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USGS's FAQ : Capped Maps

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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.