Anomalous EM Signals and Changes in Resistivity at Parkfield: Collaborative Research Between the Universities of California at Berkeley and Riverside and Oregon State University. Final Report, January, 2005-January 2006

摘要

An 18 year long experiment to capture changes in electrical resistivity prior to, during, and after a M6 earthquake on the San Andreas fault at Parkfield, California culminated with an event of that magnitude on 28 September 2004. Resistivity was monitored with a telluric array, giving precisions in measured parameters of 0.2% or less. No coseismic resistivity changes greater than 5% could have occurred or they would have been detected by the array, and any precursory changes would have been much smaller because maximum possible precursory strains were 4000 times smaller than the coseismic strain. Given that the earthquake ruptured directly beneath the telluric array, this experiment places a bound on the amount resistivity might change at the hypocentral location; changes measured with instruments farther from earthquakes can only be smaller. We cannot exclude the possibility that a measurable change with a larger earthquake is possible or that other fault geometries are capable of generating larger signals, but our results show clearly that future experiments should be designed with precisions of much better than 0.1% if they are to be successful.