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Re: Probabilistic Definition of Seismic Hazards
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- Subject: Re: Probabilistic Definition of Seismic Hazards
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: Tue, 31 Dec 1996 09:54:42 +0500
Per comments by Frank E. McClure: >Seismic hazards are usually defined ... at a specified mean annual frequency of >EXCEEDANCE, (not mean annual probability of OCCURRENCE) (e. g. 0.002 per year, >(1/500) ,or its effective equivalent (e.g. annual probability of exceedance, mean >return period, probability of exceedance in n years, e.g., 10% in 50 years). In >other words, seismic hazards are usually defined ... at a specified MEAN ANNUAL >FREQUENCY OF EXCEEDANCE, which is the reciprocal of the MEAN RETURN PERIOD >for a criterion earthquake with a specific probability of exceedance in n >years. It is important to remember that we are discussing MEAN RETURN PERIOD ( the >average return period) not the RETURN PERIOD. >We start down a very slippery slope if we starting telling our clients or the >public that the probability of occurrence can be given in terms of a certain >number of years for earthquakes of a certain magnitude or ground motion >strength. Earthquake ground motion strength parameters are usually, for >building code purposes or earthquake risk assessments, expressed in terms of >"MEAN (average) ANNUAL FREQUENCY OF EXCEEDANCE ( not OCCURRENCE). Please clarify what the difference is between annual frequency of occurrence and annual frequency of exceedance - aren't these different ways of expressing the same thing? I do prefer the method of expressing events in terms of probability of exceedance during a given period, since it gives a better sense of the risk involved, but I am not clear on what the mathematical difference is. If an event has an annual frequency of occurrence of 0.002, then it has a 500 year expected return period. Then in 50 years, doesn't it have a 10% probability of being exceeded (0.002x50 = 0.10)? and statistically, wouldn't it have a 100% probability of occurring in 500 years? The problem with using return periods to describe probabilistic events is that it seems to give a false sense of security. For example, consider design for flotation resistance against a 100-year flood. Such a flood would have a 0.01 annual probability of occurrence. For a 50 year design period, is seems to me it would have a "50% probability of being exceeded" (0.01/year x 50 years). If you are concerned with flotation of a structure, designing for a 100-year flood for a 50-year design life sounds conservative. But having a 50% risk of exceedance during the design life sounds very risky.
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