1986 - 1998

Installation of the HRSN deep (200-300m) borehole sensors initiated in 1986 and recording of triggered 500 sps earthquake data began in 1987. The HRSN sensors are 3-component geophones in a mutually orthogonal gimbaled package. This ensures that the sensor corresponding to channel DP1 is aligned vertically and that the others are aligned horizontally. 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 (Daley and McEvilly, 1990). During 1988, the original 10 station network was completed, which included a deep (572 m) sensor from the Varian well string. Data from network stations was telemetered into a central detection/recording system operating in triggered mode. Also in 1988, the Varian string system was slaved for about two years to the Vibroseis control signals, allowing simultaneous recording of vibrator signals on both systems. For several years beginning in 1991, low-gain event recorders (from PASSCAL) were installed at several of the sites to extend the dynamic range from about $M_{L}$ 1.5 to about $M_{L}$ 4.5.

The data acquisition system operated quite reliably until late 1996, when periods of unacceptably high down time developed. During this period, as many as 7 of the remote, solar-powered telemetered stations were occasionally down simultaneously due to marginal solar generation capacity and old batteries, and recording system outages of a week or more were not uncommon. In July 1998, the original data acquisition system failed permanently. This system 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 data (Vibroseis discontinued in 1994, Karageorgi et al., 1997). The system was remote and completely autonomous, and data tapes were mailed about once a month to Berkeley for processing and analysis. The old system also had a one-sample timing uncertainty and a record length limitation because the tape write system recovery after event detection was longer than the length of the record, leaving the system off-line after record termination and until write recovery was completed.