From: hough@seismo.gps.caltech.edu (Susan Hough)
Subject: Intraplate earthquakes (was Re: rain and earthquakes)
Summary:  arguing semantics?
Organization: California Institute of Technology, Pasadena
Distribution: ca
Date: Wed, 22 Jul 1992 18:18:33 GMT


In article <50981@seismo.CSS.GOV> stead@skadi.CSS.GOV (Richard Stead) writes:
>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
>
>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.

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.  

>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

Most people regard the New Madrid as a failed rift, not a currently
extending feature.  I agree that there are some semantic difficulties
with various classification schemes, but North America is widely regarded
as being a single plate.

>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

This kind of problem is, I suspect, what led Johnston et al. to look 
for a better classification than intraplate/interplate.

>> 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.

>say.  (But maybe we shouldn't get too far down some tangent that may get
>boring to everyone else).

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?
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
the paleoseismic observations that can unearth evidence of past large
earthquakes that have deformed shallow sediments.  One problem with these
studies is that they require the presence of the right kind of sediments
and the preservation of earthquake-generated features, things you don't
always have.  But this kind of study has been used to 
demonstrate the existence of previous Charleston
earthquakes, the suggestion of earthquakes to the immediate north
and south of the 1886 event, the existence of a previous large earthquake
in the Wabash Valley (Illinois-Indiana), etc.

Back to work,

Sue 

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



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