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).
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.
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.