HR: 10:25h
AN: S42A-01    [Abstracts]
TI: Topographic Slope as a Proxy for Seismic Site Amplification Correction
AU: * Wald, D J
EM: wald@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS-966 Denver Federal Center, Denver, CO 80225-0046 United States
AU: Earle, P S
EM: pearle@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS-966 Denver Federal Center, Denver, CO 80225-0046 United States
AU: Quitoriano, V
EM: vinceq@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS-966 Denver Federal Center, Denver, CO 80225-0046 United States
AB: As part of an effort to rapidly predict ground shaking and earthquake impact globally (Prompt Assessment of Global Earthquakes for Response, U.S. Geological Survey Project PAGER, also see Earle et al. paper, this meeting), we need at least a first order approximation for seismic site conditions worldwide for input into our ground motion estimation. In many seismically-active regions of the world, information about shallow geology and shear velocity does not exist, varies dramatically in quality, or is not easily accessible. Topographic elevation data, on the other hand, are available at uniform sampling globally, and here we attempt to derive site condition maps directly from this highly accurate and readily available data. For calibration, we use the California statewide topographic data and geologically-based shallow (30 m) site condition map of Wills et al (2000). The Wills et al. map is categorized by NEHRP lettering with grades at intermediate values ranging from hard rock to bay mud (i.e., B, BC, C, CD, D, DE, E). In short, we find that the slope of the topography is well correlated with the site condition map. By taking the gradient of the topography and choosing the range of slope that maximize the correlation with the existing site condition map we can recover, to first order, many of the important aspects of the map. Additionally, we assign class E (bay mud) to all flat regions with elevations between negative 3 and plus 3 meters. This assumption produces reasonable class E boundaries and avoids incorrectly assigning class E to large flat areas such as the San Joaquin Valley and to areas below sea level such as Death Valley. The largest discrepancy in geologically- and topographically-derived site conditions is between soft and hard rock. Fortunately, the differences in site amplification for these types are relatively small. Naturally, this simple assumption will break down for some topgraphic/geomorphic combinations. As an example, in glacial terrains, we cannot distinguish between topographically similar depositional (glacial till) drumlins and erosional (bedrock) roche moutonnees. Nonetheless, this approach should provide adequate first-order estimates of regional site amplification for the entire globe.
DE: 7223 Seismic hazard assessment and prediction
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting