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Site-Specific Velocity Corrections Do Not Improve Seismic Hazard Map Performance for California When Compared With Historical Shaking Data

Session: Probabilistic Seismic Hazard Assessment: Where Do We Go from Here? I

Type: Oral

Date: 4/20/2021

Presentation Time: 10:30 AM Pacific

Description: 

Probabilistic seismic hazard assessments forecast levels of earthquake shaking that should be exceeded with only a certain probability over a given period of time are important for earthquake hazard mitigation. These rely on 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).

To explore the robustness of maps’ shaking forecasts, we consider how maps hindcast past shaking. We compiled the California Historical Intensity Mapping Project (CHIMP) dataset of the maximum observed shaking from the largest Californian earthquakes over the past 162 years. Prior comparisons between the maps for a constant V_S30 (shear-wave velocity in the top 30m of soil) of 760 m/s and CHIMP based on metrics suggested that current maps overpredict shaking.

The differences between the V_S30 at the CHIMP sites and the reference value of 760 m/s could amplify or deamplify the ground motions relative to the mapped values. We evaluate whether the V_S30 at the CHIMP sites could cause a possible bias in the models. By comparison with the intensity data in CHIMP, we find that using site-specific V_S30 does not improve map performance, because the site corrections cause only minor differences from the original 2018 USGS hazard maps at the short periods relevant to PGA and hence MMI. The minimal differences reflect that the nonlinear deamplification due to increased soil damping largely offsets the linear amplification due to low V_S30.

Possible reasons for this discrepancy include limitations of the dataset, bias in the hazard models, overestimation of the aleatory variability of the ground motion, or that seismicity throughout the historical period has been lower than the long-term average. Resolving this discrepancy could improve the performance of seismic hazard maps and earthquake safety for California and worldwide. We also explore whether new nonergodic GMMs, with reduced aleatory variability, perform better than presently used ergodic GMMs compared to historical data.

Presenting Author: Molly Gallahue

Student Presenter: Yes

Authors