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Re: 97 Code is a Life Safety Standard

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Lynn Howard,

The 1997 Uniform Building Code, Section 1627 - Definitions defines what the 
Design Basis Ground Motion is for use in the 1997 Uniform Building Code.   It 
reads:
"DESIGN BASIS GROUND MOTION is that ground motion that has a 10 percent 
chance of being exceeded in 50 years as determined by a site-specific hazard 
analysis or may be determined from a hazard map."

The ground motion that has a 10 percent chance of being exceeded in 50 years 
has a MEAN RETURN PERIOD  of 475 years.

You are incorrect in thinking that the UBC code design based earthquake has a 
10% probability of occurring in 50 years.  You are also wrong in thinking 
that the UBC code design based earthquake is the same as the largest 
earthquake that you would expect to occur in any 495 (475 is the correct 
value) year period.  The UBC definition does not use the wording "occurring" 
or "occurrence."

You should use the term "MEAN" preceding the wording "RETURN PERIOD" anytime 
you use the term "RETURN PERIOD."
>
An earthquake with a MEAN RETURN PERIOD of 475 years has a 10% chance of 
being exceeded in 50 years.

I would suggest you talk to your local seismologist or ground motion 
consultant and perhaps they can explain the proper definition of the UBC 
design ground motion and terms you should be using.

I have sent you previous email messages, via the SEAOSC List Server, on this 
subject.

Frank E. McClure