← Back to Sessions

Backprojection Imaging of the 2020 Mw 5.5 Magna, Utah Earthquake

Session: Intermountain West Earthquakes in the Spring of 2020 I

Type: Oral

Date: 4/20/2021

Presentation Time: 02:30 PM Pacific

Description: 

On 18 March 2020 a moderate earthquake (M_w 5.5) struck the Salt Lake Valley in Utah, ~5 km north of the town of Magna and ~10 km west of downtown Salt Lake City. The Magna earthquake occurred in a very well instrumented area with tens of strong motion seismic stations. In this study, we present the application of a backprojection method for imaging the detailed rupture of the M_w 5.5 Magna, Utah earthquake by exploiting the very dense local strong motion network. For the backprojection 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. Backprojection resolves the epicentral location of the mainshock with an absolute error of less than 1 km, while the depth resolution is within the centroid depth range of multiple moment tensor solutions. We find an up-dip unilateral WNW-ESE ~ 10km rupture resolved from the spatial distribution of the imaged subevents, consistent with the distribution of early aftershocks. The average rupture speed for the first 3 s is 2.9–3.2 km/s, which is ~0.9 times the shear wave velocity for the area at 12 km depth [3.4–3.7 km/s], whereas for the first 2 s the rupture speed is~2.9 km/s (~0.8 Vs). We perform two tests to check that the rupture propagation is not an artifact of site effects due to stations located on soft soil or a function of station distribution. In the first test we perform the backprojection for 22 strong motion stations located on rock sites, and in the second test we backproject a M4.1 aftershock back to the source region, assuming that an earthquake of that size approximates a point source. We argue that site effects did not bias the backprojection results, specifically the rupture propagation, even though we used stations located on both rock and soft soil. This study emphasizes the need for instrumenting metropolitan areas of high seismic risk and adopting backprojection techniques in the near real-time network products immediately after a local strong moderate earthquake.

Presenting Author: Kristine L. Pankow

Student Presenter: No

Authors