A Database of Aftershock Ground Motions Recorded by the I-15 Downhole Array Following the 2020 M5.7 Magna, Utah, Earthquake

The I-15 Downhole Array (I15DA) is located near the intersection of I-15 and I-80 in Salt Lake City, Utah. It is positioned in a seismically active area near the Salt Lake City section of the Wasatch Fault and the Taylorsville section of the West Valley Fault. The downhole array is installed in soft, Lake Bonneville soil deposits where the depth to bedrock is expected to be at least 370 m. The array is made up of five, three-component force-balance accelerometers (FBA). One of the sensors was installed on the surface, and the other sensors were installed at depths of 7.6, 18.3, 48.8, and 119.8 m. Unfortunately, due to a loss of power and data buffering/telemetry issues at the site, none of these sensors fully captured the main shock of the 2020 M5.7 Magna, Utah, Earthquake, whose epicenter was located approximately 15 km north-west of the I15DA. However, the surface and deepest sensors recorded some of the aftershocks following the main event, and continuous datastreams containing approximately 61 aftershocks with magnitudes greater than 2.0 were provided to our research team by the University of Utah Seismograph Stations. The 61 aftershocks had local magnitudes (ML) that ranged from 2.0 to 4.57, and peak ground acceleration (PGA) values for the surface sensor that ranged from less than 0.001 g to 0.132 g. Unfortunately, after processing, only eight of the aftershock events generated high enough signal-to-noise ratios (SNR) on both the surface and 120-m sensors over a bandwidth of 0.5 to 10 Hz. This paper describes the ground motion processing performed on the I15DA recordings, presents empirical transfer functions (ETFs) calculated from the ground motions, and provides a reference to an open-access database where the raw and processed ground motion records can be downloaded for use in site response studies.