Earlier this month there were a couple of posts about the New Madrid Earthquake
s of 1811 & 1812. The cause of these quakes was pretty much a mystery until th
e USGS shot a seismic line over the area in the late 70s (I think). The found
what appears to be a failed rift. This is a rift or breaking apart of the eart
h's crust that started, but failed to continue. We have similar rifts today in
East Africa and if they continue an ocean will form there. If they fail to co
ntinue the area will probably get covered up, filled in, and be like New Madrid
.. Paul is correct that the Charleston quakes were caused by a similar fault sy
stem. (I assume there is seismic evidence for this area too, but I've never se
en it.) Rifts occur when the crust of the earth pulls apart (or is pulled apar
t) This is how we got the Atlantic ocean, and it is still growing. However,
North America today is under compression and the quakes along the New Madrid fa
ult zone are apparently the result of an occasion slip along these old scars.
Really large quakes like 1811-1812 are estimated to occur only every 500-700 ye
ars, so the New Madrid fault is much less active than say the San Andreas. Som
ewhere around 1984 I went to a great conference at Cape Girardeau, MO where all
this was explained. The material presented at this conference was published b
y the USGS and if you want a good reference you might try to find this in the G
ov. Documents section of your library or contact the USGS. Somebody permanentl
y borrowed my copy so I can't give you the exact reference. The inferred mecha
nism for the major plate movements that result in all these things is convectio
n in the earth's mantle and it seems unlikely that impacts of objects from spac
e would play a role. Sorry, but I can't remember the dates of the original rif
t, but I think they are older than 320 million years. There are lots of people
looking for these old crators because the dust from the impacts has been propo
sed as the cause of climate changes which in turn caused major impacts in life
on earth. (extinction of dino etc.) 320 million is in the middle of the Carbo
niferous when we were busy growing the stuff which later made coal in this part
of the country. There were minor changes causing the Carboniferous to be divi
ded into Mississippian and Pennsylvanian which occured 320 million years ago so
these impacts might be responsible for this division, although I'm not aware o
f any great extinctions at this boundary.