Parkfield Borehole Network

Introduction

The operation of the High Resolution Seismic Network (HRSN) at Parkfield, California began in 1987, as part of the U.S. Geological Survey initiative known as the Parkfield Prediction Experiment (PPE) (Bakun and Lindh, 1985).

Figure 6.1 shows the location of the network, its relationship to the San Andreas fault, sites of significance to previous and ongoing research using the HRSN, relocated earthquake locations, 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 wide bandwidth high frequency recordings (0-125 Hz), and its sensitivity (recording events below magnitude -1.0) 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 relationship 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, a homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90$\%$ right-lateral strike-slip), and the planned drilling zone and penetration and instrumentation site of the San Andreas Fault deep observatory at depth (SAFOD) installation (see: http://www.icdp-online.de/html/sites/sanandreas/objectives/proposal.html).

Figure 6.1: 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). 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 main shock is located at the large open circle. The location of the pilot hole and proposed SAFOD drill site is shown by the filled star, and the location of the 2 alternative M2 repeating earthquake targets (70 meters apart) are shown as concentric circles. Seismicity relocated using an advanced 3-D double-differencing algorithm applied to a cubic splines interpolated 3-D velocity model (Michelini and McEvilly, 1991) is also shown (grey points). Station GHIB (Gold Hill, not shown) is located on the San Andreas Fault about 8 km to the Southeast of station EADB.

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 Part III.

HRSN Overview

1986 - 1998

The HRSN was installed in deep (200-300m) 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 original network was completed to a ten station 3-component 500 sps set of stations telemetered 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 down time 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 July of 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 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 had completed.

1998 - 1999

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, 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 6.2.

Table 6.1: 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 surface 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.

Site	Net	Latitude	Longitude	Surf. (m)	Depth (m)	Date	Location
EADB	BP	35.89525	-120.42286	499	245	01/1988 -	Eade Ranch
FROB	BP	35.91078	-120.48722	542	284	01/1988 -	Froelich Ranch
GHIB	BP	35.83236	-120.34774	433	63	01/1988 -	Gold Hill
JCNB	BP	35.93911	-120.43083	559	224	01/1988 -	Joaquin Canyon North
JCSB	BP	35.92120	-120.43408	487	155	01/1988 -	Joaquin Canyon South
MMNB	BP	35.95654	-120.49586	731	221	01/1988 -	Middle Mountain
RMNB	BP	36.00086	-120.47772	1198	73	01/1988 -	Gastro Peak
SMNB	BP	35.97292	-120.58009	732	282	01/1988 -	Stockdale Mountain
VARB	BP	35.92614	-120.44707	511	572	01/1988 -	Varian Well
VCAB	BP	35.92177	-120.53424	790	200	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 6.2: Instrumentation of the 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, 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. The upgraded network uses a Quanterra 730 4-channel system. For the three new stations (bottom) horizontal orientations are approximate (N45W and N45E) and will be determined more accurately in the near future.

Site	Sensor	Z	H1	H2	RefTek 24	RefTek 72-06	Quanterra 730
EADB	Mark Products L22	-90	170	260	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
FROB	Mark Products L22	-90	338	248	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
GHIB	Mark Products L22	90	failed	unk	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
JCNB	Mark Products L22	-90	0	270	01/1988 - 12/1998	12/1998 - 06/2001	03/2001 -
JCSB	Geospace HS1	90	300	210	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
MMNB	Mark Products L22	-90	175	265	01/1988 - 12/1998	12/1998 - 06/2001	03/2001 -
RMNB	Mark Products L22	-90	310	40	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
SMNB	Mark Products L22	-90	120	210	01/1988 - 12/1998	12/1998 - 06/2001	03/2001 -
VARB	Litton 1023	90	15	285	01/1988 - 12/1998	12/1998 - 07/1999	03/2001 -
VCAB	Mark Products L22	-90	200	290	01/1988 - 12/1998	12/1998 - 06/2001	03/2001 -
CCRB	Mark Products L22	-90	N45W	N45E	-	-	05/2001 -
LCCB	Mark Products L22	-90	N45W	N45E	-	-	08/2001 -
SCYB	Mark Products L22	-90	N45W	N45E	-	-	08/2001 -

Upgrade and SAFOD Expansion

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.

We have also 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 6.1 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 6.1 and 6.2. All HRSN Q730 data loggers employ FIR filters to extract data at 250 and 20 Hz (Table 6.3).

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 has significantly increased event detection and reduced false triggers for the 60-station network data.

Figure 6.2 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.

Figure 6.2: 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.

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 SEGY format).

2001-2002 Activities

Significant efforts were made to identify and reduce noise and telemetry problems arising from the new recording, telemetry and site design this year. Detection, monitoring, and high-resolution recording of earthquakes down to the smallest possible magnitudes with the highest possible signal-to-noise (especially in the region of the proposed SAFOD drilling) is a major objective of the HRSN data collection. Consequently, elimination of all sources of unnaturally occurring noise is a primary goal. The minimization of data loss due to station outages and data-dropouts is also critical to this objective, since reduced station coverage degrades the sensitivity of network triggering.

Noise Reduction

The sophisticated HRSN data acquisition involves integration of a number of distinct components at each station (i.e., sensor, preamp, solar panels, solar regulator, batteries, Freewave radio, antenna, lightening arresters, and associated cabling, connectors and grounds).

This complex integration of station and communication components combined with a variety of associated concerns (e.g., ground loops, cable resistances, radio interference at stations and between stations, atmospheric effects on telemetry and power, the integration of older (pre-upgrade) hardware components with new upgraded components, failure of older components, and malfunctioning and unexpected performance characteristics of newer components) makes identification of specific causes of network generated (i.e. artificial) noise difficult to identify.

Over the past year, our exhaustive iterative testing of HRSN performance has identified three primary causes for the observed artificial noise. We have designed and have implemented or are in the process of implementing fixes.

Power separation

Persistent 50 and 100 Hz noise sources affecting nearly all stations to varying degrees has been found to result from the interaction of the preamp and Quanterra systems through their common connection to a single power supply system. As a fix, we have separated preamp solar and battery power from the power provided to the rest of the data acquisition system at each station.

Solar regulators

Regularly occurring spikes occurring during the daylight hours were observed in the continuous data streams and found to be due to the solar regulators. We have purchased and tested new solar regulators and are installing them at all the sites.

Preamp Noise

A significant contribution source of artificial noise is the preamp amplification levels. In the upgraded system, preamps from the older network were used. During integration of the older preamps with the increased dynamic range capabilities of the 24-bit Quanterra system, gain settings of the preamps were reduced from x10000 to x80 in order to match signal sensitivity of the new system with the older one. While these lower preamp gain levels are still within the operational design of the preamps, they are no longer in their optimal range which enhances the contribution of preamp generated noise. Initially, this was not expected to be a significant problem. However, we have subsequently found that even the small increase in preamp noise that results from the preamp gain reduction can significantly impact the sensitivity of the network for detecting and recording the very smallest events.

Figure 6.3 shows the preamp noise effect from a test done at station EADB using background noise on day 134 of year 2002. Considerable signal hash is seen at gain levels of x80 (top 3-component waveforms), and significantly reduced when gains are increased to x1000 (lower waveforms). Since we are also interested in recording on-scale as large events as possible on the unique borehole, high-frequency broadband width HRSN, simply increasing gain levels on all stations is not an option. Doing so would cause the recording system to saturate at lower magnitudes. Our plan is to redesign the preamp operation characteristics so that their operation at gain levels of x80 is optimal.

A prototype preamp has been designed and built which is to be installed on an HRSN station for testing in the near future. If testing proves successful, installation of the redesigned preamps at all 13 stations is planned.

Figure 6.3: Preamp noise reduction test. Shown are 30 seconds of 3-component background signal recorded at station EADB on day 134 of 2002 at 1520 UTC (top 3) and 1550 UTC, when gain levels were set to x80 and x1000 respectively. Note the substantial reduction in preamp generated noise at high the higher gain. Network operation currently continues at x80 gain despite the preamp noise, in order to optimize the dynamic range capabilities. A prototype redesign of the preamp with optimized operational characteristics at x80 gain has been built and is to be field tested shortly.

Telemetry Dropouts

The cause of data dropouts at one of the new SAFOD critical stations (CCRB) was particularly difficult to determine. This problem did not appear during the early operation of CCRB, but became intermittent and then rather severe during the winter season. The majority of the time the transfer of data packets from CCRB to the the central data collection site were satisfactory. However, a strong positive correlation of the times of dropouts with the occurrence of earthquakes was observed (definitely not a desirable situation). It was eventually determined that the dropout problem was the result of an interplay involving data compression, station buffer size and marginal radio connectivity. For low amplitude background signals, the compression of data packets before telemetry was sufficient to prevent exceedence of the Quanterra buffer storage between periods of radio connectivity dropouts. However, during earthquakes, data compression is lower due to the higher amplitude signals of the quakes. This resulted in exceedence of the CCRB buffer storage capacity and data loss during earthquakes. Figure 6.4 shows an example of the dropout problem at an intermediate stage of its severity.

In an initial attempt to improve radio connectivity, installation of a large antenna dish was tried, but found to be an inadequate fix. A relay of the CCRB-Gastro Peak telemetry through a new repeater site was eventually required.

Figure 6.4: Data dropout example at station CCRB. Shown are 21 seconds of vertical component (DP1) waveform data from HRSN stations CCRB (top), SMNB (middle) and VCAB (bottom). Exceedence of local buffer capacity at CCRB caused data loss at about 5.5 seconds into the earthquake first arrival due to marginal radio telemetry and the reduced data compression possible for large amplitude (i.e. earthquake) signals.

SAFOD Pilot Hole Drilling

In June of 2002, drilling began on the SAFOD Pilot Hole (PH). The Pilot Hole was drilled to a depth of approximately 2 km (drilling was completed in late July).

Noise from the drilling was clearly visible at station CCRB, which may prove crucial for guiding SAFOD drilling in the future. Figure 6.5 shows the signal spectra below 65 Hz for 30 minutes of data recorded on the DP1 (vertical) channel at 250 sps generated by the SAFOD PH drilling on June 24 of 2002. The data are high-pass filtered at 0.5 Hz. The pilot hole was drilled within several 10's of meters from the planned SAFOD scientific hole and about 2 km due north of CCRB. Significant low frequency energy above background levels are seen below about 10 Hz.

Several significant spectral peaks can also be seen at about 1.5, 4.5, 6.5, and 10 Hz. The drill-bit spectra drops off sharply above 10 Hz. Comparable spectral amplitudes and character are observed on the DP1 and DP2 horizontal channels (not shown). The horizontal orientations (N45W and N45E) are bisected by the north-south oriented path from CCRB to the Pilot Hole. The frequency band and spectral character was also observed to change over longer time periods. We infer these changes to reflect either changes of the lithology being penetrated by the drill-bit or changes in the type of drill-bit or rotary speed. These changes further demonstrate the sensitivity of the borehole sensors for imaging bit generated noise.

With the completion of the pilot hole and the deployment of the downhole sensor strings, discussions are underway between the BSL and the USGS Menlo Park regarding use of the PH data within the HRSN system for enhanced triggering capability (see the "Future Directions" section).

Figure 6.5: Signal spectra from SAFOD pilot hole drilling. Shown is the spectral amplitude below 65 Hz of 30 minutes of vertical component (DP1) data recorded by CCRB at 250 sps and high-pass filtered at 0.5 Hz. There is a marked absence of bit noise above 10 Hz, and distinct high amplitude spikes at about 1.5, 4.5, 6.5, and 10 Hz.

Data Archive

At this time, continuous data streams on all 39 components are being recorded at 20 and 250 sps on the local HRSN computer at the CDF facility and archived on DLT tape. The 20 sps data are transmitted continuously to the BSL over the frame-relay linked and archived at the NCEDC. In addition, the 13 vertical component channels at 250 sps are also transmitted continously to the BSL over the frame relay-circuit for purposes of fine tuning the triggering algorithm for detection at smallest possible magnitude levels.

An ongoing effort has been the development of a new earthquake triggering scheme, with the goal of replacing the continuous archive with triggered event gathers. 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.

In order to facilitate the archive of the HRSN events, BSL staff are developing a Graphical User Interface (GUI). The GUI will allow review of every trigger and either schedule the event to be archived or deleted (if it is noise, rather than an earthquake). The GUI will also allow the analysts to log problems, such as the noise spikes, and to characterize events based on S-P time.

Table 6.3: Data streams currently being acquired at each HRSN site. Sensor type, channel name, sampling rate, sampling mode, and type of FIR filter are given. C indicates continuous; T triggered; Ac acausal; Ca causal. "?" indicates orthogonal vertical and 2 horizontal components.

Sensor	Channel	Rate (sps)	Mode	FIR
Geophone	DP?	250.0	T	Ca
Geophone	BP?	20.0	C	Ac

Examples of Data

The upgrade of the HRSN system from a 16- to 24-bit system has greatly improved its ability to record earthquakes over a wider magnitude range. Previously, clipping of waveforms would take place around magnitude 1.5. With the new system, earthquake with magnitudes between 4 and 5 are expected to be recorded on scale.

As an example of the HRSN waveform data quality at larger magnitudes, Figure 6.6 shows waveforms from the Sept. 6, 2002 magnitude 3.9 earthquake near Parkfield, CA. As expected the signal-to-noise (S/N) is excellent at all the borehole stations. Clipping of seismograms is absent, even at station EADB located only 3 km away from the epicenter. Figure 6.6 also includes seismograms from two PH sensors (PL11, at surface, and PL21, at 1.85 km depth).

Figure 6.6: Sample HRSN and SAFOD Pilot Hole (PL11 and PL21) waveforms from the Sept. 6, 2002 magnitude 3.9 earthquake occurring some 3-4 km southeast of Parkfield, CA, at a depth of about 9.5 km. Station GHIB is located southeast of the event by about 5 km (top waveform). All other stations locate northwest of the event and are ordered according to their progressively increasing P arrival times. In general only the vertical components of the 3 component sensors are shown. Exceptionally, all three components of the station closest to the event (EADB) and the station closest to planned SAFOD drill site (CCRB) are shown. Horizontal component DP3 has been substituted for the vertical component at VARB due to a recording failure on the VARB vertical for this event. Seismograms are unfiltered and without corrections for sensor response or polarity. PH sensor PL21 is particularly deep ( 1.85 km below surface). The P arrival time of PH sensor PL11 (located at the surface) is approximately 0.39 sec. after that of PL21, so the PL11 waveform has been plotted out of arrival time order to facilitate comparison with PL21.

Figure 6.6 illustrates the power of three-component recordings in borehole installations, as the the horizontal records give much better definition of the S-arrival than the vertical component alone. Vertical and horizontal components recored at the station closest to the M3.9 (EADB) and at the new station closest to the SAFOD drill site (CCRB) are shown. The apparent S-phase as seen on the vertical components arrives noticeably later ( 0.1 sec.) than the S-phase arrival seen on the horizontal components. S-arrival time differences of this magnitude can lead to location errors on the order a km or more The later arriving apparent S in the vertical records could be attributed to near surface forward scattered energy or possibly to Fault Zone Guided Wave arrivals, known to exist at Parkfield, rather than to the true S-phase.

Although the PH sensors are currently only recording vertical motion, the recordings of the deep sensors (1.85 km) should significantly aid in the detection of the very smallest events in the penetration zone. A significant delay in the P-arrival time of the M3.9 event at PL11 relative to that at PL21 ( 0.39 sec) can be seen. This indicates that the average P-wave velocity in the top 1.85 km of the crust at the PH site is on the order of 4.7 km/sec which is in general agreement with that observed in tomographic inversion of seismic and active source experiments in the area and with velocities expected for the Salinian composition of the crust penetrated by the PH. The PL21 record also shows some slight clipping. Events in the SAFOD penetration zone are much closer to PL21 than the M3.9 event, but are in general much smaller. However, the ultimate target of the SAFOD drilling is penetration of a site of repeating M2 earthquakes. It is not expected that a M2 close-in to PL21 will also cause it to clip, but an outside possibility for such clipping to occur does exist.

Figure 6.7 illustrates the performance of the HRSN borehole stations for recording teleseismic earthquakes. Shown are records of the August 19, 2002 magnitude 7.7 Fiji Is. deep focus earthquake occurring over 8900 km away from Parkfield. The signal-to-noise in the 0.3-2 Hz band shown is very good, allowing for a variety of waveform analyses for deformation of source characteristics and whole earth structure.

Note the contrast in waveform shape in this frequency band, particularly in the coda, of the MMNB recording. Station MMNB is located directly on the surface trace of the SAF and is known to record Fault Zone Guided Waves for local events. Information on the details of the local deep fault zone structure are also contained in the wave fields of energy generated by distant teleseismic events.

Figure 6.7: Sample 1 minute length seismograms for the 19 August 2002 deep focus Fiji Islands teleseism (11:01 UT, -21.70, -179.51, 580 km deep, M 7.7). Vertical components for the 13, 3-component HRSN borehole stations are shown. The waveforms have been deconvolved to ground velocity, and 0.3-2 Hz bandpass filtered, and plotted using an absolute scale. Station VARB vertical experienced a recording failure during this event. A similar plot for the same earthquake, recorded on the Northern Hayward Fault Network, is show in Figure 5.2.

Future Directions

We are continuing to work at reducing magnitude threshold levels and improving data completeness across the network. Initiation of an automated state-of-health monitoring routine is planned soon and a semi-automated waveform and trigger review scheme (GUI based) is currently under development. These improvements will allow for rapid identification of network outages and problems with station/component specific waveform recording.

Additional efforts underway to increase event detection sensitivity include: 1) refinement of a station specific filtering scheme, 2) refinement of subnet triggering scheme to allow for 2 (instead of 3) station triggering criteria to provide detection of even smaller local earthquakes, 3) incorporation of the pilot hole array into the network triggering scheme to capture the smallest events in the SAFOD drilling area. 4) continue assessment of waveform/spectral character to search for further artificial noise sources at finer scales, and consideration and development for associated fixes.

Monitoring of the systematics of microseismic characteristics, particularly in the SAFOD drilling and target zone, is a primary objective of the HRSN data collection effort. Continued analysis of these data for detailed seismic structure, for similar and characteristic microearthquake systematics, for slip rate evolution, and for determining the source patch size and other characteristics of the SAFOD target(s) and associated earthquakes is also a primary focus that is being pursued in our ongoing research (Part III).

Acknowledgements

Thomas V. McEvilly passed away in February 2002 (Chapter 2). Tom was the PI on the HRSN project for many years, and without his dedication and hard work the creation and continued operation of the HRSN would not have been possible. His contributions continue to be appreciated in the extreme and the fruits of his labor many-fold.

This chapter was compiled by Bob Nadeau. Under Bob Nadeau's and Doug Dreger's general supervision, Rich Clymer, Wade Johnson, Doug Neuhauser, Bob Uhrhammer, John Friday, Pete Lombard, and Lane Johnson all contribute to the operation of the HRSN.

The upgrade and operation of the HRSN is partially supported by the USGS, through the NEHRP External Grants Program (01HQG00057 and 01HQGR0067). NSF provided support for the expansion of the HRSN near the SAFOD drill site (EAR-9814605).

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. Seismol. Soc. Am., 82, 1388-1415, 1992.

Michelini, A. and T.V. McEvilly, Seismological studies at Parkfield: I. Simultaneous inversion for velocity structure and hypocenters using B-splines parameterization, Bull. Seismol. Soc. Am., 81, 524-552, 1991.

Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94 720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2002, The Regents of the University of California.