Complementing the deformation measurements at Parkfield are measurements of the electrical and magnetic properties of the crust. As the crust deforms, these rock properties may change in response to changes in stress or changes in the distribution (and composition) of fluids in the crust. At Parkfield, sensors measure changes in the electrical, magnetic, and electromagnetic properties of the crust at a variety of frequencies. These data are analyzed in conjunction with the local earthquake activity and crustal deformation data.
Magnetometers
The local magnetic field is monitored with absolute total field magnetometers at 7 sites in the Parkfield region. The data are synchronized to within 1.0 second and are transmitted with 16-bit digital telemetry satellite to Menlo Park, California. Measurement precision at periods of 10 minutes and tens of days are about 0.2 and 0.7 nanotesla, respectively. Changes of 1.0 nanotesla, which correspond to stress changes of several bars according to current models, can be detected with the present instrumentation at periods greater than a day.
Magnetotelluric Observatories
In the Fall of 1995, the University of California, Berkeley installed magnetotelluric (MT) observatories at two locations along the San Andreas Fault to monitor possible changes in the electromagnetic field before, during and after earthquakes (Fraser-Smith, 1995). Since then, MT data have been recorded continuously and archived at the UC Berkeley Digital Seismic Network (BDSN). This data set provides an opportunity to investigate long-term variations of MT parameters at short periods. The MT observatories are located at Parkfield (PKD1, 300 km south of the San Francisco Bay area) and Hollister (SAO, halfway between San Francisco and Parkfield). Both sites are equipped with 3 induction coils and 2 x 100 m electric dipoles connected to a Quanterra digitizer. Time synchronization is provided by GPS. The data are sent to BDSN via telephone lines.
Electrical Resistivity
Changes in electrical earth currents (telluric currents) and in earth resistivity associated with concentrations in stress before earthquakes. Chinese researchers and other have reported substantial changes in resistivity before large earthquakes. However, the mechanism(s) responsible for these changes are not understood. The University of California, Riverside has installed a telluric current monitoring array in the Parkfield region in order to investigate variations in resistivity as a potential means for predicting earthquakes. The resistivity network has been upgraded.