Tuesday, December 7, 2010

The Physics of the Super-Shear Earthquake Part 2

Now, consider the average fault rupture exactly like setting up a dominoes fall.

It is quite difficult to set things up so that you tip one, and they all go.  When I was a kid, quite often I would have a big one set up, and then OOOPS!  Very frustrating.  For faults, the critical parameters are Critical Displacement (dc), and stress drop.  The dc is the distance the fault has to slide to shear off those last adhesion points and go into dynamic friction.  The stress drop measures how much that fault friction drops.  In dominoes, these things are set by the geometry of the pieces.

A normal earthquake has a high dc, probably over a metre.  For a rupture, the whole fault has to be set up so that there is a high stress drop, and the dc is reachable by the propagating S wave.  The P wave zooms on ahead, and is ignored.  As the rupture starts, the first stress drop goes into seismic energy, which has to be sufficient.  The wave goes ahead, and applies a force to create displacement.  More seismic energy is liberated as the dominoes fall.  These earthquakes are usually bad enough.

Now we come to this delicious paper.

Sad old Yucca Mountain!  But they put a lot of money into it.  Next episode we'll see what we got.

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