Evidence for a Listric Wasatch Fault From the 2020 Magna, Utah, Earthquake Sequence

Abstract:

The 18 March 2020 M_w 5.7 Magna earthquake near Salt Lake City, Utah, offers a rare glimpse into the subsurface geometry of the Wasatch fault—one of the world’s longest active normal faults and a major source of seismic hazard in northern Utah. As of 30 April 2020, using conventional methods, the University of Utah Seismograph Stations (UUSS) detected and located 2,103 earthquakes in the Magna sequence. We employed a matched filtering technique on all available seismic data within 40 km of the mainshock for the time period of 17 March 2020 (1 day before the mainshock) through 30 April 2020 and enhanced the original catalog by ~3 times. We applied a double-difference relocation algorithm, GrowClust, using the differential travel times from waveform cross-correlations. We resolved oblique-normal slip on a fault dipping shallowly (30–35º) to the west at ~9–12 km depth from the high-precision relative relocation of aftershocks and moment tensor solutions for the mainshock and eight of the larger aftershocks. Combined with near-surface geological observations of a steep dip (~70º), our results support a curved, or listric, fault model of the Wasatch fault. High-precision aftershock locations show the activation of multiple, low-angle (<30º) structures, indicating the existence of a complicated fault system. Our observations provide the first direct evidence for the deep structure of the Wasatch fault, and suggest that future large earthquake on this fault may generate stronger ground in the Salt Lake City metropolitan area than previously estimated.