E. M. Thompson1, D. J. Wald2, C. B. Worden3, and C. Gustafson2

1  San Diego State University, San Diego, CA
2  U.S. Geological Survey, Golden, CO
3  Synergetics, Inc., Fort Collins, CO

A Vs30 Map for California incorporating Geology, Topography, and In Situ Measurements

In many earthquake hazard applications, such as ground motion prediction equations (GMPEs), building codes, and earthquake hazard mapping, site response is often estimated through empirical correlations with the average shear-wave velocity to 30 m depth (Vs30). These applications therefore depend on availability of Vs30 maps at local regional, and global scales. Vs30 data, however, is sparse, and a second proxy is needed to estimate Vs30 at unsampled locations. We present a new Vs30 map for California, which accounts for observational constraints from multiple sources and spatial scales, such as geology, topography, and in situ Vs30 measurements. We apply the kriging-with-a-trend mathematical framework to combine these constraints for predicting Vs30. For the Vs30 trend, we start with geology-based Vs30 values and identify two distinct trends between topographic slope and the residuals from the geology Vs30 model. One trend applies to all the Quaternary alluvium (Qal) geologic units (including those in the Imperial Valley and the Los Angeles Basin) as well as Qal,fine. The second is slightly stronger and applies to Pleistocene sedimentary units (including Qoa, Qs, and QT). The krigingwith- a-trend framework ensures that the resulting map of California is locally refined to reflect the rapidly expanding database of Vs30 measurements. This method for mapping Vs30 is analogous to the method of mapping ground motion intensities that is currently employed by ShakeMap (Worden et al., 2010), where the spatial proxy (event-corrected GMPEs) is locally refined to reflect the instrumental/ macroseismic observations. We compare the accuracy of the new mapping method to previously developed maps of Vs30 for California. We also illustrate the sensitivity of ground motions to the new Vs30 map by comparing real and scenario ShakeMaps with Vs30 from our new map to those for existing Vs30 maps.

2013 SCEC Annual Meeting