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Hypothetical Structural Model for the 18 March 2020, Mw 5.7 Magna, Utah, Earthquake

Session: Intermountain West Earthquakes in the Spring of 2020 I

Type: Oral

Date: 4/20/2021

Presentation Time: 03:15 PM Pacific

Description: 

The 18 March 2020, M_w 5.7 Magna, Utah, earthquake occurred beneath western Salt Lake Valley which is near the eastern margin of the Basin and Range Province and within the Intermountain Seismic Belt. Using recent geologic mapping, cross sections, deep borehole data, and seismic and gravity geophysical data, we created a subsurface structural model to elucidate the earthquake causative fault. The model depicts a complex system of faults in the hanging wall of the west-dipping Wasatch normal fault zone, including the Harkers fault, West Valley fault zone, and newly interpreted Saltair graben. The mainshock occurred between 9.0 and 11.9 km below sea level. Numerous aftershocks occurred in two main clusters: a large western cluster near the mainshock focus and a small, shallower eastern cluster in the vicinity of the east-dipping Granger fault of the West Valley fault zone. We hypothesize that the mainshock occurred on a major listric fault, likely the Warm Springs fault of the Salt Lake City segment of the Wasatch fault zone. The Warm Springs fault has an observed surface dip of ~70° to the west, but below a depth of about 7 km the aftershock locations suggest a ~30° dip. The eastern cluster of aftershocks roughly coincides with the modeled intersection of the Granger fault and the Warm Springs fault at depth. The western aftershock cluster appears to be near the intersection of the concealed Saltair graben faults and the Wasatch fault zone. The West Valley fault zone, Harkers fault, and Saltair graben may represent an accommodation zone between the Weber and Salt Lake City segments of the Wasatch fault zone. The accommodation zone is likely reoccupying a prior zone of weakness, the inferred Transverse fault zone, a suspected Sevier-age structure. Perhaps oblique-slip components observed in the moment tensor solutions of both the mainshock and major aftershocks are related to this hypothesized accommodation zone.

Presenting Author: Adam P. McKean

Student Presenter: No

Authors