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).

Unfortunately, funding from the USGS was significantly reduced during 1999-2001 and the electromagnetic monitoring array fell into disrepair. However, on the plus side, new electric field sensors were installed in the 3 new HRSN boreholes and we hope to bring these data on-line in the coming year.

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 data loggers, shared with the collocated BDSN stations, synchronized in time by GPS and sent to the BSL via dedicated communication links.

Figure 5.1: Map illustrating the location of operational (filled squares) and closed (grey squares) MT sites.

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, sampling mode, and FIR filter type. C indicates continuous; T triggered; Ac acausal.

Sensor	Channel	Rate (sps)	Mode	FIR
Magnetic	VT?	0.1	C	Ac
Magnetic	LT?	1.0	C	Ac
Magnetic	BT?	40.0	C	Ac
Electric	VQ?	0.1	C	Ac
Electric	LQ?	1.0	C	Ac
Electric	BQ?	40.0	C	Ac

Station Maintenance

Unfortunately, the reduced funding for the electromagnetic monitoring array has had a major impact on the status of the equipment. Table 5.3 summarizes the state-of-health of the EM equipment in 2000-2001. Periods with "X" indicate equipment failure or bad data.

Table 5.3: Status of EM data channels at PKD and SAO in 2000-2001. The electric-field data have been effective offline (primarily due to the sensors drying out), while the magnetic field equipment have been somewhat more robust. X indicates equipment failure or bad data; OK indicates satisfactory equipment performance. LQ2, LQ3 = 100 m electric field dipole in west and south directions.

EM Data Status
Date	SAO		PKD
	E-field	B-field	E-field	B-field
2000
001-008	X	OK	X	OK
009-159	X	X	X	OK
160-162	X	X	X	X
163-219	X	OK	X	X
220-229	X	OK	X	OK
230	X	X	X	OK
231-339	X	OK	X	OK
340-366	X	X	X	OK
2001
001-007	X	X	X	OK
008-057	X	OK	X	OK
058-066	X	X	X	OK
067-118	X	X	LQ1-X	OK
119-194	X	X	LQ3-OK	OK
195-present	X	X	N/A	N/A

In the past few months, the BSL and Professor Morrison have agreed to expand support for the electromagnetic monitoring array. A part-time staff person has been hired to assist with returning the existing equipment to operating conditions by servicing the electrodes and induction coils as well participate in routine data processing.

Vertical Electric Field Measurements

Recent activities have been directed towards measuring the vertical electric field. This type of measurement is not common in magnetotelluric studies simply because the electric fields are very small at the surface of the earth. However, borehole electric dipole studies have been undertaken by researchers investigating electromagnetic precursors to earthquakes in the past. This proposed research is in conjunction with the study of clustering and periodic recurrence of microearthquakes currently investigated by Nadeau and McEvilly, UC Berkeley Seismological Laboratory.

Silver-chloride electrodes were fabricated and installed in three boreholes to measure the vertical electric fields in the Parkfield area. The first pair of electrodes were installed October 30, 2000 at depths of 316 ft and 712 ft for a dipole length of 396 ft (120.7 m). Electrodes for the second borehole were installed May 21, 2001 at depths of 118 ft and 732 ft for a dipole length of 614 ft (187.2 m). Electrodes for the third borehole were installed May 29, 2001 at depths of 233 ft and 729 ft for a dipole length of 496 ft (151.2 m).

Future activities include bringing the data on-line and correlating to see if there is any association with clustering and periodic recurrence of microearthquakes.

Acknowledgements

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. Sierra Boyd and 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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