Friday, November 30, 2018

The Geology of Anchorage, Alaska

There's the city, along a big basin.  The foundation is loose dirt.  I studied the 1964 earthquake extensively when I did my report on earthquake ground motions for rock and soil.  It has the distinction of being one of the highest amplifiers of pgv (peak ground velocity) in the world, perhaps next to Mexico City.

For the 1964 earthquake, the rock pgv was under 1 cm/s.  Old buildings on rock didn't have a speck of damage.  In fact, you can't tell how low that pgv was.  Anchorage had 50 to 100 cm/s, just like this earthquake.  The amplification was 50 to 100 times.

This is an exact duplicate of the Christchurch earthquake in every way.  Another M7-8 is possible, so I hope they don't 'approve' (green tag) a concrete office tower.  These things can only be approved by instrumenting before and after, and never by inspection.

You can see that it is at the end of a magnificent back-arc spreading basin.  Anything can happen on top of a subduction zone, and this rift is huge.  Expect further action, as such a basin must be maintained in light of huge sediment loads.  You can see that it is 'clean' and as such it must have m8's every few hundred years.  An m7 doesn't do much, although they will later measure that the basin went down, and the surrounding hills went up.

If I lived in a place here, that got a good hit, I would literally head for the hills, to grandma's house on solid rock.  The earthquake probably didn't even wake her.  Do not go to work in a 70's office building.  That is instant 'pancake death'.  


ps. Dear Alaska, do not read me, or attempt to interview me (ha!) because I think the usgs, US earthquake engineers, and nasa earth sciences are a bunch of idiots (to quote Sheldon).  :)

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