From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Newsgroups: ca.earthquakes
Subject: Re: "Earthquake Risks Called Exaggerated"
Date: 13 Dec 1992 07:06:13 GMT
Organization: Stanford Univ. Earth Sciences
Distribution: ca
NNTP-Posting-Host: pangea.stanford.edu

In article <51603@seismo.CSS.GOV> stead@sol.CSS.GOV (Richard Stead) writes:
>In article <1992Dec10.075333.6706@netcom.com>, bverreau@netcom.com (stargazer) writes:
>> So what do the experts have to say about all this?  I thought the seismic
>> gap theory - which relates to long term probabilities - was  pretty well
>> confirmed on different faults and quakes around the world.  Does this mean
>> that *both* short term and long term predictions are impossible?  Or are
>> there holes in the new theory?
>The seismic gap hypothesis is in tatters.  A paper in Journal of Geophysical
>Research just last year (Kagan and Jackson, 1991 pp. 21419-21431) reviewed
>the success of various seismic gap hypotheses.  The theory did not perform
>better than random.  In fact, large quakes occurred preferentially in areas
>of recent large quakes.  This is the opposite of the seismic gap hypothesis.
Stu Nishenko showed at AGU (as many of us had noticed and tried to
convince Jackson of) that the Kagan and Jackson test was not a test of
the seismic gap hypothesis.  The gap hypothesis only applies to
earthquakes on the actual plate boundary in the paper that K&J was
tested.  K&J however mushed together all sorts of other events that
were not on the plate boundaries (e.g. normal faulting events within the
subducting plate).  Dave Jackson gave the next paper and essentially
admitted that this was true.  However, he did point out that this
limited the number of events that could be predicted with a gap model.
Stu also pointed out that the gap model has been improved in the last
10 years to include local repeat times for the various plate boundary
segments.  The upshot is that 10 years just wasn't long enough to test
the model.

>Several other researchers, notably Kerry Sieh of Caltech have found evidence
>that large quakes cluster in time, with long time gaps between clusters.
This result was also questioned at the meeting although I forget by
whom.  They combined the Pallet Creek data with that from another
trench and showed that it was possible that there were some missing
events from Pallet Creek and that things could be more regular.  It
isn't clear who is correct on this one.

>Thus it is not surprising that the authors of the report in question have found
>that probability estimates of quakes are too high when implicitly based on
>the assumption that quakes are regular.  The faults with multiple large
>historic quakes have probably entered quiescent periods, so the recent
>frequency of quakes is too high.  Conversely, however, faults with no
>identified historic activity are that much more dangerous than expected.
>The Landers quake is an excellant example, and perhaps we can expect more
>activity out in the Mojave over the next couplke centuries now that the area
>has entered an active period.
I think it may be more true that regions enter active periods around
the time that their major fault segments (the ones that carry most
of the slip, e.g. the San Andreas) become active.  This happened
in the late 1800's before the 1906 earthquake.  On the other hand I
suspect we may be over-estimating the amount of irregularity that
can occur on these major faults.  Before we may have been
under-estimating it (where we=scientific consensus).

Andy
Newsgroups: ca.earthquakes