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RE: USGS vs NEHRP

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I had a similar experience. At a site in Orange County, the geotech had the
PGA for the MCE pegged at .45g, but the USGS web site returned more like
.70g. 

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Tuesday, September 13, 2005 2:45 PM
To: seaint(--nospam--at)seaint.org
Subject: USGS vs NEHRP

For a given latitude and longitude in California, the 0.2 second MCE
acceleration is 1.50 per the NEHRP/ IBC/ ASCE 7 maps (using the CD-ROM); but
the value returned by the USGS web site is 2.00.  Why is the USGS value 33%
higher?  

The documentation for NEHRP says that they modified some contours downward
in California relative to USGS probabilistic maps based on deterministic
seismic data.  But my geotechnical engineer is coming up with the same
ground accelerations as the USGS web site, based on deterministic methods
for known faults.  Thus, the deterministic method does not seem to explain
this difference? 


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

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