Simultaneous inversion for lateral velocity variations and hypocenters in the Yellowstone Region using earthquake and refraction data

A three‐dimensional upper crustal P wave velocity structure of the Yellowstone region was determined by simultaneous inversion using Pg refracted arrival times and earthquake‐generated direct P wave arrival times. The velocity structure was modeled by a two‐layer block configuration with the velocity of the surface layer fixed. The inversion was performed on a 14.4‐km‐thick second layer (5×6 blocks) using an optimal block size of 21.2×18.3 km and incorporated the method of separation of parameters. Results show volumes of higher velocities, approximately 6.3 km/s, north of the Yellowstone caldera that correlate with gravity anomaly highs and are interpreted to be associated with near‐surface crystalline basement. The upper crust beneath the caldera showed an average velocity of 5.8 km/s with prominent low‐velocity zones of 4.9 and 5.2 km/s in the northeast and southwest portions of the caldera, respectively. The 4.9‐km/s low‐velocity layer in the northeast corner of the caldera coincides with a localized‐20‐mGal Bouguer gravity low and the largest area of hydrothermal activity in Yellowstone National Park. Relocating earthquakes with the new velocity model showed a systematic migration of epicenters radially toward the caldera by an average of 1.2 km relative to locations using a homogeneous velocity model.