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RE: USGS Ground Motion Tool

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I would agree that a site along San Pablo Avenue in Berkeley is highly unlikely to be Site Class B. However, the USGS Ground Motion Program simply uses a Default of Site Class of B expecting you to change the Site Class to whatever your site class actually is. Site B represents a typical rock site in the Western US (Class A is very hard rock like they have in the Eastern US) and makes for an easy reference standard to index other site classes from. Notice that the online JAVA application says "Set [emphasis added] Site Class" on the right hand side of the Site Coefficients window. Also note that Site Class B is the only site class with both Fa and Fv = 1.0 (i.e, it is the reference value).
Bill Cain, S.E.
Berkeley, CA

> Date: Tue, 31 Aug 2010 08:33:24 -0700
> From:
> To: garcia(--nospam--at); seaint(--nospam--at)
> Subject: USGS Ground Motion Tool
> The USGS Ground Motion Program gave me a Site Class "B" where my
> common sense told me that was wrong and found that it is most likely a
> "D" Site Class.
> I understand the program may not be accurate but this is far too much!
> If I am doing something wrong, please speak up! For now I using Site
> Class "D" not the recommended "B"
> The USGS ground motion tool reports a Site Class of B for an address at:
> Latitude = 37.87924 and Longitude = -122.29585. This is along SAN PABLO
> AVE, BERKELEY 94702.
> References:
> Seismic Hazard Curves and Uniform Hazard Response Spectra using the
> Earthquake Ground Motion Tool Version 5.0.9a-10/21/2009
> I found (Site Class D), Qhaf, Alluvial fan and fluvial deposits
> (Holocene), Reference: Graymer, R.W., 2000. Geologic map and map data
> base of the Oakland Metropolitan Area, Alameda, Contra Costa, and San
> Francisco Counties, California, U.S. Geological Survey Miscellaneous
> Field Study Map 2342.
> Site Class "D" for this location also is given in: "Mapping NEHRP VS30
> Site Classes" figure 1
> Thomas L.Holzer, a) M. EERI, Amy C. Padovani, b) Michael J. Bennett, a)
> Thomas E. Noce, a) and John C. Tinsley, IIIa)
> I found a recent soils report on a nearby address that reports a Site
> Class D.
> David Merrick SE
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