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RE: USGS vs NEHRP

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Are you sure you are using the same dataset?  There are two sets of data on
the website.  The 2000 IBC & 97 NEHRP are based on the 1996 maps while the
2006 IBC are based on the 2002 maps.  

This is all speculation, but I also recall that there was some concern about
values in CA as many areas gave dramatically lower results than the UBC
does.  It may be that some of those opinions made it into the data set you
are using. 

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Tuesday, September 13, 2005 3:45 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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