A Crustal Velocity Model for the Peninsular Ranges of Baja California and Southwestern Laguna Salada, Mexico

To see any change in seismic velocities that may be associated with an abrupt change in the regional geology (granitic rock in contact with sediments), we conducted a refraction seismic study in the Peninsular Ranges of Baja California, which is in the Mexico-southwestern Laguna Salada (LS) region. We installed 30 three-component portable seismic stations, supplemented with two permanent six-component stations of the Northwest Mexico Seismic Network (RESNOM). The stations, spaced similar to 6 kmalong a refraction profile, recorded two blasts; these were the direct shot located to the south of the city of Ensenada and the reverse shot in the southwestern LS (southwest-northeast direction). Record sections show seismograms with impulsive P arrivals at nearby stations. Rays from the two blasts were modeled (using asymptotic ray theory) to obtain a P-wave velocity model from 0 to similar to 15 km depth along the refraction profile. Our modeling results are as follows: in the southwestern part of the profile (0-25 km distance), a low-velocity zone of similar to 2 km/s exists between the depths of 0 and 3.5 km; in Sierra Juarez, the mean P-wave velocity is similar to 5.6 km/s between the depths of 0 and 5 km; and in southwestern LS, a lowvelocity layer of similar to 2.5 km/s exists between the depths of 0 and similar to 3 km. We also modeled a layer of similar to 6.5 km/s between 4 and 12 km in the Ensenada-Ojos Negros region, and between the depths of 4 and 8 km below the southwestern LS. From a profile distance of 0 to 50 km, a velocity zone of similar to 6.7 km/s appears between the depths of 12 and 15 km.