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The Normal-Faulting 2020 Mw 5.8 Lone Pine, Eastern California, Earthquake Sequence


 The 2020 Mw 5.8 Lone Pine earthquake, the largest earthquake on the Owens Valley fault zone, eastern California, since the nineteenth century, ruptured an extensional stepover in that fault. Owens Valley separates two normal-faulting regimes, the western margin of the Great basin and the eastern margin of the Sierra Nevada, forming a complex seismotectonic zone, and a possible nascent plate boundary. Foreshocks began on 22 June 2020; the largest Mw 4.7 foreshock occurred at ∼6  km depth, with primarily normal faulting, followed ∼40  hr later on 24 June 2020 by an Mw 5.8 mainshock at ∼7  km depth. The sequence caused overlapping ruptures across a ∼0.25  km2 area, extended to ∼4  km2, and culminated in an ∼25  km2 aftershock area. The mainshock was predominantly normal faulting, with a strike of 330° (north-northwest), dipping 60°–65° to the east-northeast. Comparison of background seismicity and 2020 Ridgecrest aftershock rates showed that this earthquake was not an aftershock of the Ridgecrest mainshock. The Mw–mB relationship and distribution of ground motions suggest typical rupture speeds. The aftershocks form a north-northwest-trending, north-northeast-dipping, 5 km long distribution, consistent with the rupture length estimated from analysis of regional waveform data. No surface rupture was reported along the 1872 scarps from the 2020 Mw 5.8 mainshock, although, the dipping rupture zone of the Mw 5.8 mainshock projects to the surface in the general area. The mainshock seismic energy triggered rockfalls at high elevations (>3.0  km) in the Sierra Nevada, at distances of 8–20 km, and liquefaction along the western edge of Owens Lake. Because there were ∼30% fewer aftershocks than for an average southern California sequence, the aftershock forecast probabilities were lower than expected. ShakeAlert, the earthquake early warning system, provided first warning within 9.9 s, as well as subsequent updates.