Earthquake geology and the Ridgecrest earthquake

Most SCEC scientists were preparing for 4th of July celebrations when the M6.4 Ridgecrest earthquake occurred. Numerous industry, academic, and agency geologists responded quickly to capture ephemeral data, following earthquake rapid response protocols developed with SCEC participation. They began to document the left-lateral ~NE-SW trending surface rupture and secondary effects. Just 34 hours later, the M7.1 earthquake occurred along a nearly orthogonal, NW-SE trending right lateral system, cutting largely through the China Lake Naval Weapons Station, and dramatically disrupting the dinner hour and response coordination meeting of responding geologists in nearby Ridgecrest, California. Due to access constraints, US Geological Survey and California Geological Survey personnel worked closely with the US Navy to document surface rupture on the base, while the academic and industry scientists focussed their effort off the base.

Earthquake geology observations, collected in July Mojave desert heat, included documentation of offsets and mapping of the complex ground ruptures, as well as fragile geologic features that constrained strong ground motions. Field observations are augmented with a variety of  remote mapping technology, including high-resolution  photogrammetry, airborne and terrestrial laser scanning, and image and topographic differencing. 

The earthquake geology results are being published in numerous peer-reviewed venues including dedicated sections of the Seismological Research Letters and Bulletin of the Seismological Society of America. The scientific response demonstrated a well coordinated effort among many geologists from numerous institutions, including many within the SCEC collaboration. SCEC also coordinated an effort to jointly support lidar data acquisition, field geology, and geodesy efforts via an NSF RAPID award.

The M6.4 rupture was about 18 km long and had mean left-lateral displacement of 0.3-0.5 m with a maximum of 0.7-1.6 m (Figure 1). The right lateral M7.1 ground ruptures were ~50 km long with mean displacement of 1.2-1.7 m and maxima of 4.3-7.0 m. The peak displacement occurred along a 12 km portion of the rupture near the epicenter. More than 650 field-based surface displacement measurements are summarized by DuRoss, et al., 2020. Airborne laser scanning (Hudnut, et al., 2020 available at OpenTopography) and photogrammetry (Pierce, et al., 2020 and Donnellan, et al., 2020) were collected immediately after the earthquakes and provide the basis for rupture mapping, offset reconstructions, and topographic differencing.


Figure 1. Ridgecrest earthquake sequence (orange lines; from DuRoss, et al., 2020) within the California fault network (other lines from USGS Qfaults database). Red lines indicate historic earthquake ruptures: notably the 1872 M7.5 Owens Valley earthquake to the NW and the 1992 M7.3 Landers and 1999 M7.1 Hector Mine earthquake. These events illustrate the activity of the Eastern California Shear Zone, which is dominated today by right-lateral strike-slip faulting. The upper right 0.25 m per pixel lidar-derived hillshade illuminated from the NE shows the ground rupture along the southern portion of the M7.1 rupture. The lower right shows SCEC scientists including Rich Koehler (UNR) and Alana WIlliams (ASU) working along the rupture (photograph by Ian Pierce; Oxford University).

The Ridgecrest earthquakes occurred within the Eastern California Shear Zone. This zone of distributed shearing has seen much research by SCEC scientists, owing to a series of large events here over the last few decades lifetime of SCEC, and SCEC-sponsored research that revealed enigmatic patterns of strain accumulation and release. The Ridgecrest events occurred southeast of the 1872 M7.5 Owens Valley earthquake and northwest of the 1992 M7.3 Landers and 1999 M6.9 Hector Mine earthquakes. The 5th July M7.1 event stopped just short of the Garlock Fault, a major, Holocene active left lateral fault that transects the Mojave Desert, yet shows little or no geodetic evidence for strain accumulation. The conjugate faulting observed in the Ridgecrest sequence is consistent with Quaternary faulting studies as well as prior events in the region.  Limited evidence for prehistoric rupture was mapped before the 2019 events. However, a closer look at landforms along the rupture trace, and better understanding of their context provided by high-resolution topography, illuminates hints of prior events. These are now research targets for SCEC earthquake geologists. 
Along with the impressive ground rupture, the ground motions away from the fault shook fragile geologic features (FGFs; and many not-so-fragile), causing them to fail. FGFs have been the topic of study by SCEC researchers for decades and this proved a great opportunity to examine them whilst knowing the source of the shaking. SCEC scientists documented fresh damage on several of Trona Pinnacles which are about 5 km east of the southern portion of the M7.1 ruptures. Closer to the surface rupture, field geologists observed boulders that had jumped from or at least moved within their sockets. This pervasive failure of geologic features was elegantly examined for the Ridgecrest case by Sleep and Hough, 2020. The GEER team documented much of the ground deformation associated with infrastructure, such as liquefaction (Stewart, et al., 2019; Brandenberg, et al., 2020). 
The geological response to the Ridgecrest earthquake sequence was highly coordinated, with leadership guided by the U.S. Geological Survey and the California Geological Survey, and following the outlines of the SCEC Earthquake Response Plan. This team effort is a powerful example of the SCEC community coming together to collect ephemeral post-earthquake data and build new understanding about these faults, the ECSZ, and earthquake processes in general, in real-time. Blending field observations with new technologies enabled coherent digital data compilation and sharing as well as sharpened the record of the event. Much of the mapping was done using GNSS-enabled tablets. sUAS imaging and photogrammetry--where permitted--provides very high resolution (cm-scale) 3D documentation of the rupture and fragile geologic features. Even the airborne laser scanning point counts are the densest yet: 2010 El Mayor Cucupah earthquake post event laser scan (another SCEC coordinated response) was 9 pts/m2 overall whereas the Ridgecrest post event laser scan is 33 pts/m2 (and in the rupture corridors >80 pts/m2).

About the Authors

Ramon Arrowsmith is a professor in the School of Earth and Space Exploration at Arizona State University. His research focuses on the earthquake geology, paleoseismology, and geomorphology of fault zones. He is currently co-leader of the San Andreas Fault Systems group of the Science Planning Committee and co-founder and co-PI of the OpenTopography effort.
Michael Oskin is a professor in the Earth and Planetary Sciences Department at the University of California, Davis. His research focuses on active crustal deformation and its relationships to surface processes and topography. He has been the co-leader of the Earthquake Geology working group of the Science Planning Committee since 2007.


This research was supported by the Southern California Earthquake Center. SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047. Additional support was provided by NSF RAPID Award EAR-1945781. Lidar data acquisition and processing completed by the National Center for Airborne Laser Mapping (NCALM). NCALM funding for this project provided by the U.S. Geological Survey and by NSF's Division of Earth Sciences, Instrumentation and Facilities Program (EAR-1339015), and the NSF RAPID program (EAR-1945781). OpenTopography is supported by the National Science Foundation under Award Numbers 1948997, 1948994 & 1948857, and the NSF RAPID program (EAR-1945754). We thank our USGS, CGS, and SCEC colleagues for collaborating in this research.