Monday, February 2, 2009

Famous Earthquakes - Whittier Narrows 1987

The full title is 'Famous Earthquakes I Have Known'. These are earthquakes that happened during my long earthquake career, and affected the way I think about earthquakes. Needless to say, I think a lot differently than anybody else! So, I have started this series to see where I went wrong. :)

The Whittier earthquake first introduced the term 'blind thrust' to my tiny brain. The fault rupture was along a previously unknown fault that ended in an anticline. It did not surface, therefore it was 'blind'.

This was actually quite a big deal in earthquake land! Previously, all thought had been bent towards the Big One, along mapped faults. They went through all this trouble of mapping faults, and preventing building right on top of them. Here was a kick in the head!

Once again, the main damage was on the soft soils, while most of the housing that was on rock (what passes for rock!), was essentially undamaged.

Whittier was one of the first urban earthquakes that was extensively instrumented. It was only about an M6, but produced very strong ground motions at a considerable distance. Although I never saw a paper on it, I believe it to be the first instrumented 'Fist of God' earthquake, which involves super-shear (high speed) motion on the fault, and a large velocity pulse. The results from this earthquake started to show the very tight relationship between PGV and general damage.

That, combined with the other earthquakes generally convinced Shakemap to go with PGV. Conventional seismic engineering goes with PGA (bah!).

I knew from my computer modeling and blasting data that PGV was directly related to the maximum strain of the plane wave (body shear), and that this translated directly to the induced shear in a structure. Nearly all the structures at Whittier failed in a mode of applied shear. This earthquake also did a number on the suitability of 'tilt-up' construction. For although this constuction was theoretically good, it was very susceptible to rust and rot.

Finally, I was impressed by the extent of interior damage for this earthquake. This showed you could have buildings survive, but be totally useless for further occupation, because all the flimsy lights fell. For 'Experience Data' on nuclear plants, it showed the importance of securing the lights in the control room.

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