From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: rain and earthquakes
Date: 21 Jul 92 22:22:19 GMT
Distribution: ca

In article <1992Jul21.171819.14565@cco.caltech.edu>, hough@seismo.gps.caltech.edu (Susan Hough) writes:
> 
> 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.

Sure, but I guess it's a matter of what boxes you want to put your quakes
in.  To me, a classic intraplate quake would be something like the Borah
Peak quake or the quakes near Alice Springs in Australia, etc.  These are
quakes that have no direct fault links to the plate bounding faults.
Failed rifts are a direct link to the plate bounding faults and some may
represent active rifting - a plate boundary in the process of forming.
Afterall, we know that the Mississippi embayment has been progressing
north since the formation of the Atlantic, that the Saint Lawerence
seaway is a continuous region of current seismicity and the only thing
separating these two is an old suture through the craton.  Both also
terminate against plate bounding faults.  This could be the separation
of North America into two plates.  So is it intra-plate or not?  I'd like
to put it in a third box - one for faults that connect to the plate
bounding faults but are not through-going themselves.  The East Africa
Rift is a better example.  No one doubts that east Africa will eventually
rift from the continent, however the northern end of the rift is clearly
the failed thrid arm related to the opening of the Red Sea.  The continental
margin quakes are another special case - actually two of them.  One, like
Charleston, is probably related to the relic transforms in the oceanic
part of the plate that may not be so relic.  The others are normal faulting
parallel to the margin.  The stresses involved in these two are entirely
different.

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

Could be.  I like to keep an open mind on these things.  But the area has
been extensively drilled and a tremendous amount of refraction and reflection
profiling has been done.  It's hard for me to see where to put a more
significant feature than those already identified.  Afterall, the structures
on which both the Coalinga quake and the Whittier quake occurred had been
identified by drilling and seismic imaging before the quakes occurred.
They simply hadn't been identified as seismic features.  Yes, they don't
cross the surface, but they are easily seen.  I would expect that given the
lessons of these two quakes, the researchers involved at New Madrid would
have had a look for such things.  Maybe not.

> Well, this may not be the appropriate forum for a scientific debate, but

Sure it is.  Perhaps not everyone will follow the discussion (I do try
to keep jargon out of it, even when debating a point like this), but
I'm sure most of those who read the group can because they're generally
interested in quakes.  If I have to use something that may be unfamiliar,
I'll quickly explain it in the post (like the description of failed rifts
in my last post).  Most of the readers will be happy to see what you have to
say.  (But maybe we shouldn't get too far down some tangent that may get
boring to everyone else).


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