The electrical conductivity of the lithosphere and asthenosphere beneath the coastline of Southern California

Abstract

From November 1992 to March 1993, a ring-core fluxgate magnetometer was deployed on the ocean floor in 3850 m of sea-water, approximately 200 km from the coastline of southern California. Simultaneous magnetic field data were collected at a site next to the coastline, and data were also made available by the United States Geological Survey from the Tucson and Fresno Magnetic Observatories. Two of the main aims of the experiment were to measure the geomagnetic coast effect from the Californian margin and to probe the conductivity structure of the lithosphere and asthenosphere beneath. Geomagnetic depth sounding and vertical gradient sounding estimates show a strong two-dimensional geomagnetic coast effect. Finite-element modelling suggests that the continental lithosphere has a resistivity-thickness product of the order of 3 ´ 107 Wm2, which is two orders of magnitude less than the measured resistivity-thickness product of the oceanic lithosphere off California. Below the lithosphere, the modelled electrical conductivity rises at a depth of 60 km, which may mark the boundary with the asthenosphere and the seismic low-velocity zone.