Friday, April 15, 2011

New Madrid Earthquake - Still at M7.0


This is another paper by Susan Hough, pushing down the magnitudes of the New Madrid earthquake sequence.

According to The Commercial Appeal newspaper, the paper by Susan E. Hough -- a research seismologist with the U.S. Geological Survey -- says new reviews by independent experts put the magnitude of the three main earthquakes between Dec. 16, 1811, and Feb. 7, 1812, at a maximum of about 7.0. 

If you look at the seismicity map for this area, I consider that most of these earthquakes are the aftershocks of the 1811 sequence.  That puts the cross-thrust at 40 km, and the shear wing at 80 km.  An M7.0 should be about 30 km (about the length of the Hamilton fault).  Thus, I think we should give them maybe, an M7.2 ??

But, really, this is nit-picking.  The real damage comes from those very soft soils, which I think can amplify more than Christchurch!  If you have a basin with a basin, such as Mexico City, then the amplifications go well beyond a factor of 10, and approach a factor of 100.  Plus we must expect some directivity pulses, due to super-shear fault rupture.  All in all, we might get PGV's of up to 1 m/s, which will destroy anything.  The exact magnitude of the earthquake doesn't matter at all, since ground motions saturate at M7.0 anyway.  Instead of 30 km for an M7, we just get 300 km for an M8.

ps. looks like they stopped the secret injection at Arkansas, which was an ideal miniature of New Madrid.

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