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Parkfield Borehole Network

T. V. McEvilly and R. W. Clymer


Introduction

As part of the U.S. Geological Survey initiative known as the Parkfield Prediction Experiment, 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 seismicity along 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).


  
Figure 4.1: Parkfield HRSN Network and Seismicity.
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Table 4.1: Stations of the Parkfield HRSN, with channel name, sampling rate, location (referenced to NAD84), surface elevation (in meters above mean sea level), sensor depth below the surface, and the dip and azimuth of the sensors.
Stat Chan Rate (sps) Latitude Longitude elev (m) depth (m) Dip Az
EAD DPZ 500 35.89522 -120.42262 503 245 90 0
EAD DP1 500 35.89522 -120.42262 503 245 0 170
EAD DP2 500 35.89522 -120.42262 503 245 0 260
FRO DPZ 500 35.91095 -120.48688 549 284 90 0
FRO DP1 500 35.91095 -120.48688 549 284 0 338
FRO DP2 500 35.91095 -120.48688 549 284 0 248
GHI DPZ 500 35.83225 -120.34728 427 63 -90 0
GHI DP1 500 35.83225 -120.34728 427 63 0 ???
GHI DP2 500 35.83225 -120.34728 427 63 0 ???
JCN DPZ 500 35.93897 -120.43112 567 224 90 0
JCN DP1 500 35.93897 -120.43112 567 224 0 0
JCN DP2 500 35.93897 -120.43112 567 224 0 270
JCZ DPZ 500 35.92117 -120.43400 488 155 -90 0
JCZ DP1 500 35.92117 -120.43400 488 155 0 300
JCZ DP2 500 35.92117 -120.43400 488 155 0 210
MMN DPZ 500 35.95650 -120.49600 735 221 90 0
MMN DP1 500 35.95650 -120.49600 735 221 0 175
MMN DP2 500 35.95650 -120.49600 735 221 0 265
RMN DPZ 500 36.00087 -120.47772 1198 73 90 0
RMN DP1 500 36.00087 -120.47772 1198 73 0 310
RMN DP2 500 36.00087 -120.47772 1198 73 0 40
SMN DPZ 500 35.97297 -120.57988 732 282 90 0
SMN DP1 500 35.97297 -120.57988 732 282 0 120
SMN DP2 500 35.97297 -120.57988 732 282 0 210
VAR DPZ 500 35.92608 -120.44705 509 572 -90 0
VAR DP1 500 35.92608 -120.44705 509 572 0 15
VAR DP2 500 35.92608 -120.44705 509 572 0 285
VCA DPZ 500 35.92162 -120.53392 789 200 90 0
VCA DP1 500 35.92162 -120.53392 789 200 0 200
VCA DP2 500 35.92162 -120.53392 789 200 0 290
 

Network Performance and Upgrade Plans

The system has recorded more than 10000 microearthquakes in the Parkfield region along with many times that number of regional and teleseismic events. More than 5000 good quality 3-D locations make up the catalog for the 40 km stretch of the fault zone centered at Middle Mountain. More than 50 controlled-source data sets from the Vibroseis monitoring program have also been gathered from mid-1987 until its termination in 1997. Waveforms and locations are accessible on the NCEDC archive (Table 4.1).

The original data acquisition system failed in 1998. It was a modified VSP system using 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 - it could not be monitored from Berkeley - so that reliability and hands-off operation was a crucial design feature.

Thanks to emergency funding from the USGS NEHRP, we have begun to replace the present system with a modern 24-bit acquisition system, and resources requested in this proposal will complete the upgrade of the 10 stations. The new system is compatible with the data flow and archiving common to all the elements of the Berkeley Digital Seismic network (BDSN) 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. We had a one-sample timing uncertainty, and a record length limitation because the time to do a tape write after event detection is longer than the length of the record, and we were off-line for the write time. The new system, based on the Quanterra Q730 datalogger, solves all three problems: timing resolution, dynamic range, and complete detection, in addition to the advantage in the conventional data flow. We hope to add three new borehole stations to the NW edge of the network as part of the fault-zone drilling (SAFOD) project, if that initiative is successful, to improve resolution at the planned drilling target on the fault.

References

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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