The P Wave Velocity of the Uppermost Mantle of the Rio Grande Rift Region of North Central New Mexico

A network of seismograph stations has operated in north-central New Mexico since 1975. The network is approximately 200 by 300 km in size and encompasses the Rio Grande rift there. Several seismic refraction experiments have been reported in the literature for the region of the network and adjacent areas. Because all of the seismic refraction lines are unreversed, Pn velocities reported were mainly of the inverse travel time slope for the direction of the corresponding line. The values of the inverse slope for those studies range from 7.6 to 8.2 km/s. The purpose of our study is to estimate the P wave velocity of the uppermost mantle by using the time term method. First, we timed the Pn waves of strong signals from five explosions and eight shallow earthquakes recorded by the network. The main data set, which contains 87 time-distance pairs, was processed by using the time term method. The Pn velocity estimated by this method is 8.0 :!: 0.1 km/s. To corroborate this estimate, we then processed 10 subsets of the main data set in the same way. Almost all of the solutions show velocities of 7.9-8.1 km/s, in agreement with the velocity determined for the main data set. The station time terms of the main data set also are substantiated, and they suggest that the base of the crust dips northward by a few degrees in the region of the survey. The smallest value reported by other investigators for the inverse slope (7.6 km/s) appears to be related to the dip. The normal P wave velocity of the uppermost mantle of north-central New Mexico places restrictions on thermal models of the rift. For instance, the results exclude the likelihood of a wide zone of asthenosphere at the base of the crust beneath the rift, but they do not exclude a narrow such zone.