Implications of the March 18, 2020 Magna Mw5.7 Earthquake for Coulomb Stress Change, Earthquake Hazard, and Listric Structure on the Wasatch Fault, Utah, USA

Abstract:

The March 18, 2020 Mw5.7 Magna earthquake occurred 18 km west of the surface trace of the Salt Lake City (SLC) segment of the west-dipping Wasatch fault (WF) in Salt Lake Valley, on the eastern edge of the Basin and Range extensional province, USA. Approximately 1.1 million people live within 20 km of the Salt Lake segment of the WF, which is capable of producing a > M7 earthquake. The SLC segment has an average recurrence interval of ~1300 yrs and it has been ~1400 yrs since the last surface-rupturing event (Wong et al., 2016) so the fault is late in its seismic cycle. Published source mechanics (UUSS, 2020) and initial analysis of the distribution of aftershocks show the Magna earthquake was caused by normal-oblique slip on a fault that dips 30 ± 10^o west. To our knowledge, this is the first recorded moderate to large earthquake on a normal fault that dips ~30^o or less in the Basin and Range. Furthermore, if the WF is listric and dips ~30^o W in the subsurface, then the earthquake likely occurred on the down-dip extension of it. Our models of Coulomb stress change caused by the Magna earthquake indicate that if the Wasatch fault is listric, then Coulomb stress on it has increased significantly. In contrast, if the WF dips >45^o in the subsurface then Coulomb stress on it probably was reduced. For the likely scenario that the Magna earthquake occurred on the down-dip extension of the WF, we estimate Coulomb stress increased ~1 b average over ~290 km² of area on the Salt Lake segment of the WF. This is similar to estimated increases prior to events apparently triggered by slip on nearby faults (e.g., Hodgkinson et al., 1996; Nalbant et al., 2005). We apply the method of Stein et al. (1997) to estimate the change in seismic hazard on the Wasatch fault that could be expected from the calculated Coulomb stress changes. The method incorporates both a long-term and a transient effect due to Coulomb stress change into standard estimates of conditional probability made primarily from paleoseismology data. This calculation suggests that the Magna earthquake may have increased the 10-year conditional probability of a surface-rupturing earthquake on the Wasatch fault by a factor of 1.5 to 3.3 (i.e., from ~1.5% to ~2.1 - 5%), and the 50-year conditional probability by factor of 1.1 to 1.5 (i.e., from ~7.5% to ~8.5 – 12%) relative to standard conditional probability estimates.