Long‐term monitoring of ULF electromagnetic fields at Parkfield, California

[1] Electric and magnetic fields in the (10−4–1.0) Hz band were monitored at two sites adjacent to the San Andreas Fault near Parkfield and Hollister, California, from 1995 to 2007. A data window (2002–2005), enclosing the 28 September 2004 M6 Parkfield earthquake, was analyzed to determine if anomalous electric or magnetic fields or changes in ground conductivity occurred before the earthquake. The data were edited, removing intervals of instrument malfunction, leaving 875 days in the 4 year period. Frequent, spikelike disturbances were common but were not more frequent around the time of the earthquake; these were removed before subsequent processing. Signal-to-noise amplitude spectra, estimated via magnetotelluric processing, showed the behavior of the ultralow frequency fields to be remarkably constant over the period of analysis. These first-order plots make clear that most of the recorded energy is coherent over the spatial extent of the array. Three main statistical techniques were employed to separate local anomalous electrical or magnetic fields from the dominant coherent natural fields: transfer function estimates between components at each site were employed to subtract the dominant field, and look deeper at the “residual” fields; the data were decomposed into principal components to identify the dominant coherent array modes; and the technique of canonical coherences was employed to distinguish anomalous fields which are spatially broad from anomalies which occur at a single site only, and furthermore to distinguish anomalies present in both the electric and magnetic fields from those present in only one field type. Standard remote reference apparent resistivity estimates were generated daily at Parkfield. A significant seasonal component of variability was observed, suggesting local distortion due to variations in near-surface resistance. In all cases, high levels of sensitivity to subtle electromagnetic effects were demonstrated, but no effects were found that can be reasonably characterized as precursors to the Parkfield earthquake.