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Focal Mechanisms for the 2020 Magna, Utah Earthquake Sequence Using Manual and Deep-Learning First Motions From a Temporary Large-N Array and Permanent Seismic Network

Session: Intermountain West Earthquakes in the Spring of 2020 [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 11:30 AM Pacific

Description: 

On 18 March 2020 an M_w 5.7 earthquake occurred near Magna, Utah, USA, about 15 km west of downtown Salt Lake City. Immediately after the mainshock, 180 5-Hz nodal geophones, or nodals, were deployed for 30 days throughout the Salt Lake Valley. Between mid-March and mid-April 2020, there were over 300 seismic stations, including the nodals, operating within 200 km of the mainshock. The University of Utah located more than 2,500 aftershocks, ranging from M-0.6–4.6, not including those found via template matching. The aftershocks separate into two clusters: the eastern cluster, beneath the Salt Lake City International Airport, and the western cluster, in the vicinity of the mainshock. Due to a large number of first motion misfits, we were unable to resolve any focal mechanisms for the eastern cluster. The largest event in this cluster was an M3.25 with clear first motions on 158 stations, which suggests either non-double-couple sources or 3D velocity perturbations. For the western cluster, we combined manually picked first motions (MPFM) and machine learning first motions (MLFM) to create a dataset of ~54,000 first motions from 370 stations. The first motions comprise 493 focal mechanisms; 71 are from MPFM and 422 are from MLFM. There are nine MPFM focal mechanisms that also have moment tensor solutions. The maximum angle between an MPFM mechanism and moment tensor mechanism for these events is 51°, well below the 180° that indicates two mechanisms are perfectly dissimilar. There are 51 events with MPFM and MLFM focal mechanisms, and the maximum angle between the two types of mechanisms is 90°. These angles indicate good agreement between moment tensor, MPFM, and MLFM focal mechanisms. The MPFM mechanism nodal planes generally strike north-south and have a median dip of 50°, or 48° when the strike-slip events are removed. This median dip is steeper than the ~20° dip of the relocated aftershocks and the ~30° dip of the mainshock.

Presenting Author: Monique M. Holt

Student Presenter: Yes

Authors