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Published May 1, 2020 | public
Journal Article

Near-Field Ground Motions from the July 2019 Ridgecrest, California, Earthquake Sequence


The 2019 Ridgecrest, California, earthquake sequence, including an M_w 6.4 event on 4 July and an M_w 7.1 approximately 34 hr later, was recorded by 15 instruments within 55 km nearest‐fault distance. To characterize and explore near‐field ground motions from the M_w 6.4 foreshock and M_w 7.1 mainshock, we augment these records with available macroseismic information, including conventional intensities and displaced rocks. We conclude that near‐field shaking intensities were generally below modified Mercalli intensity 9, with concentrations of locally high values toward the northern and southern termini of the mainshock rupture. We further show that, relative to near‐field ground motions at hard‐rock sites, instrumental ground motions at alluvial near‐field sites for both the M_w 6.4 foreshock and M_w 7.1 mainshock were depleted in energy at frequencies higher than 2–3 Hz, as expected from ground‐motion models. Both the macroseismic and instrumental observations suggest that sediments in the Indian Wells Valley experienced a pervasively nonlinear response, which helps explain why shaking intensities and damage in the closest population center, Ridgecrest, were relatively modest given its proximity to the earthquakes.

Additional Information

© 2020 Seismological Society of America. Manuscript received 27 September 2019; Published online 26 February 2020. The authors thank Paul Friberg for supplying an ah2sac code to help us resuscitate data from the 1995 Ridgecrest sequence, and the many residents of Ridgecrest who provided information about earthquake effects. Assistance from Ken Sanger and Skip Graff, and the staffs of the Maturango Museum, Ridgecrest Animal Shelter, and Ridgecrest Independent are acknowledged with appreciation. The general cooperation of the Naval Air Weapons stations is acknowledged with appreciation. The authors thank Nicholas van der Elst, Brad Aagaard, Shane Detweiller, and two anonymous reviewers for their constructive reviews of this article, and Alison Bent for her stewardship of the journal. Data and Resources: Ground‐motion recordings of the foreshock and mainshock, peak acceleration values for both events, and site characterization information for the stations listed in Table 1 can be downloaded from https://strongmotioncenter.org (last accessed July 2019). "Did You Feel It?" (DYFI) intensity data can be downloaded from https://earthquake.usgs.gov/earthquakes/eventpage/ci38443183/dyfi/intensity and https://earthquake.usgs.gov/earthquakes/eventpage/ci38457511/dyfi/intensity (both last accessed July 2019) for the foreshock and mainshock, respectively. News reports about damage on the China Lake Naval Air Weapons Station were downloaded from https://www.navytimes.com/news/your-navy/2019/08/12/china-lake-repair-costs-might-top-2-billion/ (last accessed September 2019). Evaluating damage at China Lake base school is available at https://www.bakersfield.com/multimedia/photo-galleries/photo-gallery-evaluating-damage-at-china-lake-base-school/collection_91115390-a2a5-11e9-944f-977e60c2b209.html (last accessed September 2019). The information about the Navy facing a billion dollar tab is available at https://news.usni.org/2019/08/21/navy-facing-billion-dollar-tab-years-to-get-china-lake-fully-operational-after-quake (last accessed September 2019).

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August 19, 2023
October 20, 2023