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Parkfield-Hollister Electromagnetic Monitoring Array


Introduction

In 1995 UC Berkeley added magnetotelluric (MT) observatories to two BDSN sites located along the San Andreas Fault to monitor possible changes in the electromagnetic (EM) fields associated with earthquakes (Fraser-Smith et al., 1990). Since then MT data have been continuously recorded at 40 Hz and 1 Hz and archived at the NCEDC (Table 5.1 and 5.2).

Magnetotelluric array

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 (Figure 5.1. In 1995, initial sites were established at PKD1 and SAO, separated by a distance of 150 km, and equipped with three induction coils and two 100 m electric dipoles. PKD1 was established as a temporary seismic site, and when a permanent site (PKD) was found in 1996, a third MT observatory was installed with three induction coils, two 100 m electric dipoles, and two 200 m electric dipoles. PKD and PKD1 ran in parallel for one month in 1999, and then the MT observatory at PKD1 was closed.

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.


  
Figure 5.1: Map illustrating the location of operational (filled squares) and closed (grey squares) MT sites.
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Figure 5.2: Hx total and residual power spectra at PKD1 averaged over 8 days.
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Table 5.1: Sites of MT observatories
Site Net Latitude Longitude Elev (m) Date Location  
PKD BK 35.945171 -120.541603 583 1999/02/05 - Bear Valley Ranch, Parkfield  
PKD1 BK 35.8894 -120.426109 431.6 1995/06/06 - 1999/03/08 Haliburton House, Parkfield  
SAO BK 36.76403 -121.44722 317.2 1995/08/15 - San Andreas Obs., Hollister  
 


 
Table 5.2: Typical data streams acquired at each MT site, with channel name, sampling rate, and sampling mode. C indicates continuous; T triggered.
Sensor Channel Rate (sps) Mode
Magnetic VT? 0.1 C
Magnetic LT? 1.0 C
Magnetic BT? 40.0 C
Electric VQ? 0.1 C
Electric LQ? 1.0 C
Electric BQ? 40.0 C
 

Visual inspection in the time domain

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 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).


  
Figure 5.3: Ten seconds of raw EM and seismic time series: Hollister data on the left and Parkfield data on the right. Vertical scale is counts and horizontal scale is time. Top five traces are EM data: 2 electric field channels, 2 horizontal magnetic field channels and the vertical magnetic field. The lower five traces are seismic data. Traces 6-8 are 1 Hz sampled seismic data; traces 9 and 10 are 80 Hz sampled seismic data. From Parkfield the 1 Hz seismic data comes from both PKD1, traces 6-8, and PKD, traces 9 and 10. Note, the first EM channel from Parkfield, Ex, is down.
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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.

Acknowledgements

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.

References

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.


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Next: Bay Area Regional Deformation Up: Operations Previous: Parkfield Borehole Network

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