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Re: What does "credible" mean?

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Because of the subjectivity that goes into the process Paul described 
(and the fact that what seismologists really consider to be credible 
changes significantly over time), for code purposes such events are 
now being called "maximum CONSIDERED earthquakes."  The maximum 
considered earthquake is exactly what it purports to be--namely, the 
largest earthquake considered; there are larger events that are 
"credible" or plausible but so unlikely that they are not considered.

As used in modern codes (2000 IBC, 1997 NEHRP, FEMA 273, FEMA 310, 
etc.), the MCE response spectrum is taken as the mean spectrum with a 
2 percent probability of exceedence in 50 years, but need not be 
taken greater than 150 percent of the median spectrum for the 
characteristic event on the controlling fault.  (This allows 
deterministic values to "cap" the probabilistic values.)

**************
> Date:          Tue, 16 Nov 1999 12:29:08 -0800
> From:          Paul Crocker <PaulC(--nospam--at)ckcps.com>
> Subject:       Re: What does "credible" mean?

> "Credible" is fairly subjective.  Depending on the code you are looking at,
> the "credible" earthquake is probably something like a quake producing a
> ground acceleration with a 2% chance of being exceeded in 50 years.  This
> varies by code, and I can't remember off of the top of my head what the
> 1997 UBC corresponds to.  This boils down to a societal decision,
> attempting to weigh the increasing costs of designing for larger quakes
> against the cost of coping with the damages caused by them.  So, the codes
> pick a level they feel is an appropriate and realistic compromise between
> the two, and design for it.  Plus, "credible" sounds better than "a good
> guess".
> 
> Paul Crocker
> 
> 
> RoyLevy(--nospam--at)aol.com wrote:
> 
> > Is there a  precise  definition for the word "credible" as in "credible
> > earthquake?" . Or is this just a subjective category?
> >
> > Roy Levy

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
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