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Re: seismic forces

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On Nov 25, 2008, at 8:47 AM, Mike Jones wrote:

Walter Jon Williams wrote a fictional account of a NM event happening "Today".
Probably something like 'Lucifer's Hammer.' I used to read a lot of apocalyptic science fiction--if Williams is as good as _The Stand_ (Stephen King) it'll be pretty damn good.

In a related vein, the statistical approach to earthquake occurrence give me serious heartburn. Nuke plants all have reams of SSE and OBE response spectra, including plants in Minnesota and Nebraska and other states with no particular seismic history. I play by the rules and use them as if they were revealed wisdom, but sometimes I wonder what the reality actually is. The whole process is so non-linear, I can't imagine that any two damaging earthquakes really have any similarity. The plates seem to move in the same relative direction, but the contact geometry must really change enormously over time and certainly during an earthquake. Tack on the discovery of new fault lines from time to time and the predictability must be very iffy.

The small quakes probably do repeat to a degree, but extrapolation of small quake behavior, which necessarily doesn't change much, to large quake response which can move river beds seems like a leap of faith, not science.

It really seems like a big stretch to pronounce a certain acceleration as having a '2%-in-50 chance of exceeding an M7.1 to M7.3 earthquake.' Statistical analysis of this sort demands a fairly large population of nominally similar members and a solid quantitative data base for numerical comparison. Damaging earthquakes just don't fit these requirements, especially the big ones like the Charleston and New Madrid quakes. We really can't determine when a future earthquake may occur, let alone how big it might be, simply because we only have so few examples. The idea of imposing some notion of a time element on the really damaging quakes, to quantify repetition seems far-fetched. The validity of a 2500 year event is nonsensical. Seems to me like the do-something-even-if-it's-wrong philosophy that we've all had to sweep up after.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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