[CEUS-earthquake-hazards] Article on Central U.S. Earthquake hazard by expert panel

[Oliver Boyd]

------ Forwarded Message
From: Art Frankel <afrankel at usgs.gov>
Date: Thu, 28 Apr 2011 16:02:24 -0600
To: Oliver Boyd DOI/GD/Golden <olboyd at usgs.gov>
Subject: please post

I think it doesn't make sense to say the occurrence of the Haiti, Wenchuan,
and Tohoku earthquakes invalidates seismic hazard mapping and probabilistic
seismic hazard assessment (PSHA). It's all a matter of how good the inputs
to your PSHA are and whether you consider earthquakes with longer recurrence
times.  The inputs to the U.S. national seismic hazard maps include
paleoseismic and geologic slip rate information that provide estimates of
earthquake recurrence rates from thousands of years of data.

First of all, the 1999 GSHAP hazard map for Haiti and the Caribbean did not
even include the hazard from specific faults, such as the Enriquillo fault.
Their map only used the recent seismicity. Thus, their hazard map was not
based on the methodology used in the U.S. national seismic hazard maps. Our
new hazard maps for Haiti do include the major faults in Haiti and show
quite high hazard in Port-au-Prince, for example (see USGS Open File Report
2010-1067).  The hazard from these faults was estimated using slip rates
constrained by GPS data from Eric Calais and his colleagues. Future
paleoseismic investigations will improve estimates of these slip rates and
recurrence times.

The Tohoku and Wenchuan earthquakes apparently have long recurrence times of
perhaps a thousand years or more. It is necessary to consider paleoseismic
data and geologic slip rates in order to accurately assess the hazard from
these long-recurrence time events. For the U.S. national seismic hazard
maps, we used paleoseismic and paleotsunami data when evaluating the hazard
from the Cascadia subduction zone.  We also considered paleoseismic data to
evaluate the hazard for the New Madrid region and other areas. By using
geologic slip rates to estimate recurrence times for hundreds of Quaternary
faults, we evaluated the hazard for long recurrence rate faults that don't
exhibit substantial recent seismicity.

So the recent occurrence of these long-recurrence time earthquakes actually
supports the methodology we apply to make the U.S. national seismic hazard
maps. It also supports the idea of using low probability levels of, say, 2%
probability of exceedance  in 50 years for building codes.  Designing for
these low probability ground motions provides protection for long recurrence
time, high consequence earthquakes such as the Tohoku and Wenchuan
earthquakes. 





Art Frankel
U.S. Geological Survey
University of Washington
Dept. of Earth and Space Sciences
Box 351310
Seattle, WA 98195-1310
phone: 206-553-0626
email: afrankel at usgs.gov

------ End of Forwarded Message

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://geohazards.usgs.gov/pipermail/ceus-earthquake-hazards/attachments/20110429/ec19ae83/attachment.html>