Backprojection Imaging of the 2020 Mw 5.5 Magna, Utah, Earthquake Using a Local Dense Strong-Motion Network

Abstract

Abstract We present the application of a backprojection method for imaging the detailed rupture of the 2020 Mw 5.5 Magna, Utah, earthquake. This is the first time that this method is applied to an earthquake smaller than Mw 6 in a local scale using a dense strong-motion network. The 2020 Magna earthquake occurred in a very well instrumented area, the Salt Lake valley, with tens of strong-motion seismic stations. We use envelopes of high-frequency S waves recorded on the transverse component at 45 seismic stations that are located at distances up to 100 km from the epicenter. The nearest station is ∼4.5  km. Backprojection resolves the epicentral location of the mainshock with an absolute error of less than 1 km, whereas the depth resolution is within the centroid depth range of multiple moment tensor solutions. Spatial distribution of the imaged subevents shows an up-dip unilateral west-northwest–east-southeast rupture with a length of ∼10  km, consistent with the distribution of early aftershocks. The average rupture speed is between 2.9 and 3.2  km/s for the first 2 and 3 s, respectively. The shallow dip (∼35°) of the Wasatch fault at depth, which failed during the Magna earthquake, combined with the up-dip unilateral rupture, indicates that ground-motion scenarios for future larger earthquakes in the Salt Lake Valley should be re-evaluated. This study underlines the need for instrumenting metropolitan areas of high seismic risk and adopting backprojection techniques in the near-real-time network products immediately after a strong earthquake.