The I-15 Downhole Array (I15DA): A Database of Non-invasive Site Characterization Data and Aftershock Ground Motions 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 and 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 (DH0) and deepest (DH4) sensors recorded some of the aftershocks following the main event. 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 had sufficiently high signal-to-noise ratios (SNR) on both the DH0 and DH4 sensors over a bandwidth of 0.5 to 10 Hz. This dataset contains both the raw and processed aftershock ground motions, as well as the small-strain empirical transfer functions (ETFs) calculated between the DH0 and DH4 sensors.

This dataset also contains an extensive set of non-invasive site characterization data collected by our research group to facilitate seismic site response studies at the I15DA. The non-invasive tests consisted of Multichannel Analysis of Surface Waves (MASW), Microtremor Array Method (MAM), and Horizontal-to-Vertical Spectral Ratio (HVSR) measurements. These tests supported the development of deep (greater than 1 kilometer) one-dimensional (1D) shear wave velocity (Vs) profiles at the I15DA as well as assessments of lateral variability in fundamental site frequency (f0). This information was used to develop a pseudo-3D Vs model that extends for over 1 km in all directions around the site.

The ground motions and site characterization data from the I15DA can be used by other researchers to investigate seismic site response through empirical and numerical methods.