Tuesday, April 16, 2013

M7.8 Earthquake Iran

This is set in an area of pure compression, as the African Plate and Indian both jam up.

And therefore, we get a 'beachball' of pure compressional thrust.  Very shallow.  Most likely this was a 'super quake', similar to Kobe or Armenia.  That is when the fault rupture velocity exceeds the shear wave velocity, and essentially forms a shock wave of compression.  When that hits the surface, rocks fly in the air, and the PGV probably exceeds 2 m/s.  Nobody has been around to actually measure one of these things, so we don't know.

Update:  Pooey!  The beachball shows normal faulting.  Always get that wrong!  Forget the super-quake then, it's just average, which is enough to knock all the rubble houses down.

Update2:  I take that back.  A shallow thrust super-quake could totally screw up the beach!  I would have to double-check Armenia, if I wanted to do work.  :)

Update3:  Blah, deep extension earthquake, with no PGV.  This area is like the Sea of Japan, being wrenched apart by 2 plates with different vectors.


Christoph said...

Hmm, thrust faulting is what I expect, normal faulting is what I see. In the historic beachballs, too: http://static2.emsc.eu/Images/EVID/31/312/312831/312831.EMMA.jpg

Harold Asmis said...

Darn, I always get those things upside down! That would probably mean a lot less PGV, but in a land of rubble, you won't be able to tell the difference.

Christoph (http://www.paleoseismicity.org/) said...

D'accord! Let's keep fingers crossed that there's no second Bam happening.

Harold Asmis said...

It's so isolated and poor that total damage saturates with a very low PGV. The only hope to determine the mechanism would be detailed fault mapping and some sat radar. As with Kern County you might see some huge slabs of rock lifted, which would give you the 2 m/s PGV.