Upper crustal structure beneath the Jemez Mountains Volcanic Field, New Mexico, determined by three‐dimensional simultaneous inversion of seismic refraction and earthquake data

The upper crustal structure beneath the Jemez Mountains volcanic field (JVF), New Mexico, was determined using a simultaneous inversion of earthquake and seismic refraction travel time data. One‐, two‐, and three‐dimensional inversions were sequentially applied to derive a detailed velocity model of the upper 10 km of the crust beneath the JVF. Travel time observations from six shot points recorded at as many as 72 temporary seismograph stations and from 27 earthquakes recorded at from 6 to 12 network stations were simultaneously inverted to determine both compressional wave velocity structure and hypocentral parameters. The velocity model was described by a grid of velocity values (unknowns in the inversion procedure) with velocities between grid points given by linear interpolation. Ray tracing through two‐dimensional slices of the three‐dimensional model was performed to calculate theoretical travel times for each source‐receiver pair. A damped least squares iterative inversion was used to derive the velocity model and hypocenters from the differences between observed and theoretical travel times. The relocation of the earthquakes resulted in changes in epicentral location of up to 16 km, although most events were moved less than 7 km. Hypocentral depths of the relocated events ranged from sea level to 17 km, averaging about 8 km. The final three‐dimensional model provided a good fit to the travel time observations. The velocity model displays a prominent low‐velocity, approximately cylindrically shaped body beneath the Valles caldera within the JVF. The velocity anomaly is defined by a lateral decrease in velocity from about 6.0 km/s to as low as 5.6 km/s. The low‐velocity body has a diameter of about 15 km. We interpret the velocity anomaly beneath the Valles caldera to be the result of the combined effects of a silicic intrusion in the upper crust and velocity reduction caused by elevated temperature.