Tuesday, November 25, 2008

Re: seismic forces

Chris,
 
This goes to the bottom of my original post.
Thank you, 
 
V. Steve Gordin, SE
Irvine CA
 
 
----- Original Message -----
Sent: Tuesday, November 25, 2008 09:08
Subject: Re: seismic forces


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@skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 
1864)
http://www.skypoint.com/members/chrisw/



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