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Probabilistic Definition of Seismic Hazards

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Dave Evans,        December 28, 1996
In your recent posting to SEAOC List Server, concerning "Seismic Evaluation
of Buildings--unsafe for occupancy",  you mentioned "probabilistic methods
could help inform the decision, with consideration of the likelihood of a
significant seismic event occuring in the time before remedial work, the risk
of collapse due to that event relative to the risk of collapse of some other
"typical" building during a similar exposure period, etc."

Seismic hazards are usually defined by ground motion strength parameters
(e.g. PGA or SA) at a specified mean annual frequency of EXCEEDANCE, (not
mean annual probability of OCCURENCE) (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),  taken
and abridged from ATC 35-2, (1995) "National Earthquake Ground Motion Mapping
Workshop" page 6-1, "An Advocation: Map Probabilistically Derived Quantities"
by C. Allin Cornell.  He goes on to state: "I have intentionally emphasized
"mean (annual) frequency of exceedance" rather than "probability" in the
definitions above, first, because I believe today that this is proper index
to use in specfying structural safety norms and more importantly here,
because it makes it very clear that Probabilistic Seismic Hazard Analysis
(PSHA) results carry with them (no matter how presented) the units of time."

In other words, seismic hazards are usually defined using ground motion
strength parameters (e.g. Peak Ground Acceleration or SA, such as the 1994
UBC or NEHRP Provisions Response Spectra values) 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.   

The definition of seismic hazards are not usually defined in terms, such as
the "likelihood of occurence" in given number of years or the "recurrence
interval"  (like the return of Haley's Comet) of a criterion earthquake of a
certain magnitude or ground motion strength.  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 occurence 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 OCCURENCE).

 Frank E. McClure          FEMCCLURE(--nospam--at)aol.com

P.S.  You did not answer my earlier question to you: "Is it good PUBLIC
POLICY for the requirements of FEMA 273 for the seismic evaluation and
guidelines for the rehabilitation - retrofit- of existing buildings to be
more restrictive than the 1994 NEHRP Provisions for the seismic design of new
buildings."  I would appreciate your or anyone elses' views on this matter.