[ghsc-seminars] GHSC Seminar TOMORROW—Molly Gallahue, Northwestern University, Tuesday, April 26th, 2022, 10–11am MDT

[Rengers, Francis K]

Exploring the discrepancy between seismic hazard maps for California and historic shaking observations

Speaker: Molly Gallahue
               Earth and Planetary Sciences
               Northwestern University
Location: online-only
Date/Time: TOMORROW, Tuesday, April 26th, 2022, 10–11am MDT

Abstract: Probabilistic seismic hazard assessment (PSHA), which forecasts levels of earthquake shaking that should be exceeded with only a certain probability over a given period of time, is one of the most important products produced by the seismological community for earthquake risk mitigation. Seismologists have recently begun assessing how well PSHA models and corresponding maps forecast the shaking that actually occurs. Salditch et al. (2020) found that current PSHA maps for California predict higher shaking than that observed by the California Historical Intensity Mapping Project (CHIMP) dataset of maximum observed shaking from the largest California earthquakes between 1857 and 2019. Qualitatively similar results have been obtained in Italy and Japan. We examine three possible causes for the discrepancy in California: 1) site effects due to variations in VS30 (time-averaged shear-wave velocity in the top 30m of soil), which could amplify or deamplify ground motions relative to hazard maps for a reference site condition, 2) incompleteness of the CHIMP data set, and 3) aleatory variability in conversion from peak ground acceleration (PGA) to Modified Mercalli Intensity (MMI). We find that incorporation of site-specific VS30 does not appreciably change maps and hence their inconsistency with CHIMP data, because at the short periods that control PGA (and MMI), nonlinear deamplification due to increased soil damping largely offsets linear amplification due to low VS30.  We show that the other two factors also cannot explain the discrepancy between predicted and observed shaking. We outline future work that will be undertaken to understand the discrepancy. Remaining possible explanations include but are not limited to two-step regression in ground motion intensity conversion equations, use of mean rather than median hazard to characterize PSHA, and overestimation of aleatory variability in modern ground motion models.

[cid:1271ab17-4ea2-4d64-9469-b9b3015bb70f]

Bio: Molly Gallahue is a 4th year PhD candidate at Northwestern University (in Evanston, IL) studying seismic hazard. Before Northwestern, she received a Bachelor of Science in geology and math from St. Norbert College (Green Bay, WI). She is advised by Seth Stein, Norman Abrahamson, Sue Hough, and Leah Salditch.


GHSC Seminar Committee: Oliver Boyd <olboyd at usgs.gov>, Josh Rigler <erigler at usgs.gov>, Francis Rengers <frengers at usgs.gov>

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