Publication Date:
2005
Description:
Despite providing an exceptionally clear example of the basics of
probabilistic seismic hazard analysis (PSHA), Wang and Ormsbee [2005] nevertheless
conclude that "... using PSHA for risk analysis is not only confusing, but is also
inappropriate." I argue here that (1) the results of a PSHA analysis are not confusing
and have physical meaning, and (2) the authors' basis for declaring PSHA "inappropriate"
is misguided. I note in passing that the authors consistently confuse "risk" with
"hazard." Both PSHA and flood frequency analysis provide estimates of hazard. Risk is
the product of hazard, vulnerability, and exposure. This discussion is only concerned
with hazard. The authors reveal the basis for their confusion about a physical
interpretation of PSHA in the statement, "Because it is impossible for the three
earthquakes to occur at exactly the same time (1.5*10^-19 probability at the same hour),
the predicted Peak ground acceleration [(peak ground acceleration) at a point of
interest] corresponding to the total annual probability of exceedance is a statistical
measure and does not have a clear physical meaning" (words in brackets are mine(. The
total annual probability of exceedance (P) from PSHA is not conditioned on all three
earthquakes occurring at once. Using the authors' example, it is the sum of the
independent probabilities that any one of the three faults will cause Peak ground
acceleration to exceed 0.97g. Summing the probabilities simply produces the annual
probability that the Peak ground acceleration will be exceeded in a year. It does not
imply the three earthquakes are concurrent
Keywords:
Earthquake hazard
;
PSHA
;
Statistical investigations
;
7212
;
Seismology:
;
Earthquake
;
ground
;
motions
;
and
;
engineering
;
seismology
;
7223
;
Earthquake
;
interaction,
;
forecasting,
;
and
;
prediction
