Wednesday, March 14, 2012

Famous earthquakes: 1952 Kern County M7.5

Some rather high estimates of possible ground motions for the next big Cascadia slip, have led me to look for high ground motions with historical earthquakes.  Invariably these earthquakes are 'Superquakes', or my own term 'Fist of God'.  These are deep faults, near vertical thrusts, and they are so smooth when they rupture, that the rupture velocity approaches the speed of sound in rock (P wave velocity).  Most fault ruptures just trundle along at S-wave velocity or less.  I have written extensively about this.

The White Wolf Fault only had a length of 34 miles, and was thought to be only a minor threat.  This is a typical length for an M7'ish earthquake.  In actuality it was quite amazing that this fault showed its face at all, since most of these types of faults are deeply buried.

The highest intensity was MM XI, which has a PGV of about 2 m/s.  At this level outstanding things tend to happen:  things fly, trees and poles snap, etc.  There was absolutely nobody living around there, so just the railways got the worst of it.  These superquakes only produce the 'super motion' in a small zone around the fault.

Had there been buildings in the thrust zone, nothing would have survived.  Thus, we can see that the biggest threat in terms of ground motion are these superquakes at around M7.  M8's are just a string of M7's, which you can see if you look at the detailed rupture velocity maps.  M9's are a series of M8's.  :)  Oceanic M9's do not produce high ground motions.

Why the sweet spot at M7?  That's because the fault rupture length is 30 km or so, which is the usual depth to Moho (molten rock).  This is the maximum length where we can get a pure thrust pulse.  When you get to the M8 300 km, you can see that it is a long thin spaghetti on the crust, and has to rip like a zipper.

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