Parkfield Borehole Network

Introduction

As part of the U.S. Geological Survey initiative known as the Parkfield Prediction Experiment (Bakun and Lindh, 1985), the operation of the High Resolution Seismic Network (HRSN) at Parkfield, California, and the collection and analysis of its recordings began in 1987. Figure 4.1 shows the location of the network and its relationship to the San Andreas fault. The HRSN records exceptionally high-quality data, owing to its 10 closely spaced three-component borehole sensors, its very broad band recordings (0-125 Hz), and its sensitivity (recording events below magnitude -0.5) due to the extremely low attenuation and background noise levels at the 200-300 m sensor depths (Karageorgi et al., 1992). Several aspects of the Parkfield region make it ideal for the study of small earthquakes and their relation to tectonic processes. These include the fact that the network spans the expected nucleation region of a repeating magnitude 6 event and the transition from locked to creeping behavior on the San Andreas fault, the availability of three-dimensional P and S velocity models, a very complete seismicity catalogue, a well-defined and simple fault segment, and a homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90$\%$ right-lateral strike-slip).

In a series of journal articles and Ph. D. theses, we have presented the cumulative, often unexpected, results of this effort. They trace the evolution of a new and exciting picture of the San Andreas fault zone responding to its plate-boundary loading, and they are forcing new thinking on the dynamic processes and conditions within the fault zone at the sites of recurring small earthquakes. Recent results are described in Chapter III.

  

Figure 4.1: Map showing the location of the Parkfield HRSN stations (filled diamonds) and the 3 planned sites (open diamonds), along with the BDSN stations PKD and PKD1 (filled squares). The location of the proposed SAFOD drill site is shown by a star.

 

Table 4.1: Stations of the Parkfield HRSN. Each HRSN station is listed with its station code, network id, location, date of initial operation, and site description. The latitude and longitude (in degrees) are given in the WGS84 reference frame, the elevation (in meters) is relative to mean sea level, and the depth to the sensor (in meters) below the surface. Coordinates and station names for the 3 planned sites are approximate.

Site	Net	Latitude	Longitude	Elev (m)	Depth (m)	Date	Location
EAD	BP	35.89525	-120.42286	499	254	01/1988 -	Eade Ranch
FRO	BP	35.91078	-120.48722	542	258	01/1988 -	Froelich Ranch
GHI	BP	35.83236	-120.34774	433	370	01/1988 -	Gold Hill
JCN	BP	35.93911	-120.43083	559	335	01/1988 -	Joaquin Canyon North
JCS	BP	35.92120	-120.43408	487	332	01/1988 -	Joaquin Canyon South
MMN	BP	35.95654	-120.49586	731	510	01/1988 -	Middle Mountain
RMN	BP	36.00086	-120.47772	1198	1125	01/1988 -	Gastro Peak
SMN	BP	35.97292	-120.58009	732	450	01/1988 -	Stockdale Mountain
VAR	BP	35.92614	-120.44707	511	-61	01/1988 -	Varian Well
VCA	BP	35.92177	-120.53424	790	590	01/1988 -	Vineyard Canyon
CCR	BP	35.957	-120.544			Planned	Cholame Creek
LCC	BP	35.977	-120.507			Planned	Little Cholame Creek
SCY	BP	36.016	-120.538			Planned	Stone Canyon

 

 

Table 4.2: Instrumentation of the Parkfield HRSN. Most HRSN sites have L22 sensors and were originally digitized with a RefTek 24 system. After the failure of the WESCOMP recording system in Dec 1998, PASSCAL RefTek recorders were installed. In July of 1999, 6 of the PASSCAL systems were returned to IRIS and 4 were left at critical sites. The new Quanterra-based systems will be installed in the fall of 2000.

Site	Sensor	Z	H1	H2	RefTek 24	RefTek 72-06
EAD	Mark Products L22	-90	170	260	01/1998 - 12/1998	12/1998 - 07/1999
FRO	Mark Products L22	-90	338	248	01/1998 - 12/1998	12/1998 - 07/1999
GHI	Mark Products L22	90	failed	unk	01/1998 - 12/1998	12/1998 - 07/1999
JCN	Mark Products L22	-90	0	270	01/1998 - 12/1998	12/1998 -
JCS	Geospace HS1	90	300	210	01/1998 - 12/1998	12/1998 - 07/1999
MMN	Mark Products L22	-90	175	265	01/1998 - 12/1998	12/1998 -
RMN	Mark Products L22	-90	310	40	01/1998 - 12/1998	12/1998 - 07/1999
SMN	Mark Products L22	-90	120	210	01/1998 - 12/1998	12/1998 -
VAR	Litton 1023	90	15	285	01/1998 - 12/1998	12/1998 - 07/1999
VCA	Mark Products L22	-90	200	290	01/1998 - 12/1998	12/1998 -

Network Background

The HRSN was installed in boreholes beginning in 1986. In November 1987, the Varian well vertical array was installed and the first VSP survey was conducted, revealing clear S-wave anisotropy in the fault zone. During 1988, the network was completed to ten 3-component 500 sps radio-telemetered stations into a central detection/recording system, incorporating a deep (572 m) sensor in the Varian well string into the network. The Varian system was slaved in 1988, for about two years, to the Vibroseis control signals, allowing simultaneous recording of vibrator signals on both systems. In 1991, low-gain event recorders (from PASSCAL) were installed to extend the dynamic range to M_L about 4.5. The data acquisition system operated quite reliably until late 1996, when periods of unacceptably high downtime developed, with as many as 7 of the remote, solar-powered telemetered stations down due to marginal solar generation capacity and old batteries, and recording system outages of a week or more became common. In 1998 it failed permanently. The original acquisition system that failed was a modified VSP recorder acquired from LBNL, based on a 1980- vintage LSI-11 cpu and a 5 MByte removable Bernoulli system disk with a 9-track tape drive, configured to record both triggered microearthquake and Vibroseis (discontinued in 1997) data. The system was remote and completely autonomous - tapes were mailed to Berkeley. The old system had one-sample timing uncertainty, and record length limitation because the tape write after event detection was longer than the length of the record, and we were off-line for the write time.

 

Table 4.3: Typical data streams acquired at each HRSN site, with channel name, sampling rate, and sampling mode. C indicates continuous; T triggered.

Sensor	Channel	Rate (sps)	Mode
Geophone	DP	500.0	T

 

Station Maintenance

In fall 1998, the original HRSN acquisition system was replaced by 10 PASSCAL RefTek systems with continuous recording. This required the development of a major data handling procedure, in order to capture microearthquakes as small as M = -1.0 are not seen on surface stations, since continuous telemetry to the BSL was not an option at that time.

In July, 1999 we had to reduce the network to four RefTeks at critical sites that would ensure continuity in the archive of characteristic events and temporal variations in recurrence.

Network Upgrade

Thanks to emergency funding from the USGS NEHRP, we are replacing the original 10-station system with a modern 24-bit acquisition system (Quanterra 730 digitizers, spread-spectrum telemetry, Sun Ultra 10/440 central processor at the in-field collection point, with frame relay, ultimately T1, connectivity to Berkeley). The new system should be online late in 2000, assuming we can find a suitable new central site given the USGS lease problems at the old Haliburton facility.

The upgraded system will be compatible with the data flow and archiving common to all the elements of the BDSN/HFN and the NCEDC. This will provide remote access and control of the system and produce data with better timing accuracy and longer records flowing seamlessly into NCEDC. The new system solves the problems of timing resolution, dynamic range, and missed detections, in addition to the advantage in the conventional data flow (the old system recorded SEG-Y format).

SAFOD Expansion

We are adding three new borehole stations at the NW end of the network as part of the deep fault-zone drilling (San Andreas Fault Observatory at Depth - SAFOD) project, with NSF support, to improve resolution of at the planned drilling target on the fault. Figure 4.1 illustrates the location of the proposed drill site (star) and the new borehole sites. These three new stations will use similar hardware to the main network, with the possible addition of extra channels for electrical, pore-pressure and strain signals. In addition, the remoteness of the drillsite and new stations will require a telemetry collection point and wideband transmission from the RMN site. We are trying to combine this link with the data flowing from the temporary 15-station surface array being deployed around the drillsite. The new borehole installations are also planned for the fall of 2000.

More information about SAFOD project is available on the Web at http://www.earthscope.org/safod.com.html.

Acknowledgements

Under Tom McEvilly's general supervision, Rich Clymer, Bob Nadeau, Wade Johnson, and Doug Neuhauser contribute to the operation of the HRSN. Tom McEvilly and Lind Gee contributed to the preparation of this chapter.

References

Bakun, W. H., and A. G. Lindh, The Parkfield, California, prediction experiment, Earthq. Predict. Res., 3, 285-304, 1985.

Karageorgi, E., R. Clymer and T.V. McEvilly, Seismological studies at Parkfield. II. Search for temporal variations in wave propagation using Vibroseis, Bull. Seism. Soc. Am., 82, 1388-1415, 1992.

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