WHAT ELECTRICAL MEASUREMENTS CAN SAY ABOUT CHANGES IN FAULT SYSTEMS

Earthquake zones in the upper crust are usually more conductive than t he surrounding rocks, and electrical geophysical measurements can be u sed to map these zones. Magnetotelluric (MT) measurements across fault zones that are parallel to the coast and not too far away can also gi ve some important information about the lower crustal zone. This is be cause the long-period electric currents coming from the ocean graduall y Leak into the mantle, but the lower crust is usually very resistive and very Little leakage takes place. If a lower crustal zone is less r esistive it will be a leakage zone, and this can be seen because the M T phase will change as the ocean currents leave the upper crust. The S an Andreas Fault is parallel to the ocean boundary and close enough to have a lot of extra ocean currents crossing the zone, The Loma Prieta zone, after the earthquake, showed a lot of ocean electric current le akage, suggesting that the lower crust under the fault zone was much m ore conductive than normal. It is hard to believe that water, which is responsible for the conductivity, had time to get into the lower crus tal zone, so it was probably always there, but not well connected. If this is true, then the poorly connected water would be at a pressure c lose to the rock pressure, and it may play a role in modifying the flu id pressure in the upper crust fault zone. We also have telluric measu rements across the San Andreas Fault near Palmdale from 1979 to 1990, and beginning in 1985 we saw changes in the telluric signals on the fa ult zone and east of the fault zone compared with the signals west of the fault zone. These measurements were probably seeing a better conne ction of the lower crust fluids taking place, and this may result in a fluid flow from the lower crust to the upper crust. This could be a f actor in changing the strength of the upper crust fault zone.