From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Intraplate earthquakes (was Re: rain and earthquakes)
Date: 24 Jul 92 18:29:38 GMT
Distribution: ca

In article <1992Jul22.181833.4044@cco.caltech.edu>, hough@seismo.gps.caltech.edu (Susan Hough) writes:
> This may well be disintegrating into a question of semantics.  What
> Johnston & co. has done is to first identify what they term the
> "stable continental regions" around the world.  These definitions are
> not based on seismicity but on an assessment of tectonic structure.
> According to their classification, Borah Peak (in the Basin and Range,
> a region of current active extension) does not qualify as "scr", although
> Australia does.  The African rift zone also does not qualify because it
> is an active tectonic feature.  The New Madrid failed rift zone
> qualifies because it is a failed rift; no longer an active tectonic
> feature.  This classification differs from the classic intraplate/interplate
> distinction primarily because it focusses exclusively on those regions that
> should be stable from a tectonic point of view.  The difference between
> Borah Peak and New Madrid is that in the former case we know that the
> crust is being pulled apart; in the latter case there are no similar large-scale
> tectonic forces currently at work.  

Indeed a question of semantics.  All that has been done here is shift the
ambiguity from what one considers "intraplate" to what one considers an
"active tectonic feature".  Why does the E. African Rift count as active and
not the Mississippi embayment?  It seems to me they differ only in degree.

> >> 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
> 
> How about a shallow-angle detachment?  (i.e., a nearly-flat lying zone
> of weakness that would have no surface expression).  There really is
> a problem figuring out how relatively short identified segments in New
> Madrid could account for the amount of very long-period energy that must have
> radiated from those earthquakes to account for some of the long-distance
> effects that were observed.

Shallow-angle detachments are much easier to spot these days.  Afterall, that's
basically the kind of structure that the Whittier Narrows quake occurred on.
Similarly, the Coalinga quake occurred on a shallow-angle core thrust of
an anticline, where the thrust is rooted in a detachment.  These are known.
The California seismic refraction consortium CALCRUST has spent some of its
time mapping the detachments in the western Mojave (I was involved a bit very
early on in those experiments).  There's a big problem with appealing to
detachments out in Missouri - where does the detachment terminate?  Remember,
a detachment is essentially a shallow-angle thrust.  There has to be a region
that the upper sheet is moving away from, and a region that it is moving
toward.  A classic detachment is the basal Appalachian detachment - the source
of the movement was collision with Africa.  As the oceanic crust under the
continental margin was subducted, the overlying sediments were thrust toward
North America.  A basal detachment formed that carried the deforming upper
sheet in toward the continent.  The folded Appalachians represent the terminus
of the thrust - the differential motion between the old basement and the upper
sheet was taken up by folding the upper sheet.  Where do you find such present
day structures to appeal to around New Madrid?  There is no known active folding
or subduction in the vicinity, so there is no place that a detachment might
have a source or terminus.

> I think we're slipping along that tangent already.  I guess the questions
> of general interest are:  Is the New Madrid region so unusual that we don't
> have to worry about that type of earthquake elsewhere in the US?

The other area that could be similar is the St. Lawerence area, but that's
probably a long shot.

> Is the Charleston region also unusual?  The former region is associated
> with a particular feature that might or might not be termed unusual, but
> certainly others exist in the US and elsewhere in supposedly stable
> regions.  That latter region is more arguably not unusual for the Atlantic
> coast.  The best way to answer these questions, in my opinion, is with

Right, the same thing occurs all up and down the Atlantic margin.  I grew up
a mile from a similar fault zone (called the Huntington Valley fault where
I lived) in Pennsylvania just north of Philly.  There are known zones near
Boston, New York, Baltimore and S. Virginia as well.  Pretty common and often
very close to major cities that have no quake safety codes.  Most are thought
to be incapable of generating very large quakes, but few have been studied
to the detail that New Madrid and Charleston have.

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