High-resolution seismic imaging of the plate boundary in northern Baja California and southern California using double-pair double-difference tomography

Abstract

We image seismic properties of the plate boundary through northern Baja California and southernmost California using a double-difference tomography and earthquake relocation algorithm that incorporates both event-pair and station-pair data. The data are derived from seismic networks in southern California (SCSN) and northern Baja California (CICESE). It comprises >700,000 travel times (P and S) associated with ∼200 seismic stations and >21,000 local earthquakes from 2003-2018, along with >2,000,000 event-pair and >1,500,000 station-pair double-differences derived from these travel times. In the upper crust, low VP values in the resultant models imply deep sediments and sedimentary rocks northwest of the Salton Sea and around the northern end of the Imperial fault. Low VS values indicate regions of rock damage at the northern ends of the Sierra Juarez fault zone and Laguna Salada fault, and the southern end of the San Jacinto fault zone (SJFZ). Shallow seismicity (d<956 km) highlights the Salton Buttes and Cerro Prieto volcanoes as high heat-flow thin crust regions. At mid to lower crustal depths, the sharpest contrast in crustal VP and most prominent low VS anomaly (V∼S3.4 km/s, VP/VS ∼ 1.9) occurs along a corridor extending northwestward from the Cerro Prieto fault (CPF), east of and subparallel to the Sierra El Mayor/Cucapah and into the southern SJFZ, indicating a continuous weak fluid-rich zone. At ∼25 km depth, we observe uppermost mantle velocities (V∼P7.8 km/s) in the eastern Salton Trough, crustal velocities (V<P7 km/s) around the Sierra El Mayor/Cucapah and Peninsular Ranges, and near mantle velocities (7.4 < VP < 7.8 km/s) beneath Laguna Salada. These results illuminate a thin crust in the Salton Trough, except for the Sierra El Mayor/Cucapah, and a thicker crust in the Peninsular Ranges. In all, high seismic activity, rapid changes in crustal structure and the prevalence of damage/deformation and fluids in a corridor extending from the CPF to the southern SJFZ emphasize this zone as a key component of the active plate boundary.