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What the “L”? Unraveling (and Responding to) the 2019 Ridgecrest, California, Earthquake Sequence

Susan E. Hough, & . The Ridgecrest Response Team

Published August 14, 2019, SCEC Contribution #9613, 2019 SCEC Annual Meeting Talk on Mon 1030

The 2019 Ridgecrest, California, earthquake sequence included a M6.4 event on 4 July and a M7.1 ~34 hours later. The two earthquakes occurred over a holiday weekend and had surface ruptures mostly within a high-security Navy base, which posed challenges for scientific response. I give a brief overview of the USGS/SCEC/Caltech response to the sequence, including fieldwork undertaken to map surface rupture within the base. Challenges notwithstanding, the response was facilitated by response exercises undertaken by the USGS Pasadena in spring, 2018, including exercising newly developed protocols to develop talking points. While several factors complicated the response to the Ridgecrest sequence, one key factor made the response much easier than it might have been: the limited toll taken by the earthquakes on life and property. The earthquakes did cause damage in the town of Trona, and possibly claimed one life in Nevada; damage was ,however, in the nearby town of Ridgecrest (pop. 28,000), and overall losses were far lower than would have occurred from comparable earthquakes in a more densely populated part of California. I also discuss briefly the factors that may have controlled near-field ground motions. The M6.4 and M7.1 earthquakes were recorded by 12 instruments within 40 km of the ruptures. These data can be supplemented by macroseismic data, including conventional intensities and displaced rocks, to characterize near-field ground motions from the M6.4 and M7.1. Near-field shaking intensities from both events were generally below MMI 9, with three concentrations of locally high (MMI 9-10+) values along the M7.1 mainshock rupture. Relative to near-field ground motions at hard-rock sites, instrumental ground motions at alluvial near-field sites were depleted in energy at frequencies higher than 2-3 Hz. Within 20 km, shaking intensities from the M7.1 were also higher than intensities from the M6.4 by only about 0.5 intensity units, rather than 1.3 units as predicted by the regional Intensity Prediction Equation. We discuss the factors that can account for these observations, including source effects and site effects. Both the macroseismic and instrumental observations suggest that sediments in the Indian Wells Valley experienced a pervasively non-linear response, which could help explain why shaking intensities and damage in the closest population center, Ridgecrest, were relatively modest given its proximity to the earthquake.

Key Words
Ridgecrest earthquakes, ground motions

Hough, S. E., & The Ridgecrest Response Team, .. (2019, 08). What the “L”? Unraveling (and Responding to) the 2019 Ridgecrest, California, Earthquake Sequence. Oral Presentation at 2019 SCEC Annual Meeting.

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Ground Motions