From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: "Earthquake Risks Called Exaggerated"
Date: 13 Dec 92 20:27:41 GMT
Distribution: ca

In article <1gend5INNf92@morrow.stanford.edu>, andy@pangea.Stanford.EDU (Andy Michael USGS Guest) writes:
> In article <51603@seismo.CSS.GOV> stead@sol.CSS.GOV (Richard Stead) writes:
> >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.

First, I'll admit that "tatters" was theatrical and certainly stretched.
However, I still have to side a little with Jackson in that it is
certainly true that if we restrict the tests to just plate bounding
faults, 10 years is far too short, and it may take 100 years to get
any reliable statistics.  Also, the seismic gap hypothesis is based
in part on pre-instrumental quakes, which may or may not have occurred
on the plate bounding fault.  Finally, if the seismic gap hypothesis
is restricted to plate-bounding faults its use in seismic hazard
estimation and prediction becomes limited as well.  I agree that
the hypothesis is designed with a single fault in mind, but nature
is rarely so cooperative.  If it turns out to be true when properly
limited, it may become little more than a scientific curiosity.

> >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.

However, this would also imply that either: 1) the characteristic quakes
at Pallet Creek are smaller than previously thought or 2) there is a lot
more total slip in the area than is recognized.  The reason I say that
is that Sieh and others have tried to analyze the slip implied by quakes
in the region to show it is well within the expected range.  Several
missed quakes would likely change that balance.  Sieh has found
evidence of bunches elsewhere, and the historic record in long-populated
areas (like the Mediterranean, China and Japan) may also indicate
bunching.

From theoretical grounds, I prefer bunching because I think it would
be hard to avoid in a stick-slip model with healing.  The idea here
would be that at low stress, the fault's healing processes can keep
ahead of the stress buildup, allowing a large stress to accumulate.
However, once that finally gives way, the fault would still be at
relatively high stress and healing will not procede quickly enough
to prevent more large quakes until the regime returns to a very low
stress.  Then the pattern would more or less repeat.

> 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).

True, science can swing around between two extremes before finding the
answer in the middle, and "we" don't yet know where the true answer is in
this case.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov