Amid the pandemic lockdown fatigue and media frenzy surrounding the presidential election in November, field work was a most welcome distraction. Socially distancing and soaking in the balmy Mojave Desert air, we reconnected through our work—trenching along the surface ruptures caused by the M6.4 and M7.1 Ridgecrest earthquakes that occurred July, 2019.

Funded by SCEC, our team from the Nevada Bureau of Mines and Geology at the University of Nevada Reno (Rich Koehler), Arizona State University (Ramon Arrowsmith), and early career researchers (Alana Williams and Ian Pierce) set out to gather evidence on the timing and recurrent behavior of the faults that caused the Ridgecrest earthquakes. We want to understand their role in releasing strain within the Eastern California Shear Zone and southern Walker Lane region. Did the Ridgecrest faults rupture together in the past (as in 2019), or did they act as independent seismic sources? The results of this paleoseismic study could further our understanding of conjugate ruptures and triggered slip, and provide useful information for future seismic hazard models.

Our team participated in the multi-agency post-earthquake rupture mapping reconnaissance and contributed data to document the location, style, and variability of the surface ruptures (Dawson et al., in review; Ponti et al., 2020), and the surface slip distribution (DuRoss et al., 2020). During our initial mapping (Pierce et al., 2020), we noted numerous tectonic geomorphic features (e.g., scarps, pressure ridges, deflected drainages) along both ruptures that suggest previous faulting. Similarly, Thompson Jobe et al. (2020) documented evidence for prior faulting along the entire length of both ruptures and noted that the fault zones consisted of dense networks of faults—not all of which ruptured in 2019. If not all strands rupture in every earthquake, then selecting paleoseismic sites that capture evidence of prior events poses some challenges. Although detailed studies have been conducted northwest of Ridgecrest along the Little Lake and Airport Lake faults (Amos et al., 2013; Roquemore, 1981), trenching has not been conducted along the causative faults of the Ridgecrest earthquakes.

Base camp, where trench orthophoto mosaics were processed at night and projected onto the wall of the camper.

We excavated a total of six trenches across the Ridgecrest surface ruptures. Using a rented backhoe (red machine in image above), we hunted for “the penultimate event” that occurred before the 2019 Ridgecrest earthquake sequence. We assessed the stratigraphic and structural relations exposed in each trench and pursued additional excavations if initial exposures were not productive. We documented trenches using traditional paper logging techniques, orthophotomosaic logging on screens, and iPad lidar scanning. Over dinner at base camp each night, we discussed possibilities while viewing photomosaics of the trenches projected onto the side of the camper.

We excavated two trenches across the M6.4 rupture (Site B, along the left-lateral Salt Wells Valley fault), located about 0.8 km north of Randsburg Wash Road and above the 685 m highstand of pluvial Searles-China Lake. Bedrock and alluvial sediments were faulted in 2019 and also exhibited carbonate lined shear zones and fissures that did not rupture in 2019, providing evidence for prior faulting. Based on the degree of soil development, the penultimate event may have occurred tens of thousands of years ago.

(A) orthophotomosaic and (B) preliminary interpretive log of the Site C trench exposure. The upward fault terminations and progressive warping of lacustrine sediments are evidence for multiple deformation events.

Four trenches were excavated across the M7.1 rupture along the right-lateral Paxton Ranch fault. Three trenches were located southeast of the crest of the Spangler Hills (Site A), below the pluvial Searles-China Lake high stand where the 2019 rupture is characterized by left-stepping en echelon scarps, mole tracks, and fissures. The trenches at Site A showed faulted bedrock and lacustrine and alluvial deposits. Upward fault terminations and fissures buried by young alluvium provide evidence for prior faulting. The abundant shell hash and macrofossils in the lacustrine deposits were sampled for radiocarbon age analyses. The last trench was excavated across the M7.1 rupture (Site C) within lacustrine and fluvial sediments deposited in the Searles Lake basin. Here, the deposits thicken towards the fault, suggesting they were deposited against a paleo scarp. Clear upward fault terminations provide evidence for at least two Holocene or latest Pleistocene ruptures in addition to the 2019 earthquake.

We were unable to trench across all of the traces that ruptured in 2019. For example, a prominent rupture extends along the rangefront northeast of Site A, but the steep topography precluded trenching there. Therefore, while the excavated trenches all revealed evidence of paleoearthquakes, the combined record may not be complete. 

Over 25 samples were collected for radiocarbon and optically stimulated luminescence analyses. We are currently pursuing funding for analyses to place age constraints on the timing of paleoearthquakes and calculate recurrence intervals.

During the trenching study, our virtual home base was a converted military ambulance and a pop top camper, sheltering us from harsh winds and freezing nightly temperatures. Our field work concluded with a trench review with CGS, USGS, NGPO, and PG&E participants. And after the sites were backfilled and restored, we headed home in time to give thanks that—for two weeks in November—our only worry was bad weather disrupting a day in the trenches.

About the Authors

Rich Koehler is an assistant professor at the Nevada Bureau of Mines and Geology at the University of Nevada, Reno, where he conducts research on Quaternary geology and paleoseismology, primarily focussed on active faults in Nevada and California. He currently serves on the Western States Seismic Policy Council (WSSPC) and has participated in numerous post-earthquake investigations as a member of Geotechnical Extreme Events Reconnaissance (GEER).

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 SCEC Science Planning Committee and co-founder and co-PI of the OpenTopography effort.

Acknowledgements

This research was supported by the Southern California Earthquake Center (Award No. 20103). SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047. We thank Ian Pierce and Alana Williams for their leadership and efforts  in the field and our colleagues Gordon Seitz (CGS), Brian Olson (CGS), Devin McPhillips (USGS), Chris Madugo (PG&E), John Helms (High Desert Consulting), and Kelly Blake and Andrew Sabin (Navy Geothermal Program Office, NGPO) for valuable discussions in the trenches.

References

• Amos, C.B., Brownlee, S.J., Rood, D.H., Fisher, G.B., Burgmann, R., Renne, P.R., and Jayko, A.S. (2013). Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California, Geol. Soc. Am. Bull., 125, no. 7-8, 1187-1202. https://doi.org/10.1130/B30803.1
• DuRoss, C, Gold, R.D., Dawson, T.E., Scharer, K.M., Kendrick, K.J., Akciz, S.O., Angster, S.J., Bachhuber, J., Bacon, S., Bennett, S.E.K., et al. (2020). Surface Displacement Distributions for the July 2019 Ridgecrest, California, Earthquake Ruptures, Bull. Seism. Soc. Am. 110, 4, 1400-1418. SCEC Contribution 10110
• Dawson, T., and 72 others (in review). Field-based Observations of Surface Ruptures Associated with the 2019 Ridgecrest Earthquake Sequence, Bull. Seism. Soc. Am.
• Pierce, I., Williams, A., Koehler, R.D., and Chupik, C. (2020). High resolution structure-from-motion models and orthophotos of the southern sections of the 2019 Mw7.1 and Mw6.4 Ridgecrest, earthquakes surface ruptures, Seismol. Res. Lett. 91, 4, 2124-2126. SCEC Contribution 10114
• Ponti, D.J., Blair, J.L., Rosa, C.M., Thomas, K., Pickering, A.J., Akciz, S., Angster, S., Avouac, J.P., Bachhuber, J., Bacon, S. et al. (2020). Documentation of surface fault rupture and ground deformation features produced by the Ridgecrest M6.4 and M7.1 earthquake sequence of July 4 and 5, 2019, Seismol. Res. Lett. 91(5), 2942-2959. SCEC Contribution 10852
• Roquemore, G.R. (1981). Active faults and associated tectonic stress in the Coso Range, California, Naval Weapons Center China Lake Technical Publication 6270, 112 p. http://hdl.handle.net/11714/1651
• Thompson Jobe, J.A., Philibosian, B., Chupik, C., Dawson, T., Bennett, S.E.D., Gold, R., DuRoss, C., Ladinsky, T., Kendrick, K., Haddon, E., et al. (2020). Evidence of previous faulting along the 2019 Ridgecrest, California, earthquake ruptures, Bull. Seismol. Soc. Am. 110, 1427-1456. https://doi.org/10.1785/0120200041 (Also see SCEC2020 Poster #020)