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Mild Displacements of Boulders during the 2019 Ridgecrest Earthquakes

Sleep, Norman H. and Hough, Susan E. (2020) Mild Displacements of Boulders during the 2019 Ridgecrest Earthquakes. Bulletin of the Seismological Society of America . ISSN 0037-1106. (In Press) https://resolver.caltech.edu/CaltechAUTHORS:20200522-145616023

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Abstract

Strong seismic waves from the July 2019 Ridgecrest, California, earthquakes displaced rocks in proximity to the M 7.1 mainshock fault trace at several locations. In this report, we document large boulders that were displaced at the Wagon Wheel Staging Area (WWSA), approximately 4.5 km southeast of the southern terminus of the large M 6.4 foreshock rupture (hereafter “the large foreshock”) and 9 km southwest of the nearest approach of the M 7.1 mainshock surface rupture. Some boulders appear to have slid along essentially flat surfaces, which suggest that dynamic stresses overcame the coefficient of friction. Other boulders appear to have rocked within their sockets. In both cases, we use simple mechanical models to estimate total peak dynamic accelerations between 0.5 and 1g, commensurate with modified Mercalli intensity 9. It is unclear if the strongest shaking at this location occurred during the large foreshock or the M 7.1 mainshock. The inferred accelerations are higher than predicted mainshock ground motions at WWSA, although local high accelerations could have been generated by path, site, or source effects. Gaps between boulders and their sockets are easily visible in the immediate aftermath of earthquakes and provide a quick indication of strong shaking. More importantly, the gaps quickly fill with surficial organic debris, including seeds and leaves of the year, that quickly become entombed. Boulders may thus potentially be extracted to examine gap fillings associated with past earthquakes, providing a new datable paleoseismic method.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1785/0120200029DOIArticle
ORCID:
AuthorORCID
Hough, Susan E.0000-0002-5980-2986
Additional Information:© 2020 Seismological Society of America. Manuscript received 17 January 2020; Published online 19 May 2020. Mark Quigley and William Ellsworth quickly answered the authors’ queries. Two anonymous BSSA reviewers and U.S. Geological Survey (USGS) reviewers Andy Barbour and Devin McPhillips provided helpful comments. The authors are benefited from the 2019 Southern California Earthquake Center (SCEC) Workshops on Fragile Geological Features and on the Ridgecrest earthquakes. This research was supported by the SCEC. SCEC is funded by National Science Foundation (NSF) Cooperative Agreement EAR‐0106924 and USGS Cooperative Agreement 02HQAG0008. The SCEC contribution number for this article is 10060. Data and Resources: We have placed photos used in our figures, nearby photos from bursts, and their metadata in the Stanford Digital Repository. The bursts with obvious shadows may be useful for documenting future 3D movements of the boulders (https://purl.stanford.edu/tm319hn6368, last accessed April 2020). The rest of data used in this article came from published sources listed in the references. U.S. Geological Survey (USGS) ShakeMaps for the large foreshock and mainshock were downloaded from the websites available at http://earthquake.usgs.gov/earthquakes/eventpage/ci38443183/map and https://earthquake.usgs.gov/earthquakes/eventpage/ci38457511/map. These websites were last accessed in January 2020.
Funders:
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)UNSPECIFIED
NSFEAR‐0106924
USGS02HQAG0008
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center10060
Record Number:CaltechAUTHORS:20200522-145616023
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200522-145616023
Official Citation:Sleep, N. H., and S. E. Hough (2020). Mild Displacements of Boulders during the 2019 Ridgecrest Earthquakes, Bull. Seismol. Soc. Am. XX, 1–10, doi: 10.1785/0120200029
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103428
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:22 May 2020 22:57
Last Modified:22 May 2020 22:57

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