In 1995 UC Berkeley installed magnetotelluric (MT) observatories at two locations along the San Andreas Fault to monitor possible changes in the electromagnetic (EM) fields associated with earthquakes (Fraser-Smith, 1994). Since then MT data have been continuously recorded at 40 Hz and 1 Hz and archived at the NCEDC. During this past year EM data during the largest and closest earthquake to date were recorded. On August 12, 1998 the MT array consisting of two sites, Parkfield (PKD1) and Hollister (SAO), measured the electric and magnetic fields during a Mw 5.1 earthquake in San Juan Bautista. These data are of particular interest due to the earthquake's large magnitude and close proximity to the Hollister site, SAO. EM and seismic time series will be presented below.
This last year has also seen the relocation of the temporary Parkfield site from Haliburton Ranch, PKD1, to a permanent site at Bear Valley Ranch, PKD. In early February 1999, a permanent MT observatory was installed 12 km from the temporary Parkfield site, PKD1. For just under a month, the array of three MT observatories simultaneously recorded data until the temporary site was removed.
The MT observatories are located at Parkfield (PKD1, PKD) 300 km south of the San Francisco Bay Area and Hollister (SAO), halfway between San Francisco and Parkfield. Sites PKD1 and SAO, separated by a distance of 150 km, are each equipped with three induction coils and two 100 m electric dipoles. The permanent Parkfield site (PKD) is similarly equipped with an additional pair of 200 m electric dipoles for noise cancellation purposes. All data are fed to Quanterra dataloggers, shared with collocated BDSN stations, synchronized in time by GPS and sent to NCEDC 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 6.1. 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.
A Mw 5.1 earthquake occurred at 14:10:25.11 UTC (36.7533 -121.4618) August 12 1998 on the San Andreas Fault 12 km SSE of San Juan Bautista at a depth of 9.2 km. There were several foreshocks including a ML 3.14 event at 14:02:47.21 UTC (36.7517 -121.4557, depth 8.6 km), 7.6 minutes before and a ML 2.80 event at 12:54:04.59 UTC (36.7525 -121.4580, depth 8.11 km) 1.3 hours before the mainshock (Uhrhammer et al., 1999). During these earthquakes all five EM sensors were functioning at Hollister. However, only three of five data channels from Parkfield were useable as a result of equipment failure.
A visual inspection of the 40 Hz Hollister EM time series reveals some interesting features due to both cultural and natural sources. One such feature includes a 3.5 to 8 Hz sinusoidal signal on the horizontal electric field channels starting the third week in July 1998 and ending sometime during the last week in August 1998. The signal occurs approximately every 20 minutes and is about 4 minutes in duration, suggesting a man-made source. The sinusoidal signal is preceded by a spike of 40 mV and appears stronger on the north dipole. The frequency of the signal gradually increases and then abruptly ends. There is no corresponding signal on the magnetic field data.
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 6.2. 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 6.2. 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 6.2, 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 6.2, 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., 1999). 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.
This work was funded by USGS grant number 1434-HQ-97-GR-03076.
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