The Ridgecrest Earthquake Sequence began on July 4, 2019 with a Mw 6.4 earthquake at 10:33 am PDT at a depth of 8.7 km. The epicenter was located about 18 km east-northeast of the City of Ridgecrest within the Naval Weapons Station China Lake (NWSCL) property. This event was preceded by several small foreshocks a few days prior to the event. Surface rupture from this event was expressed as a zone of surface faulting over 17 km long, consisting of several strands with en-echelon stepovers, striking northeast-southwest with left-lateral displacement of the ground surface. Rupture appears to have propagated from the epicenter toward the southwest.

Aftershock patterns following the Mw 6.4 event followed the northeast-southwest trend, however a perpendicular northwest-southeast L-shaped pattern developed near the epicenter at the north end of the fault zone. This northwest-southeast aftershock pattern appeared to be weakly coincident with a discontinuous zone of northwest striking, previously mapped Holoceneactive faults. The pattern of faulting and seismicity hinted at the possibility of cross-fault triggering, similar to what was observed in other earthquake sequences such as the 1987 Elmore Ranch - Superstition Hills earthquake sequence (Hudnut and others, 1989).

About 34 hours after the Mw 6.4 event and numerous aftershocks, some of which were M5+, the Mw 7.1 mainshock event occurred at 8:19pm PDT. The epicenter of this mainshock was located approximately 10 km northwest of the Mw 6.4 epicenter, at a depth of 10 km. Surface rupture from this event occurred along a northwest-southeast striking fault zone, roughly coincident with the northwest -southeast seismicity observed north of the M 6.4 rupture prior to the mainshock. Displacement was right-lateral and extended bilaterally away from the epicenter over a distance of ~50 km.

This presentation will include a discussion of the overall earthquake response by geologic teams, surface rupture characteristics and slip measurements compiled to date for both Mw 6.4 and Mw 7.1 earthquake events, including an overview of data collection methods.

Janis Hernandez is a Senior Engineering Geologist (Supervisor) for the California Geological Survey and is manager of the Southern California Regional Office.

Janis obtained her B.S. in Geology from California State University, Fullerton in 1995, and is a Registered Professional Geologist, and Certified Engineering Geologist. Janis worked as a staff geologist, and as a project geologist and client relations manager for geotechnical consulting firms in southern California for 9 years prior to coming to CGS in 2001.

Her professional experience includes geologic mapping, fault and landslide investigations, groundwater well construction management, post-landslide, -fire, and -earthquake event response work. Her career at the California Geological Survey has encompassed several programs including Seismic Hazards Zonation, Alquist-Priolo Earthquake Fault Zone mapping (AP maps), geologic mapping for STATEMAP, and Seismic Hazards Review for the Division of the State Architect. She has also collaborated with geophysicists from the U.S. Geological Survey in performing seismic studies on the Santa Monica, Raymond, and Hollywood Faults.

Janis is currently performing new detailed geologic mapping in the Peninsular Ranges Batholith within the Borrego Valley 100k map area. As part of this mapping effort, she is also working with others in the LA office to update the A-P maps for the Elsinore and Earthquake Valley fault zones.