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USGS's FAQ : Capped Maps[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: USGS's FAQ : Capped Maps
- From: Eddie Gonzalez <Eagonzal(--nospam--at)ENG.CI.LA.CA.US>
- Date: Wed, 11 Nov 1998 15:02:51 -0800
Mr. Dave Perkins of USGS was also kind enough to send me a copy of another proposed FAQ on the above subject which they may post in their website ( http://geohazards.cr.usgs.gov/eq/faq ). I though those on this list may find it interesting. Also, perhaps, some of you may share some insights on this issue which I hear has going on for some time on capped vs. uncapped maps ********** Edward There are two sets of maps: 1. The national probabilistic ground motion hazard maps. These are straiaght probabilistic ground motion and HAVE NOT BEEN CAPPED. 2. The maps prepared by the USGS Golden GIS staff, supervised by members of Frankel's group, according to the recipe given by FEMA-BSSC subcommittees, to be used as design maps in the FEMA Recommendations for Seismic Standards for New Construction. The FEMA-BSSC recipe includes capping the ground motions on the maps in such a way that the resulting design ground motions do not exceed the largest values in the previous code, EXCEPT that when deterministic ground motions from faults exceed the cap values, the deterministic ground motions appear on the map. The maps used as a basis for the code maps are the 2 percent probability of exceedance spectral acceleration maps for 0.2 and 1.0 sec period, but recontoured so that certain contours correspond to break points in the code. Design values are 2/3 of the map values. The result (in uncapped areas) is that design values are approximately 500-yr return period ground motions in the west and approximately 1000-yr return period ground motions in the east. I hope this clears up your question. Maybe we should be distinguishing between national hazard maps and national design maps. At the site we have put up ONLY the uncapped maps. FEMA is publishing the capped maps. The California/Nevada maps at the site represent only a "zoom in" on that part of the maps.
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