Long-Period Magnetotelluric Measurements Near the Central California Coast: A Land-Locked View of the Conductivity Structure Under the Pacific Ocean

SUMMARY Telluric data from Hollister, California were combined with magnetic data from the Fresno, California magnetic observatory to determine the MT response at long periods. The shifted eigenstate analysis of LaTorraca et al. (1986) was used to determine the maximum apparent resistivities and corresponding EIH phases for periods from 1 to 30 h. The apparent resistivity and phase at Hollister are greatly influenced by the ocean, with the mantle branch beginning at a period of about 12 h. In typical continental areas, the mantle branch usually begins at periods of 10s or less. A maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters. The data required a resistivity-thickness product of approximately 1 x lo9 Q-m2 (1 x lo6 Q-m-km) for the oceanic lower crust and upper mantle. The oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth, and a rapid conductivity increase below 640km. The sensitivity analysis shows the data are most sensitive to the oceanic parameters, except for the electric field amplitudes which are also sensitive to the resistivities under the observation site.