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Parkfield Borehole Network

Subsections

Introduction

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).

Figure 4.1: Map showing the San Andreas Fault trace, the location of the original 10 Parkfield HRSN stations (filled diamonds) and the 3 new sites (open diamonds), along with the BDSN station PKD (filled square). Seismicity is also shown (grey points) and the locations of the 8 source points for the Vibroseis wave propagation monitoring experiment are represented by small black triangles. The epicenter of the 1966 M6 Parkfield mainshock is located at the large open circle. The location of the proposed SAFOD drill site is shown by the filled star. Station GHI (Gold Hill, not shown) is located on the San Andreas Fault about 8 km to the Southeast of station EAD.
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\epsfig{file=hrsn_map.ps, width=15cm}\end{center}\end{figure*}

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.

Network Background

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 $M_{L}$ 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.

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. Properties of the 10 original sites are summarized in Table 4.1.


Table 4.1: Instrumentation of the original 10 station 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.
Site Sensor Z H1 H2 RefTek 24 RefTek 72-06
EADB Mark Products L22 -90 170 260 01/1988 - 12/1998 12/1998 - 07/1999
FROB Mark Products L22 -90 338 248 01/1988 - 12/1998 12/1998 - 07/1999
GHIB Mark Products L22 90 failed unk 01/1988 - 12/1998 12/1998 - 07/1999
JCNB Mark Products L22 -90 0 270 01/1988 - 12/1998 12/1998 - 06/2001
JCSB Geospace HS1 90 300 210 01/1988 - 12/1998 12/1998 - 07/1999
MMNB Mark Products L22 -90 175 265 01/1988 - 12/1998 12/1998 - 06/2001
RMNB Mark Products L22 -90 310 40 01/1988 - 12/1998 12/1998 - 07/1999
SMNB Mark Products L22 -90 120 210 01/1988 - 12/1998 12/1998 - 06/2001
VARB Litton 1023 90 15 285 01/1988 - 12/1998 12/1998 - 07/1999
VCAB Mark Products L22 -90 200 290 01/1988 - 12/1998 12/1998 - 06/2001


Network Upgrade

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.

Figure 4.2: HRSN data flow is illustrated in this figure. 6 stations are acquired directly at the CDF facility while the other 7 send data to a router at Gastro Peak. These data are aggregated and transmitted to the CDF site over a microwave radio link. The HRSN computer system runs a modified version of the REDI software and event files with waveforms are created and transmitted to the BSL over the frame-relay link.
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Table 4.2: Planned data streams to be acquired at each HRSN site after the upgrade. Sensor type, channel name, sampling rate, sampling mode, and type of FIR filter are given. C indicates continuous; T triggered; Ac acausal; Ca causal.
Sensor Channel Rate (sps) Mode FIR
Geophone DP? 250.0 T Ca
Geophone BP? 20.0 C Ac


Data and Implications

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 $\sim $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.

Figure 4.3: Waveforms from a magnitude 0.0 earthquake and 2 of its aftershocks (magnitudes -0.8 and -1.3). The P and S phase arrivals are indicated as P1, P2, P3 and S1, S2, S3 for magnitude 0.0, -0.8, and -1.3 events respectively. Uppermost seismogram is the M0.0 quake recorded by the NCSN vertical short period surface station PVC (component VHZ) located in Vineyard Canyon near Parkfield. Second seismogram is the same event recorded on vertical component channel (DP1) by the HRSN borehole station VCAB. The PVC-VHZ data are sampled at 100 sps and VCAB-DP channels at 250 sps. PVC is 78 meters from VCAB. Third seismogram contains waveforms of the M-0.8 and M-1.3 aftershocks recorded on VCAB-DP1 (phases P2 and P3). These events occurred 6.7 seconds apart. Aftershock records are scaled x5 to aid in the comparison. Horizontal seismograms from VCAB (components DP2 and DP3) are shown in the subsequent four seismograms for all 3 events. Apparent S-phases as seen on the vertical channels are indicated by A. The same phases on the horizontal seismograms are indicated by A'. Arrow on the uppermost record shows the S arrival time pick used by the NCSN catalog. Bottom three seismograms are 3-components of the M-0.8 and M-1.3 aftershocks recorded at the new HRSN station CCRB.
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\epsfig{file=hrsn_wfs.ps, width=15.00cm}\end{center}\end{figure*}

SAFOD Expansion

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


Table 4.3: 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 new sites are given at the bottom (elevations to be determined).
Site Net Latitude Longitude Elev (m) Depth (m) Date Location
EADB BP 35.89525 -120.42286 499 254 01/1988 - Eade Ranch
FROB BP 35.91078 -120.48722 542 258 01/1988 - Froelich Ranch
GHIB BP 35.83236 -120.34774 433 370 01/1988 - Gold Hill
JCNB BP 35.93911 -120.43083 559 335 01/1988 - Joaquin Canyon North
JCSB BP 35.92120 -120.43408 487 332 01/1988 - Joaquin Canyon South
MMNB BP 35.95654 -120.49586 731 510 01/1988 - Middle Mountain
RMNB BP 36.00086 -120.47772 1198 1125 01/1988 - Gastro Peak
SMNB BP 35.97292 -120.58009 732 450 01/1988 - Stockdale Mountain
VARB BP 35.92614 -120.44707 511 -61 01/1988 - Varian Well
VCAB BP 35.92177 -120.53424 790 590 01/1988 - Vineyard Canyon
CCRB BP 35.95716 -120.55161 601 251 05/2001 - Cholame Creek
LCCB BP 35.98006 -120.51423 637 252 08/2001 - Little Cholame Creek
SCYB BP 36.00942 -120.53661 947 252 08/2001 - Stone Canyon



Table 4.4: Instrumentation of the upgraded and expanded Parkfield HRSN. Most HRSN sites have L22 sensors. The upgraded network uses a Quanterra 730 4-channel system and dates when Quanterra quality data collection began are given. Orientations of the vertical (vert.) and horizontal components (N45W and N45E) are given. Their polarities are yet to be determined.
Site Sensor Z H1 H2 Quanterra 730
EADB Mark Products L22 -90 170 260 03/2001
FROB Mark Products L22 -90 338 248 03/2001
GHIB Mark Products L22 90 failed unk 03/2001
JCNB Mark Products L22 -90 0 270 06/2001
JCSB Geospace HS1 90 300 210 03/2001
MMNB Mark Products L22 -90 175 265 06/2001
RMNB Mark Products L22 -90 310 40 03/2001
SMNB Mark Products L22 -90 120 210 06/2001
VARB Litton 1023 90 15 285 03/2001
VCAB Mark Products L22 -90 200 290 06/2001
CCRB Mark Products L22 vert. N45W N45E 05/2001
LCCB Mark Products L22 vert. N45W N45E 08/2001
SCYB Mark Products L22 vert. N45W N45E 08/2001


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.

Acknowledgements

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.

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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