Seismotectonic Interpretation of an Expansive Set of Earthquake Focal Mechanisms from First Motions and Amplitude Ratios on the Yellowstone Plateau

We present high-quality focal mechanisms and stress inversions based on a refined earthquake location catalog for the Yellowstone Plateau. The relocation process is based on non-linear search techniques that use three-dimensional velocity models. The original catalog contained 18,940 events between January 2010 and March 2021 and was obtained from the University of Utah Seismograph Stations. We successfully relocated 7735 earthquakes and used this information to calculate the focal mechanisms using two approaches. First, we only consider the P-wave first motion polarities and use the HASH program. Second, we include the S-wave/P-wave amplitude ratios and evaluate if they can improve the computed focal mechanisms obtained using only P-wave first motion polarities. We filtered the results to create a subset of data with the best quality solutions. After obtaining results from the focal mechanisms calculation, we ran joint inversion for stress and fault orientations from focal mechanisms using the STRESSINVERSE package. We divided the study area into smaller sections to analyze the spatial and temporal variations of the seismic stress field. The inclusion of the amplitude ratios increased the number of available solutions and allowed the improvement of the low-quality events. However, this addition was inefficient at refining focal mechanisms with previously established good quality solutions. The stress field in this area displays temporal and spatial variations. To describe these variations, we use the minimum principal stress directions that are horizontal or near horizontal in all the cases as expected for an extensional regime dominated by NE-SW Basin and Range extension in this area. The minimum stress direction rotates from an orientation near to N-S near Hebgen Lake fault zone to NE-SW near Norris Junction. In addition, all the regions that fall within the 0.64 Ma caldera display orientations ENE-WSW.