Sacramento AEG February Meeting: Alex Morelan 

Please join us on Tuesday, February 23, 2021 on Zoom! We will be hosting Alex Morelan from California Geological Survey, who will be giving a presentation titled Geologic Observations of the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence.

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Geologic Observations of the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence

Alex Morelan, PhD

 California Geological Survey (CGS)

Alex.Morelan@conservation.ca.gov

Abstract

The Ridgecrest earthquake sequence began on July 4, 2019 with a M_w 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, that consisted of several strands of en-echelon stepovers striking northeast-southwest with left-lateral displacement.  Rupture appears to have propagated from the epicenter toward the southwest. Aftershock patterns of the M_w 6.4 event largely followed the northeast-southwest surface rupture trend with a perpendicular northwest-southeast L-shaped pattern that developed near the epicenter at the northeast end of the fault zone.  This northwest-southeast aftershock pattern appeared to be weakly coincident with a discontinuous zone of northwest-striking, previously mapped Holocene-active faults. This pattern of orthogonal faulting and seismicity hinted at the possibility of cross-fault triggering, like what was observed in other earthquake sequences such as the 1987 Elmore Ranch - Superstition Hills earthquake sequence (e.g. Hudnut et al., 1989). About 34 hours after the M_w 6.4 event and numerous large (M5+) aftershocks, the M_w 7.1 mainshock event occurred at 8:19pm PDT.  The epicenter of the mainshock event was located approximately 10 km northwest of the M_w 6.4 epicenter at a depth of 10 km.  Surface rupture from this event occurred along a northwest-southeast striking fault zone coincident with the northwest -southeast M_w6.4 aftershock seismicity. Displacements on the M_w 7.1 trace were primarily 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.

Speaker Bio

Alex Morelan is an engineering geologist with the California Geological Survey (CGS) in Sacramento. He received both his BS (2011) and PhD (2019) at UC Davis, after which he started at CGS. Alex’s dissertation focused on tectonic geomorphology and earthquake geology. His focus areas included slip rates along the southern San Andreas fault, alluvial fan deposition along active range fronts in eastern California and Nevada, and using photogrammetry to archive ephemeral earthquake offsets after the 2014 Napa earthquake.