"anomalous Em Signals and Changes in Resistivity at Parkfield: Collaborative Research between the Universities of California at Berkeley and Riverside and Oregon

Fluctuations of resistivity have been monitored with an array measuring natural electric currents (telluric currents) in Parkfield. This array is designed to detect short-term relative changes of resistivity of 1% over days to weeks and long-term variations of 0.1% over months to years. Fractional daily variations of the telluric coefficients are computed and then compared to the earthquake record from Parkfield in order to determine if significant changes occurred prior to or at the time of local earthquakes. Changes in the telluric coefficients are related to changes in resistivity, albeit in a complicated manner because the earth is heterogeneous. While there were two earthquakes with ML > 3.5 within the area monitored by the array during 2001-2002, neither of these appears to have caused resistivity to fluctuate above our minimum sensitivity levels of ~1%. This is consistent with prior observations that earthquakes up to ML=5.0 do not produce significant fluctuations of telluric coefficients. Little progress has been made in adapting robust MT processing techniques to produce more stable coefficient estimates, but this work is not completed. So far, the existing analysis techniques are as good as the robust MT methods. Finally, work has begun on the construction of an updated 3-D resistivity model for Parkfield.