Tuesday, December 4, 2012

Alaska Earthquake M5.8 - The Cluster Gets Serious

We've had quite the weird cluster in Alaska, mainly veering away from the subduction trench and going right under Anchorage.

Unlike my favourite harmless cluster in the Virgin Islands, this one is escalating, with the fault plane solution showing it is right on the slab, and loosening things up.  Wish I had the pattern before the big one in '64.

As we see here, Anchorage is not happily situated with regard to earthquakes.  You are either on dirt, or on an eroding mountain.

They will be lucky to get out of this with an M7 or 8.  Here's where the rubber hits the road with regard to soft soil.  For 40 years, the US engineers have said that soft soil is wunderbar for earthquakes because the peak acceleration goes down.  This they attribute to 'soil damping'.  Going with this flow, the city is golden!

But, if you realize that peak acceleration is garbage, then you use peak ground velocity (PGV).  And soft soil amplifies PGV by a factor of more than 10, perhaps up to 100.  Because of this, all the building codes are wrong when it comes to soft soil.  This has been proven many times by other earthquakes, not in the US, so they don't really exist.  :(

I really hate for Anchorage to be the proof of the soil pudding, but somebody has to be, Virginia wasn't strong enough.

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