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How Much Concentrated Earthquake Damage is due to Site Effects Versus Basin Edge Focusing?

Paul M. Davis

Published August 15, 2017, SCEC Contribution #7718, 2017 SCEC Annual Meeting Poster #244

One of the goals of constructing earthquake hazard maps is to predict the shaking in advance of an earthquake, so that regions where damage will concentrate can be identified beforehand, and be given priority for minimizing hazard such as retrofitting. The problem addressed is how site effects, or their proxies such as VS30 (Stewart et al 2014, Yong et al 2015), need to be supplemented by knowledge of structural focusing to give improved predictors of hazard. At the times of the Whittier Narrows (197) and Northridge (1994) earthquakes red-tagged building damage was concentrated near faults (e.g., Whittier, Santa Monica, Hollywood, Las Cienegas) associated with the LA basin edge. Anomalous damage in Santa Monica was attributed to basin edge focusing (Gao et al 1996, Davis et al 2000). The focusing phenomenon has been proposed for damage for 5 earthquakes, and the question remains how ubiquitous is it? In order to address the role of deep basin focusing, we pursue 2 developments since we worked on Northridge. The SCEC community velocity model (CVMH) has been constructed from sonic logs and industry reflection data in the Los Angeles basin (Suess and Shaw 2003) superimposed on basement models from regional tomography. Further, in 2014 we conducted a seismic experiment, LASSIE, across the LA basin and Puente Hills, a region where the SCEC velocity model shows a steep basin edge and near where Whittier Narrows damage occurred.
High frequency 3D finite differences are used to model focusing for comparison with estimates of site effects. We find evidence in the LASSIE data of structural focusing from the 3 km deep geology variation associated with the Whittier fault, causing amplification several times that of the site effects. Modeling using the SCEC model suggests focusing could have been important in the damage distribution of the Whittier Narrows earthquake. Similar modeling of the Northridge Earthquake suggests focusing may have been a factor in the Hollywood and Las Cienegas regions as well as Santa Monica. Focusing depends both on frequency and azimuth. Adding azimuth-dependent amplification factors to VS30 compilations will give engineers an improved tool for hazard maps.

Key Words
Site basin edge focusing earthquake damage

Davis, P. M. (2017, 08). How Much Concentrated Earthquake Damage is due to Site Effects Versus Basin Edge Focusing?. Poster Presentation at 2017 SCEC Annual Meeting.

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