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USGS vs NEHRP

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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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