Sacramento AEG February Meeting!

Please join us on Tuesday, February 23 on Zoom! Our speaker will be Mr. Alex Morelan will give a presentation titled Geologic Observations of the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence  The event will begin at 5:30PM.

About the Topic:

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 w 6.4 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.

About the Alex Morelan:

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.

Please register here!

Once registered, you will receive a Zoom link.

We hope to virtually see you there!

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