From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: "Hot, dry weather causes earthquakes"
Date: 24 Jul 92 19:16:29 GMT
Distribution: ca

In article <1992Jul23.155821.17008@cco.caltech.edu>, hough@seismo.gps.caltech.edu (Susan Hough) writes:
> Sorry, but my sympathies lie with Paul on this one.  A supposed mechanism
> to link weather and earthquake rupture?  How about large-scale atmospheric
> pressure fronts?  How about tidal conditions (which can be linked to weather)
> influencing earthquakes?  A very careful recent study of the San Jacinto
> fault showed a definate correlation between the tidal cycle and
> earthquakes.  Earthquakes and weather may not be linked on a one-to-one

I'm not familiar with that study - what's the reference?  I have seen several
careful studies over the years that have looked for a tidal trigger and have
never found a correlation.  If San Jacinto shows a correlation, I would say
it is the exception, not the rule, since so many other studies have never
found a correlation elsewhere.  (A reference?  How about two right out of
the same office you work in:  Harzell and Heaton, 1989, Bulletin of the
seismological Society of America Vol. 79, p. 1282 and Heaton, 1982, BSSA,
Vol. 72, p.2181.)

As for atmospheric pressure, that has always been a pet theory of mine.  Yet
a simple pressure front alone is insufficient.  Remember, the nucleation of
quakes occurs between about 5 to 15 km deep (and the biggest quakes usually
are at the deepest end of that range).  You have to affect the stress regime
down there and on the fault.  Clearly, it would be difficult to use
air pressure to generate a shear stress - air is a fluid and does not support
shear.  You would have to use some strange pattern of pressure on the surface
and then appeal to a strongly laterally varying crustal structure underneath
to produce a shear.  I have dabbled in that a bit and found no satisfying
way to generate a large shear force at depth using any reasonable structure.
Therefore, I feel it is unlikely that air pressure can trigger a quake by
forcing a fault, however, it may be possibly for pressure to trigger a
quake by unloading normal stress from a fault, reducing "friction" and allowing
the existing shear stress to move the fault.  This would require a lot of
computations (and a lot of assumptions) to verify.

> there's an awful lot that seismologists don't understand--one relavent
> item is exactly what trigger causes an earthquake to be at 5:00 am on
> Sunday rather than 11:00 am on Tuesday.  (Ever notice how many earthquakes
> occur right at dawn, or right at dusk?).  Is there a "last straw"?  We

Whoa.  That ain't the case.  Just group the quakes from the southern California
catalog by hour of day if you'd like to see it.  There is no favoritism
for dawn or dusk.  Actually if you just take M>6 events before 1983, they
concentrate in the morning (a quick check I did - 20 of the 36 M>6 events
in the catalog before 1983 occurred between 3am and 11am local time, but
the distribution is flat - no peak in that 8 hours at dawn.).  If you consider
a larger number of quakes, the morning peak disappears.

> just don't know.  Given that, I think that Paul is right when he says
> that scientists tend to be too quick (too arrogant?) in dismissing
> especially anything that smells like old wive's tales.  Electromagnetic
> precursors is another good example.

I don't think any scientist has dismissed Electromagnetic precursors.  Where
did you get that idea?  The results have been published in peer-reviewed
geophysical journals.  I have seen no attacks by other scientists against
the research.  However, I agree there is much skepticism of it.  One of the
keys of science is skepticism.  The problem with EM precursors is that there
is only one well-documented datapoint - Loma Prieta.  So the first question
is "Is Loma Prieta an exception?"  That is, if the signals are definitely
quake-related, it could be that they only happened for the Loma Prieta
quake, and maybe no other quake does that.  Another question is "Have
all other possible sources been eliminated?"  If not, the signals could
just be coincidental.  Another is "How often do such signals occur followed
by no earthquake?"  Very important, because if this happens all the time,
how do you tell when it is a precursor to a quake and when it is something
else.  Finally, "What's the mechanism?"  We're scientists, not mystics.
These things don't "just happen".  There has to be some reasonable
mechanism to explain the effect.  I have seen many explanations for this
one, but it would be nice to see someone actually run a model to see
if the explanation can actually generate the signals observed.  It may
turn out that when the mechanism is known, there may be a better way to
monitor it.  It has been suggested that the signal may be due to groundwater
movement - that could be monitored directly in wells.  It has been suggested
that it could be due to vibration - moving the receiving antenna in the earth's
magnetic field and inducing current that way - a seismometer is a better way
to measure vibration.

I see scientists disparaged in the popular press al the time, I don't like
but I like it even less here.  I think the EM precursor issue has been
treated rather fairly on all sides, and that there's no truth to the assertion
that "scientists have dismissed" this work.

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