Analyses of site effects in the 2020 M 5.6 Magna mainshock and aftershocks and development of shallow site amplification factors for Salt Lake Valley, Utah

The 18 March 2020 M 5.6 Magna, Utah earthquake near Salt Lake City was recorded by more than 160 strong motion/broadband stations. The ground motions compare favorably with the NGA-West2 ground motion models (GMMs) using estimates of V S30 but there are numerous exceptions. The purpose of this study was to evaluate the site effects at 26 seismic stations located in the Salt Lake Valley. We evaluated the data from the mainshock and seven largest aftershocks ( M 3.9 to 4.6) and performed a one-dimensional (1D) random vibration theory equivalent-linear site response analysis (SRA). We compared our predicted site amplification with the recorded ground motions to evaluate which features in the data could be predicted using a 1D approach. We first updated the 2008 Utah Geological Survey shallow shear-wave velocity (V S ) database. Shallow V S profiles were assigned to each seismic station and extended to the source depth of 7 km using the Wasatch Front Community Velocity Model. Next an inversion was performed to evaluate stress drops, kappa, and Q(f) using recordings from 109 earthquakes ( M 2.9 to 5.6). The resulting parameters were then used in the SRA for four Salt Lake Valley site categories to compute linear amplification factors (AFs). These factors were then compared with the AFs (response spectra) implied by the NGA-West2 GMMs for the 26 seismic stations. Horizontal-to-vertical spectral ratios were also calculated from the recorded data and compared with the AFs (Fourier amplitude spectra) from the SRA to evaluate the consistency of the predicted site resonant frequencies. Our predicted AFs for the mainshock generally agree with the NGA-West2 AFs except for the softest unit where we predict more soil nonlinearity and higher long-period amplification. Our results also show an underprediction at short periods and close-in distances which may be due to a source effect, for example, radiation pattern and/or rupture directivity.