HR: 0800h
AN: S21A-0263    [Abstracts]
TI: Progress Toward Quantifying CISN ShakeMap Uncertainty
AU: * Lin, K
EM: klin@consrv.ca.gov
AF: California Geological Survey/SMIP, 801 K St., MS 13-35, Sacramento, CA 95814 United States
AU: Wald, D J
EM: wald@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046 MS-966, Denver, CO 80225 United States
AU: Worden, B
EM: bruce@gps.caltech.edu
AF: U.S. Geological Survey, 525 S Wilson Ave, Pasadena, CA 91106 United States
AB: We are developing and testing algorithms to quantify uncertainties associated with ShakeMap ground motions through efforts by the California Integrated Seismic Network (CISN) ShakeMap Working Group. There are multiple sources of uncertainty in producing a ShakeMap, including sparse ground motion measurements, approximate representation of fault finiteness and directivity, empirical ground motion predictions, numerical interpolation, and site corrections. Although significant contributors to the uncertainties can be reduced through the release of successively more accurate versions of ShakeMap that contain improved fault representation and newly recovered data, it is imperative to provide a clear mechanism to convey the estimates of uncertainty for each successive map. To this end, we quantify the uncertainties of the maps on a point-by-point basis, by combining the separate, but related, contributions of uncertainty using the following sequence. For a small (point source) event, uncertainty at any point on the map away from a station is controlled by the (inter- and intra-event) variability associated with the ground motion attenuation estimates. If there are stations close by, we further reduce the total sigma using a spatial variance equation as a function of inter-station spacing to account for spatial correlations. For extended sources, and for sites in the near-fault region, additional complexity is required. The total sigma is adjusted upward for an event without defined fault dimensions due to the range of ground motions possible given the range of potential distances a given site can be from the source (ranging from epicentral to the closest station-to-fault measure). At several source dimensions, these distance measures converge and this additional uncertainty is small; likewise when the source dimension is specified (source modeling, aftershocks, etc.), this contributor to the total sigma is removed. Again, any nearby stations significantly reduce the total sigma locally, and as expected, the potential uncertainty associated with sites in the near-fault region can be huge when stations are sparse. From this exercise we are converting ShakeMap ground motion predictive estimates to a median distance measure when the source dimensions are unconstrained; the associated median motions then more accurately reflect the range of potential peak motion values there. Finally, we show results of estimates of uncertainty for ShakeMap for both real and scenario earthquakes (weak and strong ground motions) in California and with/without defined fault traces. We discuss future developments and plans for integration of these uncertainty measures, both quantitative and qualitative, into the online system and user interfaces.
DE: 7223 Seismic hazard assessment and prediction
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting