As part of the U.S. Geological Survey initiative known as the
Parkfield Prediction Experiment (PPE) (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,
its relationship to the San Andreas fault, sites used in controlled
source Vibroseis experiments, earthquake locations since 1987, and
the epicenter of the 1966 M6 earthquake that motivated the PPE.
The HRSN records exceptionally high-quality data, owing to its
13 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).
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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.
The HRSN was installed in boreholes beginning in 1986. 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. During 1988, the network was completed to a ten station
3-component 500 sps radio-telemetered stations into a central
detection/recording system operating in triggered mode and 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 
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.
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. Properties of the 10 original sites are summarized in Table 4.1.
Thanks to emergency funding from the USGS NEHRP, we have replaced the original 10-station system with a modern 24-bit acquisition system (Quanterra 730 4-channel digitizers, advanced software using flash disk technology, spread-spectrum telemetry, Sun Ultra 10/440 central processor at the in-field collection point, with 56K frame-relay connectivity to Berkeley). The new system is now online and recording data continuously at a central site located on the California Department of Forestry (CDF) fire station in Parkfield.
Figure 4.2 shows the telemetry system for the upgraded HRSN. The HRSN stations use SLIP to transmit TCP and UDP data packets over bidirectional spread-spectrum radio links between the on-site data acquisition systems and the central recording system at the CDF. Six of the sites transmit directly to a router at the central recording site. The other seven sites transmit to a router at Gastro Peak, where the data are aggregated and transmitted to the central site over a 4 MBit/second digital 5.4 GHz microwave link. The microwave link was installed to support the current IRIS PASSCAL broadband array deployment in Parkfield, and is shared by the HRSN and PASSCAL. All HRSN data are recorded to disk at the CDF site. A modified version of the REDI real-time system detects events from the HRSN data, creates event files with waveforms from the HRSN and PASSCAL networks, and sends the event data in near real-time to UC Berkeley.
The upgraded system is compatible with the data flow and archiving common to all the elements of the BDSN/NHFN and the NCEDC, and is providing remote access and control of the system. It is also providing data with better timing accuracy and longer records which are to eventually flow seamlessly into NCEDC. The new system solves the problems of timing resolution, dynamic range, and missed detections, in addition to providing the added advantage of conventional data flow (the old system recorded SEG-Y format).
Significant efforts were made to identify and reduce noise sources arising from the new recording, telemetry and site design. The most significant contributors to noise have been identified and fixes have been developed and implemented at the 6 critical stations surrounding the SAFOD drilling target. Fixes for the remaining 7 stations, (some requiring the purchase of additional equipment) is currently underway. Identification and development of fixes for the lesser noise sources is continuing.
At this time data streams on all 39 components are being recorded continuously at 20 and 250 sps. These data are being archived on DLT tape, and 6 250 sps vertical channels from the critical stations surrounding the SAFOD drilling target are being continuously sent to Berkeley over the frame relay-circuit for purposes of fine tuning the triggering algorithm for detection at smallest possible magnitude levels.
Plans are to replace the continuous archiving of 250 sps data with triggered data event gathers which are to be sent in near real time to Berkeley over frame relay to be archived on the NCEDC. Plans are to replace the 6 250 sps channels now being continuously sent to Berkeley with continuous 20 Hz data for investigations of borehole based very high frequency teleseismic and regional event investigations (Table 4.2) and to archive these data on the NCEDC as well.
An ongoing effort, since initiation of data collection from the upgraded network, has been the development of a new earthquake triggering scheme. A first cut version of the new scheme has been implemented and is already detecting earthquakes at an increased rate-about 3 times the number of earthquakes detected before the upgrade.
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As an example of the HRSN data quality, Figure 4.3 shows waveforms from a magnitude 0.0 earthquake and 2 of its aftershocks as recorded on stations of the short-period regional NCSN and local 3-component borehole HRSN. All three of these earthquakes occurred within a 31 minute time period on Sept 1, 2001. The 2 aftershocks have estimated magnitudes -0.8, and -1.3, respectively, and do not appear in the NCSN earthquake catalog. The greater detection threshold of the borehole HRSN has important implications for studies of earthquake and fault physics, imaging, monitoring, scaling, earthquake hazards and fault slip rates at depth.
Like many NCSN stations, PVC is instrumented only with a vertical
component sensor. Figure 4.3 illustrates the power of
three-component recordings in a borehole installation, as the
the horizontal records from VCAB give much better definition of
the S-arrival than the vertical component alone. The apparent S-phase
as seen on the vertical components (designated A) arrives
significantly later than the S-phase arrival seen on the horizontal
components (A'). The arrow on the uppermost record shows the S
arrival time pick which was used by the USGS for locating the M0.0
event. This pick is 
0.25 to 0.5 seconds later than what would
be picked using the horizontal components of the HRSN. Arrival time
errors of this magnitude can lead to location errors on the order of 2
to 3 km. Depth of the M0.0, as given in the NCSN catalog, is 12.28
km. The apparent S in the vertical records might be attributable to
near surface back scattered energy or possibly to Fault Zone Guided
Wave arrivals-known to exist at Parkfield.
As can be seen in the Figure, waveform similarity of all 3 events is very high on all three components. This indicates the events are nearly co-located (within 100 m) and have similar mechanisms.
Despite their similarities, however, these events are not considered characteristically repeating earthquakes, since subtle differences in their S-P times exclude exact co-location and there magnitude differences and temporally clustered occurrence are contrary to properties commonly associated with characteristic event sequences. At this magnitude level, identification of repeating quakes will require several months of data collection in order to span typical repeating quake recurrence times.
All 39 geophone channels of the upgraded and expanded 13 station HRSN are currently recording data. An example of 3-component seismograms of the magnitude -0.8 and -1.3 aftershocks recorded at one of the 3 recently added stations (CCRB) is shown at the bottom of Figure 4.3. Here again both the P and S phases can be identified. Multiple arrivals at numerous well distributed stations insure that events can be accurately located even at very low magnitudes and at relatively great depth.
Events targeted by SAFOD are much shallower (3-4 km) and therefore closer to the HRSN sensors. Extrapolation using assumed geometrical spreading suggests that events with magnitudes below magnitude -2.0 can be detected and located in proximity of the SAFOD target.
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We have added 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 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 use similar hardware to the main network, with the addition of an extra channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 4.3 and 4.4
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The remoteness of the drill site and new stations required the intermediate data collection point at Gastro Peak, with a microwave link to the CDF facility. We are sharing this link with the PASSCAL broadband array deployed around the drill site by the University of Wisconsin and the Rensselaer Polytechnic Institute. We are using the HRSN triggering algorithm in a joint triggering scheme which will allow the 60-station array to identify events on the lower noise, greater sensitivity of the borehole network. This will significantly increase event detection and reduce false triggers for the 60-station network data.
More information about the SAFOD project is available on the Web at
http://www.earthscope.org/safod.com.html.
Under Tom McEvilly's general supervision, Rich Clymer, Bob Nadeau, Wade Johnson, Doug Neuhauser, and John Friday contribute to the operation of the HRSN. Bob Nadeau contributed to the preparation of this chapter.
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.