From: hough@seismo.gps.caltech.edu (Susan Hough)
Subject: Re: rain and earthquakes
Summary: careful 
Organization: California Institute of Technology, Pasadena
Distribution: ca
Date: Tue, 21 Jul 1992 17:18:19 GMT

In article <50977@seismo.CSS.GOV> stead@skadi.CSS.GOV (Richard Stead) writes:
>
>
>In a reply, Richard Ottolini felt this was "erroneous and alarming".
>I disagree.  He is correct in that recurrence rates for intraplate
>quakes are generally difficult or impossible to determine.  This is
>not true for the New Madrid area.  Several of the faults cut the surface,
>and drilling has mapped them at depth very well.  New Madrid is
>probably the second most studied fault region in the US after the San
>Andreas.  There is a tremendous amount of structural information and
>a lot of paleoseismic data.  I can't vouch for the 150 year figure -
>I don't have a reference handy - but it sounds short to me.  I thought
>average recurrence was closer to 300 years there.  The average recurrence rate
>has little meaning in this area since the intervals between quakes vary
>a lot.

Well, this may not be the appropriate forum for a scientific debate, but
I have to differ with some of this.  There are three "identified" faults
in the New Madrid regions that are generally thought to correspond to
the 3 1811-12 earthquakes, but there are problems with this interpretation--
the fault segments are awfully short to generate such large earthquakes,
and while present day seismicity is along these trends, the California
analogy says that background seismicity does *not* occur along the rupture
areas of future large events (i.e., seismic gap theory).  The paleoseismic
evidence is ambiguous--some people have claimed that there's no evidence
for any previous rupture going back some 10,000 years, while other people
do find equivocal evidence for past earthquakes on the order of 5000 yrs
ago.  I agree that the recurrence interval may vary widely--there's talk
now of possible bursts of seismicity--earthquakes that pop up out of 
nowhere in intraplate regions (where there may have been no seismicity
for 100,000 yrs), to be followed in relatively short intervals
(hundreds of years) by more activity.  

>One of the reasons that New Madrid is an exception to general rules regarding
>intraplate quakes is that it's not really a characteristic intraplate
>problem.  The New Madrid area is at the tip of a major failed third
>arm of rifting related to the opening of the atlantic.  (Rifting is

Careful!  Arch Johnston has led a thorough study of intraplate earthquakes
around the world, and concludes that most large intraplate earthquakes
occur at either failed rifts or along continental margins.  A failed
rift may not be typical intraplate crust, but there are plently of them
lurking throughout the world, and they easily could account for the
typical intraplate earthquake.  Mary Lou Zoback of the USGS in Menlo
Park has done some nice recent work to show how failed rifts will effect
the local stress fields, and shows that their presence can either increase
or decrease the likelihood of an earthquake, depending on the various
orientations involved.

>very slowly.  There are a lot of theories as to why these quakes are so
>powerful when the faults are comparatively short, but none can really
>be proven or disproven at this point.

Maybe we don't have the right faults identified!!  

Sue Hough
USGS, Pasadena
Newsgroups: ca.earthquakes