at Morgan Hill although
now we are getting into cases where the fault has activity on it
that surrounds the eventual mainshock.  The good question here
is how to interpret moderate amounts of seismicity.  Do they
increase before earthquakes?  Do they decrease?  Or both?  The
best study I know of shows that both behaviors occur.  So while
there may be a discernable pattern before Landers in retrospect
it may not be true that we can quickly transfer that pattern to
all other faults, even within California.

The best reviews of California seismicity will be found in either
"The San Andreas Fault, California" editted by Robert Wallace,
USGS Professional Paper 1515.  This is truly a great book for
those interested in the California earthquakes and is written
for the lay person that is technically oriented although I rely
on it a lot too.  Should be available at USGS Map Sales offices
or through 1-800-USA-MAPS.  The other source would be the
appropriate volume from the Geological Society of America's
mammoth series DNAG or the Decade of North American Geology.
I can't remember which volume covers seismicity but any good
geology library should have it.

More later,
Andy