U017-05
Assessment of seismic hazard map performance for California through comparison with historical shaking data

Tuesday, 15 December 2020: 05:47
Molly Margaret Gallahue1, Leah Salditch2, Madeleine Lucas2, James Neely3, Susan Elizabeth Hough4, Seth Stein5, Norman Abrahamson6 and Tessa Williams7, (1)Northwestern University, Department of Earth & Planetary Sciences, Evanston, IL, United States, (2)Northwestern University, Earth and Planetary Sciences, Evanston, IL, United States, (3)Northwestern University, Evanston, United States, (4)USGS, Pasadena, CA, United States, (5)Northwestern University, Earth & Planetary Sciences, Evanston, IL, United States, (6)University of California Berkeley, Civil and Environmental Engineering, Berkeley, CA, United States, (7)University of California Berkeley, Berkeley, CA, United States
Abstract:
Probabilistic seismic hazard assessments are important for earthquake hazard mitigation. They forecast levels of earthquake shaking that should be exceeded with only a certain probability over a given period of time. The U.S. Geological Survey (USGS) develops seismic hazard models and maps using assumptions about when and where earthquakes will occur, their size, and the resulting shaking as a function of distance as described by ground motion models (GMMs) that cover broad geologic regions. Despite many studies worldwide, map assumptions are hard to accurately estimate, posing major challenges.

To explore the robustness of maps’ shaking forecasts, we consider how the maps hindcast past shaking. We have compiled the California Historical Intensity Mapping Project (CHIMP), a dataset of the maximum observed seismic intensity of shaking from the largest Californian earthquakes over the past 162 years. Comparisons between the maps and CHIMP based on several metrics suggest that current maps overpredict shaking. Possible reasons for this discrepancy include limitations of the dataset, a bias in the hazard models, or that seismicity throughout the historical period has been lower than the long-term average, perhaps purely by chance due to the variability of earthquake recurrence. Resolving this discrepancy could improve the performance of seismic hazard maps and thus earthquake safety for California and, by analogy, worldwide.

A possible bias in the models may be that the maps assume a uniform VS30 (shear-wave velocity in the top 30 m of soil). This parameter is a proxy for local site conditions that can amplify or deamplify seismic shaking. Although uniform VS30 is a useful approximation, VS30 varies considerably based on geologic conditions. We examine how varying VS30 will affect the predicted seismic shaking from seismic hazard maps. Through comparison with the observed intensity data in CHIMP, we study how map performance changes when site-specific VS30 are used.

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