Rapid post-earthquake reconnaissance and paleoseismic trenching preliminary results for the M6.4 and M7.1 Ridgecrest earthquake sequence, Southern California

Ozgur Kozaci, Christopher M. Madugo, Jeffrey L. Bachhuber, Christopher Hitchcock, Albert R. Kottke, & Kaherine Herr

Published August 15, 2019, SCEC Contribution #9844, 2019 SCEC Annual Meeting Poster #221

A sequence of surface rupturing events started with a magnitude M6.4 earthquake on July 4th, 2019 near Ridgecrest, California. The coseismic deformation associated with this event is identified by an approximately 22-km-long, east-west oriented surface rupture with up to 1 m of left-lateral displacement. The aftershock sequence of the M6.4 earthquake included a rupture of 5-km-long north-south oriented fault strand which one day later ruptured an approximately 55-km-long conjugate structure with up to 4 meters of coseismic right-lateral displacement. These conjugate ruptures are within the Little Lake fault zone; however, were not mapped as active faults for the most part. Our field reconnaissance within 18 hours of the M6.4 event concentrated mostly south of Highway 178 near utility infrastructure and revealed a complex and relatively wide deformation zone of up to a few kilometers wide. A day later we observed similar characteristics for the M7.1 earthquake. Both ruptures are generally expressed as en echelon mole tracks and complex restraining bends or stepovers associated with warping where lithologic changes are present. These fault locations were expressed as subdued topographic and vegetation lineaments in the pre-rupture aerial photographs and satellite imagery. On July 10th we excavated three fault normal trenches across the surface ruptures south of HWY 178 to document the subsurface width and variability of deformation across multiple traces. A 30-m-long, 1,2-m-deep trench (M64-T1) across the M6.4 rupture exposed 0.7 m of left-lateral displacement across the primary fault planes. This trench also provided evidence for penultimate rupture expressed as multiple upward fault truncations beneath a fluvial deposit. 10-m-long M71-T1 and 20-m-long M71-T2 trenches were excavated normal to the primary fault zone of the M7.1 rupture, also south of HWY 178. These trenches exposed multiple fault splays and an insight into the coseismic deformation width and variability. Furthermore, M71-T2 exposed multiple evidences (upward fault truncations and a paleo-sandboil) for a penultimate event. Our preliminary analysis based on the stratigraphic correlation of the penultimate event horizons suggest that similar triggered ruptures on conjugate systems may have occurred in the past, possibly during Holocene.

Key Words
Ridgecrest Earthquakes, surface rupture, paleoseismic trenching

Citation
Kozaci, O., Madugo, C. M., Bachhuber, J. L., Hitchcock, C., Kottke, A. R., & Herr, K. (2019, 08). Rapid post-earthquake reconnaissance and paleoseismic trenching preliminary results for the M6.4 and M7.1 Ridgecrest earthquake sequence, Southern California. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes