Geologic Setting, Ground Effects, and Proposed Structural Model for the 18 March 2020 Mw 5.7 Magna, Utah, Earthquake

Abstract

The 18 March 2020 Mw 5.7 Magna, Utah, earthquake was the largest earthquake in Utah since the 1992 ML 5.8 St. George earthquake. The geologic setting of the Magna earthquake is well documented by recent geologic mapping at 1:24,000 scale and 1:62,500 scale at and near the epicenter northeast of Magna, Utah. Subsurface fault modeling from surficial geologic mapping, structural cross sections, deep borehole data, and geophysical data reveals a complex system of faulting concentrated in the hanging wall of the Weber and Salt Lake City segments of the Wasatch fault zone including the Harkers fault, the West Valley fault zone, and the newly interpreted Saltair graben. Based on geologic and geophysical data (seismic and gravity), we interpret the mainshock of the Magna earthquake as having occurred on a relatively gently dipping part of the Salt Lake City segment, with aftershocks concentrated in the Saltair graben and West Valley fault zone. Postearthquake rapid reconnaissance of geological effects of the Magna earthquake documented liquefaction near the earthquake epicenter, along the Jordan River, and along the Great Salt Lake shoreline. Subaerial and subaqueous sand boils were identified in regions with roadway infrastructure and artificial fill, whereas collapse features were noted along the shores of the Great Salt Lake. Potential syneresis cracking and pooling in large areas indicated fluctuating groundwater likely related to earthquake ground shaking. The moderate magnitude of the Magna earthquake and minimal geological effects highlight the critical importance of earthquake research from multidisciplinary fields in the geosciences and preparedness on the Wasatch Front.

Stress Perturbations From Hydrological and Industrial Loads and Seismicity in the Salt Lake City Region

“…Overall, previous studies (e.g., Baker et al., 2021; DuRoss & Hylland, 2015; Pang et al., 2020; Kleber et al., 2020; Pollitz et al., 2021; UUSS [@UUSSquake], 2020a; Wong et al., 1995) support a shallowly west‐dipping listric WFZ at the bottom of cluster W , on which the M5.7 Magna Earthquake nucleated; however, the interpretations of structures in the hanging wall contained in the Saltair zone diverge and are often speculative. Given such complexity, we do not attempt to validate the exact geometric and kinematic characteristics of the fault system.…”

“…In the reanalysis of the legacy gravity data, together with surficial geologic mapping, structural cross sections, deep borehole data, and seismic data, Kleber et al. (2020) reaffirmed this interpretation of the Saltair graben. They proposed a connection between the east‐dipping western bounding fault of the Saltair graben and the SE‐dipping Harkers fault to the south.…”

“…Kleber et al. (2020) reprocessed the legacy gravity data and interpreted the “Saltair structure” to be a graben with its western bounding fault dipping to the east and its eastern bounding fault likely an extensional reactivation of the Ogden thrust fault. They found that the geometries and properties of the faults underneath the tailings impoundment are further complicated by the inferred Harkers fault and the Transverse fault zone.…”

A Broad, Distributed Active Fault Zone Lies beneath Salt Lake City, Utah

“…These three faults compose the 40 km long SLC segment of the WFZ. Little is known about the subsurface fault geometry of any of these faults, but geologic mapping, structural models, gravity data, industry seismic data, and constraints from the 2020 Magna earthquake suggest a ∼70°fault near the surface and listric fault geometry at depth (e.g., Smith and Bruhn, 1984;Bruhn et al, 1992;Pang et al, 2020;Kleber et al, 2021). The ∼3 km left step between the active EBF and the WSF lies beneath the most densely urbanized parts of Utah, and the slip distribution and interactions between the fault strands are unclear (DuRoss et al, 2016).…”

3D Shear Wave Velocity Model of Salt Lake Valley via Rayleigh Wave Ellipticity across a Temporary Geophone Array

“…In this study, we explore the passive data recorded by the temporary Magna aftershock array to image the Salt Lake basin structure. The M w 5.7 normal-faulting Magna earthquake occurred on 18 March 2020 near Magna, Utah, and was the largest earthquake within the Salt Lake Valley since the 1962 M L 5.2 Magna earthquake (Kleber et al, 2021). A 168-station nodal array was rapidly deployed across Salt Lake Valley following the mainshock to monitor and record the aftershock sequence (Pang et al, 2020).…”