Long Term Electromagnetic Monitoring at Parkfield CA

Electric and magnetic fields in the ULF bandwere monitored at two sites adjacent to the SanAndreas Fault near Parkfield andHollister California. A data window enclosing theSeptember 28 2004 M6 Parkfield earthquake wasanalyzed to determine if anomalous electric ormagnetic fields or changes in ground conductivityoccurred before the earthquake. Threemain statistical techniques were employed to separatelocal anomalous electrical or magnetic fields fromthe dominant coherent natural fields: transferfunction and residual field estimates betweencomponents at each site; principal componentanalysis; and canonical coherence analysis wereemployed to distinguish anomalous fields which arespatially broad from those which occur at asingle site only and to distinguishsignals present in both the electric andmagnetic fields from those which are present in onlyone field type. Standard remote reference apparentresistivity estimates were generated daily atParkfield. In all cases high levels of sensitivityto subtle electromagnetic effects were demonstratedbut no effects which can be reasonably characterizedas precursors to the Parkfield earthquake were found.