Structural Engineering: RE: seismic forces

Chris,

Depending on your POV, Williams may even be better. As a talented but not bestselling SF/Fantasy author, Williams, unlike Mr. King, still gets edited...

(And I have read THE RIFT. Very well written.)

Gary

Gary J. Ehrlich, PE
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-----Original Message-----
From: Christopher Wright [mailto:chrisw@skypoint.com]
Sent: Tuesday, November 25, 2008 12:08 PM
To: seaint@seaint.org
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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