Data at the MT sites are fed to Quanterra dataloggers, shared with the collocated BDSN stations, synchronized in time by GPS and sent to the BSL via telephone lines. During the overlap period when all three sites were operational, the seven additional channels of data from the new site enabled us to measure a decrease in the residual noise power spectrum at PKD1 as seen in Figure 5.2. The residual power is basically the threshold for detecting anomalous signals that might be generated by a local source. As intended the array is providing sensitivity to such anomalous fields at roughly 30 times the sensitivity of field detection at a single station. The black curve at the top is the total power spectrum of the west component of the horizontal magnetic field, Hx, at the temporary site in Parkfield, PKD1. The other curves are the residual power spectra derived from pairs of channels, either electric or magnetic, from all three sites. The minimum residual power spectrum shown in green is the difference between Hx at PKD1 and the predicted Hx using the horizontal magnetic fields from PKD, 12 km away. This demonstrates the advantage of having at least two sites in close proximity of one another to increase the sensitivity of detecting anomalous signals.
The EM sensors are also sensitive to ground motion. As the surface seismic waves propagate, the energy displaces both the electrode dipole cables and the induction coils, creating signals. For example, when the electrode line is moved by wind or ground motion, a signal is generated by the motion of the wire in a magnetic field. Similarly, when the coil tilts as a result of ground motion, the magnetic flux through the coil varies creating an induced signal.
Ten seconds of raw EM and seismic time series from Hollister and Parkfield are presented in Figure 5.3. There is a correlated signal associated with the mainshock and subsequent ground shaking at Hollister seen in the Hollister EM and seismic time series, left plot Figure 5.3. The upper five traces are EM data starting with the horizontal electric fields followed by three components of the magnetic field. The fifth trace is the vertical component of the magnetic field which suffers the largest excursion most likely as a result of the sensor tilting in the hole. Below the EM data are low and high sampled seismic data. On the right plot are EM data from PKD1 shown in the top five traces and seismic data from both PKD1 and PKD, shown below. There is an anomalous spike on the magnetic field channels at Parkfield over 100 miles away from the earthquake as shown on the right side of Figure 5.3, in particular the vertical component, fifth trace. Could this be an electromagnetic signal generated coseismically at Hollister and propagated in the air to PKD1? The mechanism for such an EM field is presently unknown. This is followed by a delayed chirp-like pulse on the three accelerometers located 100 m away from the PKD1 coils shown on the same plot of Figure 5.3, traces 6-8. One explanation for these signals could be local ground motion at PKD1 since there is no corresponding signal on seismic traces 9 and 10 from PKD which is 12 km away. Approximately 20 seconds after the mainshock at Hollister, the seismometers and accelerometers at PKD recorded ground motion, from the Hollister event, followed by the EM sensors and accelerometers at PKD1 (data not shown here).
The analysis of intersite transfer functions revealed the influence of the Bay Area Rapid Transit (BART) system as far away as Parkfield and also indicated that BART may actually influence the generation of fields in the magnetosphere (Egbert et al., 2000). The extraction of such information from the array data is another indication of the high sensitivity to anomalous EM fields.
This work was done in collaboration with G.D. Egbert, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR.
Under Frank Morrison's general supervision, Sierra Boyd, Bill Karavas, John Friday, Dave Rapkin, and Doug Neuhauser contribute to the operation of the MT observatories. Lind Gee contributed to the preparation of this chapter.
Egbert, G.D., M. Eisel, O.S. Boyd and H.F. Morrison, DC trains and Pc3s: Source effects in mid-latitude geomagnetic transfer functions, Geophys. Res. Lett., 27, 25-28, 2000.
Egbert, G.D., Robust Multiple-Station Magnetotelluric Data Processing, Geoph. J. Int., 130, 475-496, 1997.
Fraser-Smith, A.C., A. Bernardi, P.R. McGill, M.E. Ladd, R.A. Helliwell and O.G. Villard, Jr., Low Frequency Magnetic Field Measurements near the Epicenter of the Ms 7.1 Loma Prieta Earthquake, Geophys. Res. Lett., 17, 1465-1468, 1990.