Kinematic slip models of four large Intermountain West earthquakes of 2020 and 2021
Fred Pollitz
USGS
- Date & Time
- Location
- Online-only seminar via Microsoft Teams
- Host
- TBD
- Summary
Four large earthquakes struck the Intermountain West and Basin and Range Province in 2020 and 2021: M5.7 March 18, 2020 Magna, Utah; M6.5 March 31, 2020, Stanley, Idaho; M6.5 May 15, 2020 Monte Cristo Range, Nevada; and M6.0 July 8, 2021 Antelope Valley, California, earthquake. As noted by Wesnousky (2020 SRL), each of the first three occurred in areas of relatively high background seismicity and geodetic strain rate; the same is true for the Antelope Valley earthquake. The events sample different tectonic environments with distinct fault geometries, leading to unique rupture characteristics of each event. We explore kinematic slip models of these earthquakes based on observations of geodetic static offsets and seismic waveforms. The Magna and Antelope Valley earthquakes are both normal faulting events that did not approach Earth’s surface (centroid depths ~8-10 km). However, their fault dips differ substantially — 30 deg. and 50 deg., respectively, reflecting likely differing tectonic stress fields. The Monte Cristo Range event involves predominantly left-lateral strike slip on steeply dipping faults and normal slip on a small near-surface shallowly dipping fault; it is the only event of the four to have ruptured the surface. The Stanley event is the deepest of the four, with significant slip concentrated at depth ~8 to 16 km. It involves both strike slip and normal slip, with a gradual transition between the two in the along-strike direction, reflecting a spatially variable tectonic stress field. These events highlight the significant seismic hazards of the Intermountain West area and the importance of the geological record for understanding regional differences.
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