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Revisiting California’s Past Great Earthquakes and Long-Term Earthquake Rate

Susan E. Hough

Published August 5, 2020, SCEC Contribution #10275, 2020 SCEC Annual Meeting Poster #060

We revisit the three largest historical earthquakes in California – the 1857 Fort Tejon, 1872 Owens Valley, and 1906 San Francisco earthquakes -- to review their published moment magnitudes, and compare their estimated shaking distributions with predictions using modern ground motion prediction equations (GMPEs) and ground motion intensity conversion equations. Currently accepted moment magnitude estimates for the three earthquakes are, respectively, 7.9, 7.6, and 7.8. We first consider the extent to which the intensity distributions of all three earthquakes are consistent with a moment magnitude towards the upper end of the estimated range. We then propose and apply a GMPE-based method to estimate the magnitudes of large historical earthquakes. The intensity distribution of the 1857 earthquake is too sparse to provide a strong constraint on magnitude. For the 1872 earthquake, consideration of all available constraints suggests that it was a high stress drop event, implying a magnitude on the higher end of the range estimated by scaling relationships from inferred rupture parameters. For the 1906 earthquake, based on our analysis of regional intensities and the detailed intensity distribution in San Francisco, along with other available constraints, we estimate a preferred magnitude of 7.9, consistent with the published estimate based on geodetic and instrumental seismic data. These results suggest that, while there can be a tendency for historical earthquake magnitudes to be overestimated, the accepted catalog magnitudes of California’s largest historical earthquakes could be too low. Given the uncertainties of the magnitude estimates, the seismic moment release rate between 1850-2019 could have been either higher or lower than the average over millennial time scales. Because the uncertainties permit higher magnitudes than currently estimated for the three largest earthquakes, it is not possible to reject the hypothesis that California seismicity is described by an untruncated Gutenberg-Richter distribution with a b-value of 1.0 for moment magnitudes up to 8.0.

Key Words
historical earthquakes, 1906 earthquake

Hough, S. E. (2020, 08). Revisiting California’s Past Great Earthquakes and Long-Term Earthquake Rate. Poster Presentation at 2020 SCEC Annual Meeting.

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