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

Abstract

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 ∼6 km along 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 ∼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 ∼2 km/s exists between the depths of 0 and 3.5 km; in Sierra Juarez, the mean P‐wave velocity is ∼5.6 km/s between the depths of 0 and 5 km; and in southwestern LS, a low‐velocity layer of ∼2.5 km/s exists between the depths of 0 and ∼3 km. We also modeled a layer of ∼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 ∼6.7 km/s appears between the depths of 12 and 15 km.