Long Term Electromagnetic Monitoring at Parkfield CA

Electric and magnetic fields in the ULF band were monitored at two sites adjacent to the San Andreas Fault near Parkfield and Hollister, California. A data window enclosing the September 28, 2004 M6 Parkfield earthquake, was analyzed to determine if anomalous electric or magnetic fields, or changes in ground conductivity, occurred before the earthquake. Three main statistical techniques were employed to separate local anomalous electrical or magnetic fields from the dominant coherent natural fields: transfer function and residual field estimates between components at each site; principal component analysis; and canonical coherence analysis were employed to distinguish anomalous fields which are spatially broad from those which occur at a single site only, and to distinguish signals present in both the electric and magnetic fields from those which are present in only one field type. Standard remote reference apparent resistivity estimates were generated daily at Parkfield. In all cases, high levels of sensitivity to subtle electromagnetic effects were demonstrated, but no effects which can be reasonably characterized as precursors to the Parkfield earthquake were found.