Several expeditions to the San Diego Trough (SDT) offshore San Diego resulted in the acquisition of ten marine magnetotelluric (MMT) seafloor sites in a profile across the SOT. The MMT method measures the passive electric and magnetic signals from the Earth's interaction with the Sun, and results in calculations of the subsurface resistivity at the location of the seafloor sites. The SDT is an 1100 m deep basin located 30 miles west of the city of San Diego and flanked by the Thirtymile Bank to the west and the Coronado Bank to the east. These bathymetric features are part of the California continental borderland, the broad tectonic plate boundary between the Pacific and North American plates. Previous geophysical and geological studies conducted in the region have resulted in the mapping and interpretation of the shallow (top 2-3 km below the seafloor) geologic structure, but no studies have been able to image the deeper structure of the SDT area. The data from this MMT investigation suggest a geoelectric structure that agrees with the seismic data in the top 2-3 km but introduces new evidence for deeper structure. Several faults are identified in the San Diego Trough and evidence suggests that the Coronado Bank Fault Zone is active, perhaps similar to the "creeping" segment of the San Andreas Fault in Parkfield, Califomia.
