From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Newsgroups: ca.earthquakes
Subject: Re: Newly Mapped "San Jose Fault?"
Date: 17 Dec 1992 08:08:49 GMT
Organization: Stanford Univ. Earth Sciences
Distribution: ca
NNTP-Posting-Host: pangea.stanford.edu

In article <1gll3tINNdsr@gap.caltech.edu> hough@seismo.gps.caltech.edu (Susan Hough) writes:
>>> In article <-288709981@hpcupt1.cup.hp.com> stachnik@hpcupt1.cup.hp.com (George Stachnik) writes:
>>> >2)      The article indicated that this fault has not 
>>> >	moved in a long time (1000+ years?  My memory fails
>>> >	me...).  Would that suggest a fault that is
>>> >	dangerously locked?  Or one that's dormant?  (The
>>> >	article characterized it, if my memory serves me
>>> >	well, as "active".  How can it be active if it
>>> >	hasn't moved in 1000 years?)  
>I've deleted lots of the discussion to focus on this one question:
>The magnitude 7.4 1992 "Landers" earthquake occurred on 
>a series of fault segments that probably had not ruptured 
>in the last 3000-5000 years.  The faults that produced the
>1811-12 New Madrid earthquakes probably have a repeat time
>on the order of 5000-10,000 years.  Repeat times of active fault 
>segments can easily exceed several thousand years.
>I think one definition of "active" may be any Holocene movement.
>
>An interesting point to chew on is as follows:  If the Landers and
>New Madrid earthquakes really are beasts that comes along only 
>once in 5000+ years, were we just extraordinarily lucky (or 
>unlucky) to have seen them during the historic record? If you
>think about it, the answer almost certainly has to be no--there is
>no reason to think that the last 200 years were in any way special;
>rather there must be _lots_ of faults lurking out there with long
>repeat times but potential to produce large earthquakes.  In any
>one 200-yr interval (or in any lifetime for that matter), a number
>of these faults are likely to pop off.  With continued geologic
>and geophysical investigations, more and more of these faults will
>be identified.  But the fuss over one or two more faults in the
>Bay Area (or L.A.) strikes me as overblown--we already know that
>California is a patchwork of different faults; it isn't that big
>a deal to identify a couple of them.  The picture that emerges of
>seismic hazard is admittedly not very comforting--much less a story
>of quasi-periodic rupture along a narrow plate boundary and much
>more a story of almost fractal fracturing.  Some people would go
>as far as to say that any magnitude earthquake is possible anywhere,
>just with very low probability in some areas.

I just thought it was worth saying that I agree with this view
of the seismic hazard with one small modification.  Regardless of
the short term periodicity of the faults the San Andreas will have
most of the really big earthquakes in CA over the long term.  It
is responsible for most of the deformation and it can't do that
without lots of earthquakes.  In the last 200 years the two largest
earthquakes by far were both on the San Andreas (1857 and 1906).
Both of these events appear to have happenned before from trench
data and the large offsets along the San Andreas.

>Doing my part to help you all sleep better at night,
Sounds like a good idea.

Andy