Subsections


Parkfield Borehole Network (HRSN)

Introduction

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

Figure 41.1 shows the location of the network, its relationship to the San Andreas fault, sites of significance from previous and ongoing research using the HRSN, double-difference relocated earthquake locations from 1987-1998, routine locations of seismicity from August 2002 to July 2003, nonvolcanic tremor locations from January 2001 through April 2005, and the epicenter of the 1966 and 2004 M6 earthquakes that motivated much of the research. The HRSN records exceptionally high-quality data, owing to its 13 closely spaced three-component borehole sensors (generally emplaced in the extremely low attenuation and background noise environment at 200 to 300 m depth (Table 41.1), its high-frequency wide bandwidth recordings (0-100 Hz; 250 sps), and its low magnitude detection threshold (below magnitude $0.0$ Ml).

Several aspects of the Parkfield region make it ideal for the study of small earthquakes and nonvolcanic tremors and their relationship to tectonic processes and large earthquakes. These include the fact that the network spans the SAFOD (San Andreas Fault Observatory at Depth) experimental zone, the nucleation region of earlier repeating magnitude 6 events and a significant portion of the transition from locked to creeping behavior on the San Andreas fault, the availability of three-dimensional P and S velocity models (Michelini and McEvilly, 1991), the existing long-term HRSN seismicity catalogue that is complete to very low magnitudes and that includes at least half of the M6 seismic cycle, a well-defined and simple fault segment, the existence of deep nonvolcanic tremor (NVT) activity, and a homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90$\%$ right-lateral strike-slip).

Figure 41.1: Map showing the San Andreas Fault trace and locations of the 13 Parkfield HRSN stations, the repeating M2 SAFOD targets (a 4 km by 4 km dashed box surrounds the SAFOD zone), and the epicenters of the 1966 and 2004 M6 Parkfield main shocks. Also shown are locations of the recently discovered nonvolcanic tremors, routine locations of earthquakes recorded by the expanded and upgraded 13 station HRSN (small open circles) and locations of events recorded by the earlier vintage 10 station HRSN relocated using an advanced 3-D double-differencing algorithm (gray points) applied to a cubic splines interpolated 3-D velocity model (Michelini and McEvilly, 1991).
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In a series of journal articles and Ph.D. theses, we have presented the cumulative, often unexpected, results of UC Berkeley's HRSN research efforts (see: http://www.seismo.berkeley.edu/seismo/faq/parkfield_bib.html). 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 and deep nonvolcanic tremors (Nadeau and Dolenc, 2005).

The Parkfield area has also become an area of focus of the EarthScope Project (http://www.earthscope.org) through the SAFOD experiment (http://www.icdp-online.de/sites/sanandreas/news/news1.html), and the HRSN is playing a vital role in this endeavor. SAFOD is a comprehensive project to drill into the hypocentral zone of repeating M $\sim $ 2 earthquakes on the San Andreas Fault at a depth of about 3 km. The goals of SAFOD are to establish a multi-stage geophysical observatory in close proximity to these repeating earthquakes, to carry out a comprehensive suite of down-hole measurements in order to study the physical and chemical conditions under which earthquakes occur, and to monitor and exhume rock, fluid, and gas samples for extensive laboratory studies (Hickman et al., 2004).

HRSN Overview

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 upward 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.

1998 - 1999

In December 1998, the original HRSN acquisition system was replaced by 10 stand-alone PASSCAL RefTek systems with continuous recording. To process these data, development of a major data handling procedure will be required in order to identify the microearthquakes below Ml = 0.0, since continuous telemetry to the Berkeley Seismological Laboratory (BSL) and application of a central site detection scheme was not an option at that time.

In July 1999, the network was reduced, due to limited instrument availability, to four RefTeks at critical sites that would ensure continuity in monitoring at low magnitudes and the archive of characteristic events for studying the evolution of their recurrence intervals. Properties of the 10 original sites are summarized in Table 41.2.


Table 41.1: 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 SAFOD sites are given at the bottom.
Site Net Latitude Longitude Surf. (m) Depth (m) Date Location
EADB BP 35.89525 -120.42286 466 245 01/1988 - Eade Ranch
FROB BP 35.91078 -120.48722 509 284 01/1988 - Froelich Ranch
GHIB BP 35.83236 -120.34774 400 63 01/1988 - Gold Hill
JCNB BP 35.93911 -120.43083 527 224 01/1988 - Joaquin Canyon North
JCSB BP 35.92120 -120.43408 455 155 01/1988 - Joaquin Canyon South
MMNB BP 35.95654 -120.49586 698 221 01/1988 - Middle Mountain
RMNB BP 36.00086 -120.47772 1165 73 01/1988 - Gastro Peak
SMNB BP 35.97292 -120.58009 699 282 01/1988 - Stockdale Mountain
VARB BP 35.92614 -120.44707 478 572 01/1988 - 08/19/2003 Varian Well
VARB BP 35.92614 -120.44707 478 298 08/25/2003 - Varian Well
VCAB BP 35.92177 -120.53424 758 200 01/1988 - Vineyard Canyon
CCRB BP 35.95718 -120.55158 595 251 05/2001 - Cholame Creek
LCCB BP 35.98005 -120.51424 640 252 08/2001 - Little Cholame Creek
SCYB BP 36.00938 -120.53660 945 252 08/2001 - Stone Canyon



Table 41.2: 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 as available field time permits.
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 -


2001 Upgrade and SAFOD Expansion

Thanks to emergency funding from the USGS NEHRP, we 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) in 2001. The new system is now online and recording data continuously at a central site located on California Department of Forestry (CDF) fire station property in Parkfield.

We have also added three new borehole stations, with NSF support, at the NW end of the network as part of the SAFOD project to improve resolution of the structure, kinematics and monitoring capabilities in the SAFOD drill-path and target zones. Figure 41.1 illustrates the location of the drill site, the new borehole sites, and locations of earthquakes recorded by the initial and upgraded/expanded HRSN.

The three new SAFOD stations have a similar configuration as the original upgraded 10 station network and include an additional channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 41.1 and 41.2. All HRSN Q730 dataloggers employ FIR filters to extract data at 250 and 20 Hz (Table 41.3).


Table 41.3: 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 and C Ca
Geophone BP? 20.0 C Ac


The remoteness of the drill site and new stations required an installation of an intermediate data collection point at Gastro Peak, with a microwave link to the CDF facility. 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. All HRSN data are recorded to disk at the CDF site.

The upgraded and expanded system is compatible with the data flow and archiving common to all the elements of the BDSN/NHFN and the NCEDC (Northern California Earthquake Data Center), and is providing remote access and control of the system. It has also provided triggered data with better timing accuracy and longer records, which are to eventually flow seamlessly into NCEDC. The new system also helps minimize 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).

Present Status

Because of limitations in telemetry bandwidth, however, not all continuous waveform data are currently transmitted to BSL. Instead, all continuous data are archived on DLT tapes which are brought to BSL every several weeks and uploaded to the NCEDC. A modified version of the REDI system (this report) was used to detect events in the HRSN data, extract waveform triggers, and transmit the waveform segments to the BSL in near-real-time. The December 22, 2003 San Simeon earthquake and its aftershocks, however, sent the HRSN into a nearly continuous triggering state. As a result, BSL staff had to disabled the transmission of triggered data.

At present, all 38 continuous 20 sps data streams are telemetered to the BSL. All continuous 250 sps data are migrated periodically from HRSN computer in Parkfield to DLT tape. These tapes are then mailed periodically to the BSL and are then processed and archived at the NCEDC. Seven vertical 250 sps channels are also telemetered to the NCEDC for purposes of quality control and SAFOD related activities. These data are archive temporarily (for 10 days) and then removed. Copies of the data are later restored for permanent archiving during uploading of the 38 250 sps continuous data streams from the DLT tapes.

A feature of the new system that has been particularly useful both for routine maintenance and for pathology identification has been the Internet connectivity of the central site processing computer and the station dataloggers with the computer network at BSL. Through this connection, select data channels and on-site warning messages from the central site processor are sent directly to BSL for evaluation by project personnel. If, upon these evaluations, more detailed information on the HRSN's performance is required, additional information can also be remotely accessed from the central site processing computer at Parkfield. Analysis of this remotely acquired information has been extremely useful for trouble shooting by allowing field personnel to schedule and plan the details of maintenance visits to Parkfield. The connectivity also allows certain data acquisition parameters to be modified remotely when needed, and commands can be sent to the central site computer and dataloggers to modify or restart processes when necessary.

The network connectivity also allows remote monitoring of the background noise levels being recorded by the HRSN stations. For example, shown in Figure 41.3 are power spectral density plots of background noise for vertical components of the 7 HRSN stations that are most critical for monitoring seismicity in the region containing SAFOD. The PSD analysis gives a rapid assessment of the HRSN seismometer responses across their wide band-width. By routinely generating these plots with data telemetered from Parkfield, changes in the seismometer responses, often indicating problems with the acquisition system, can be easily identified, and corrective measures can then be planned and executed on a relatively short time-frame.

Data Flow

Initial Processing Scheme. Continuous data streams on all 38 HRSN components are recorded at 20 and 250 sps on disk on the local HRSN computer at the CDF facility. The 20 sps data are transmitted continuously to the Berkeley Seismological Laboratory (BSL) over a frame-relay link and then archived at the NCEDC. In addition, the vertical component channels for the 7 stations critical to resolving seismicity in the SAFOD area are also being transmitted continuously at 250sps over a frame-relay circuit to the USGS and have been integrated into their NCSN (Norther California Seismic Network) trigger detection scheme to increase the sensitivity of the NCSN in the SAFOD area. The 7-channel 250 sps data is also being transmitted to the BSL for purposes of quality control and fine tuning the triggering algorithm for the detection of the smallest possible events around SAFOD. These telemetered 250 sps data are archived on disk for only about 1 week at the BSL and are then deleted. When the local HRSN computer disk space is full, the continuous 250 sps data on the HRSN local computer are migrated onto DLT tape, and the tapes sent to Berkeley for long-term storage and for upload to disc into the NCEDC archive. Efforts are currently underway to transmit all 38 HRSN channels to the USGS and BSL over a T1 line to enhance NCSN detection further and to make the data web-available in near-real-time.

Shortly after being recorded to disk on the central site HRSN computer, event triggers for the individual station data are determined, and a multi-station trigger association routine then processes the station triggers and identifies potential earthquakes. For each potential earthquake that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is assigned. Prior to San Simeon earthquake of December 22, 2003, 30 second waveform segments were then collected for all stations and components and saved to local disk as an event gather, and event gathers were then periodically telemetered to BSL and included directly into the NCEDC earthquake database (dbms) for analysis and processing.

Because of its mandate to detect and record very low magnitude events in the Parkfield area, the HRSN is extremely sensitive to changes in very low amplitude seismic signals. As a consequence, in addition to detecting very small local earthquakes at Parkfield, the HRSN also detects numerous regional events. For example spot checks of aftershocks following the M6.5 San Simeon earthquake of December 22, 2003 using continuous data and HRSN event detection listings have revealed that the overwhelming majority of HRSN detections following San Simeon resulted from seismic signals generated by San Simeon's aftershocks despite the HRSN's $\sim $ 50 km distance from the events. Data from the California Integrated Seismic Network (CISN) show that there were $\sim $ 1,150 San Simeon aftershocks with magnitudes $>$ 1.8 in the week following San Simeon, and during this same period, the number of HRSN event detections was $\sim $ 10,500 (compared to an average weekly rate before San Simeon of 115 detections) This suggests that despite the $\sim $ 50 km distance the HRSN is detecting San Simeon aftershocks well below magnitude 1.

Since the beginning of the network's data collection in 1987 and up until recently, the local and regional events were discriminated based on analyst assessment of S-P times, and only local events with S-P times less than $\sim $ 2.5 sec at the first arriving station were picked and located as part of the HRSN routine catalog. However, because of the large swarms of aftershocks from the San Simeon and M6 Parkfield earthquake of September 2004 and because of declining funding levels, this approach has had to be abandoned.

Current Processing. Subsequent to the M6.5 San Simeon earthquake on December 22, of 2003, our long-standing data handling procedure was no longer viable due to the enormous rate of San Simeon aftershock detections (Figures 41.2) In the first 5 months following the San Simeon mainshock, over 70,000 event detections were made by the HRSN system (compared to an average 5 month detection rate of 2500 prior to San Simeon). In the first month following the 28 September 2004 Parkfield M6 quake, over 40,000 detections were also made. Numerous additional (false) detections have also been occurring as a result of drilling activities associated with SAFOD drilling.

Figure 41.2: Shown are the number of HRSN triggers per hour for a period beginning with the San Simeon earthquake and continuing through several months after the Parkfield earthquake. For comparison, before these two large events, the average number of hourly HRSN triggers was less than 0.5 (i.e., about 10 per day). ``Eyeball" fits of the decay curves for both events are also shown. The cumulative number of HRSN triggers in the first 5 months following San Simeon exceeded 70,000, and trigger levels continued to be over 150 triggers per day through the Parkfield quake. In the first month following the Parkfield quake nearly 20,000 triggers were recorded. Extrapolation of the decay curve indicates that daily trigger levels will not return to near pre-San Simeon levels until well into 2007. At the same time, funding to support analyst's time for routine processing and cataloging of the events has virtually dried-up, requiring the development and implementation of a new scheme for cataloging events.
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The dramatic increase in event detections vastly exceed the HRSN's capacity to process both the continuous and triggered event waveform data. To prevent the loss of seismic waveform coverage, processing of the triggered waveform data has been suspended to allow archiving of the 250 sps continuous data to tape to continue uninterrupted. Cataloging of the event detection times from the modified REDI real-time system algorithm is also continuing, and the continuous 250 sps waveform data is currently being periodically uploaded from the DLT tape archive onto the NCEDC for access to the research research community.

Funding to generate catalogs of local events from the 10's of thousands of aftershock detections has not been forthcoming, and as a consequence major changes in our approach to cataloging events have had to be implemented, which involves integration of HRSN data into NCSN automated event detection and cataloging (with no analyst review) combined with a high resolution procedure now being developed to automatically detect, pick, locate and determine magnitudes for similar and repeating events down to very low magnitudes (i.e., below magnitude -1.0Ml). These new schemes are discussed in more detail in the activities section below.

2005-2006 Activities

In addition to the routine operations and maintenance of the HRSN (California's first and longest operating borehole seismic network), research into: 1) How to process the enormously increased rate of network detections 2) similar and repeating aftershocks from the 28 September 2004 Parkfield M6 earthquake, 3) ongoing non-volcanic tremors in the Parkfield-Cholame area and 4) SAFOD related activities have been the primary driving forces behind most of the HRSN project's activities this year.

Operations and Maintenance

Routine maintenance tasks required this year to keep the HRSN in operation, include cleaning and replacement of corroded electrical connections, grounding adjustments, cleaning of solar panels, re-seating, resodering and replacement of faulty pre-amp circuit cards, the testing and replacement of failing batteries, and insulation and painting of battery and datalogger housings to address problems with low power during cold weather.

Remote monitoring of the networks health using the Berkeley Seismological Laboratory's SeisNetWatch software are also performed to identify both problems that can be resolved over the Internet (e.g. rebooting of data acquisition systems due to clock lockups) and more serious problems requiring field visits.

Over the years, such efforts have paid off handsomely by providing exceptionally low noise recordings (Figure 41.3) of very low amplitude seismic signals produced by microearthquakes (below magnitude 0.0Ml) and nonvolcanic tremors (Nadeau and Dolenc, 2005).

Figure 41.3: Typical background noise PSD for the 250 sps vertical component channels of the HRSN borehole stations as a function of frequency. The data are from 2 AM Local time on 6/18/2006 (Sunday morning). Note the relatively low PSD levels and the overall consistency for all the HRSN stations. The 2 Hz minimum for the sensors occurs because of the 2 Hz sensors used at these sites. Below 2 Hz, noise levels rise rapidly and the peak at 3 sec (.3 Hz) is characteristic of teleseismic noise observed throughout California. EADB, GHIB and SCYB have a 60 Hz noise peak in the PSD, which is indicative of a ground loop problem. The PSD (dB) ranking of the stations of the stations at 2.9 Hz (near minimum PSD for most of the stations) is:
SCYB.BP.DP1 -171.05231
LCCB.BP.DP1 -170.58481
MMNB.BP.DP1 -168.70798
JCNB.BP.DP1 -167.85416
EADB.BP.DP1 -165.73283
SMNB.BP.DP1 -164.71182
FROB.BP.DP1 -163.79599
CCRB.BP.DP1 -163.56433
GHIB.BP.DP1 -161.44427
VCAB.BP.DP1 -159.84996
RMNB.BP.DP1 -156.86127
VARB.BP.DP1 -154.02579
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Enhancing HRSN Performance

Detection, monitoring, and high-resolution recording of low-amplitude seismic signals (e.g., nonvolcanic tremors and earthquakes down to the smallest possible magnitude) with the highest possible signal-to-noise (especially in the region of SAFOD drilling) are major objectives of the HRSN data collection effort. The minimization of data loss due to station outages and data-dropouts is also critical to these objectives.

Over the previous several years, we have had a serious decline in the robustness of the power system components (primarily the aging solar panels and batteries that have been in use since initiation of the network in 1987) of the network. Simultaneous outages at multiple stations are now becoming an all too frequent occurrence and are seriously affecting efforts to monitor tremor and micro- and repeating earthquake activity in the Parkfield area.

For example, during the winter of late 2004/early 2005, monitoring for nonvolcanic tremor activity using a standard detection set of 8 HRSN channels revealed significant (and sometimes catastrophic) gaps in the data. Figure 41.4 illustrates the seriousness of the problem with an example from tremor monitoring during periods of overcast weather. During the 7 day period shown, all 8 stations used for monitoring tremor activity were out simultaneously for over 50% of the time. The remaining 50% of the time, outages occurred for at least some of these 8 stations, resulting in significantly degraded capability for unambiguous detection of the low-amplitude tremor activity.

Figure 41.4: Stacked root-mean-square seismograms for the 8 stations of the HRSN used in monitoring tremor activity. Shown are 7 days of data starting at Hour 00 (UTC) of day 7 of 2005. Times when relative RMS amplitudes (REL-AMP) are 1.0 indicate periods when all 8 stations were out simultaneously.
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As suspected, further investigation, both remotely and on site, showed that these gaps occurred due to insufficient battery re-charge at many of the network's stations, which are remote solar powered installations. In previous years, similar but less severe data gaps have occurred during the winter months and have been attributed to overcast skies during the rainy season. In the winter of 2005 exceptionally heavy rainy season exacerbated the outage problem to an intolerable level, and to avoid a potential repeat of the situation, efforts were undertaken to refurbish and upgrade the solar power systems.

Specifically, the following steps were and continue to be taken:

1) replacement of the oldest batteries and switching of the remaining old batteries to the less power consuming pre-amplifiers;

2) improvement of the wiring scheme along the lines suggested by the solar power representative;

3) upgrade/replacement of solar panels. (Solar panels degrade at $\sim $ 1% per year, and newer versions have improved output. Since the installation of the HRSN over 18 years ago, the same size/format panel has gone from 40 watts to 55). This is a relatively easy field task, and should gain us 20-30% capacity at each site.

Among the three newer sites (CCRB, SCYB, LCCB), both the batteries and solar panels are relatively new. Nonetheless, stations CCRB and LCCB both had some outages last winter, which is most likely explained by the limited sunlight in these areas due to hilly terrain. We have, therefore, added one more solar panel at each of these sites to enhance their power system robustness.

The table shown in figure 41.4 summarizes the tasks of the power system upgrade effort, and shows the state of completion of the tasks as of the end of 2005. To date all tasks have now been completed.

Figure 41.5: Table of power upgrade tasks undertaken since early 2005. Red indicates tasks yet to be completed as of the end of 2005. These tasks have now been completed, and as expected data drops out and gaps that had plagued the network during the winter months have been effectively eliminated.
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Tremor Monitoring

The HRSN data played an essential role in the discovery of nonvolcanic tremors along the San Andreas Fault (SAF) below Cholame, CA (Nadeau and Dolenc, 2005). This location occupies a critical location between the smaller Parkfield ($\sim $ M6) and much larger Ft. Tejon ($\sim $ M8) rupture zones of the SAF. Because the time-varying nature of tremor activity is believed to reflect time-varying deep deformation and presumably episodes of accelerated stressing of faults, and because an anomalous increase in the rate of Cholame tremor activity preceded the 2004 Parkfield M6 by $\sim $ 21 days, we are continuing to monitor the tremor activity observable by the HRSN to look for anomalous rate changes that may signal an increased likelihood for another large SAF event to the SE. Results of monitoring effort are described further in the "Research" section of this report.

High Resolution Similar Event Catalog

As described in the "Data Flow" section above, circumstances relating to the dramatic increase in HRSN event detections spawned by larger earthquakes and by SAFOD drilling activity have required new thinking on how to catalog microearthquakes detected by the HRSN. One action taken to help address this problem has been to integrate HRSN data streams into the NCSN event detection and automated cataloging process (described below).

This approach has been successful at discriminating small events in the local Parkfield area from other types of event detections and for providing automated locations of a significantly increased number of small events in the local area (approx. double that of the NCSN network alone). However, the rate of local events from the HRSN sensitized NCSN catalog is still only catching about 1/2 the number of local events previously cataloged by the HRSN, and waveforms for the small events are not typically made available. In addition, unlike the previous HRSN catalog, the additional events added by the NCSN-HRSN integration are not reviewed by an analyst nor do they generally have magnitude determinations associated with them. In some cases, the selection rules used for the integrated catalog also result in exclusion of events that are otherwise included by the NCSN.

These limitations severely hamper efforts relying on similar and characteristically repeating microearthquakes. They also reduce the effectiveness of research relying on numerous very small magnitude events in the SAFOD zone (e.g. for targeting the SAFOD targets).

To help overcome these limitations, we have embarked on an effort to develop an automated similar event cataloging scheme based on cross-correlation and pattern scanning of the continuous HRSN data now being archived. The method uses a small number of reference events whose waveforms, picks, locations, and magnitudes have been accurately determined, and it automatically detects, picks, locates and determines magnitudes for events similar to the reference event to the level of accuracy and precision that only relative event analysis can bring.

The similar event detection is also remarkably insensitive to the magnitude of the reference event used, allowing similar events ranging over several magnitude units to be fully cataloged using a single reference event. It also does a remarkably good job even when seismic energy from multiple events is superposed. Once a cluster of similar events has been cataloged, it is a relatively straight forward process to identify characteristically repeating microearthquake sequences within the cluster (frequently a single similar event "cluster" will contain several sequences of repeating events).

Application of the method using one of the SAFOD target events as a reference is illustrated in Figure 41.6. The magnitude of the reference event is $\sim $ 2.2. This event was scanned through 5 years of continuous data, and 67 other events occurring within a zone of $\sim $  150 m were detected (including 3 very small quakes that were not even by the HRSN REDI-type system). The magnitudes of these events ranged down to magnitude -1.2 Ml. In addition to the SAFOD target sequence from which the reference was derived, several other repeating sequences within the 150m zone were also identified (5 of which had not previously been known to exist).

Figure 41.6: Map (top) and along fault depth section (bottom) views of double-difference locations resulting from application of the similar event pattern scanning and automated cataloging method using one of the SAFOD target events (green circles) as a reference. The magnitude of the reference event is $\sim $ 2.2. This event was scanned through 5 years of continuous data, and 67 other events occurring within a zone of $\sim $  150 m were detected (including 3 v. small quakes that were not even by the HRSN detection scheme). The magnitudes of the 67 events ranged from 2.2 down to -1.2 Ml. In addition to the SAFOD target sequence from which the reference was derived, several other repeating sequences within the 150m zone were also identified (5 of which had not previously been known to exist).
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The procedure is still being refined to capture even smaller events, events over a larger area and for increased processing speed. Eventually, a composite catalog of similar event groups from throughout the HRSN coverage zone is planned.

The approach also holds promise in other applications where automated and precise monitoring of bursts of seismic activity to very low magnitudes is desirable (e.g. in aftershock zones or in volcanic regions) or where automated updates of preexisting repeating sequences and their associated deep slip estimates are desired.

Efforts in Support of SAFOD

An intensive and ongoing effort by the EarthScope component called SAFOD is underway to drill through, sample and monitor the active San Andreas Fault at seismogenic depths and in very close proximity (within a few 10's of km or less) of a repeating magnitude 2 earthquake site. The HRSN data plays a key role in these efforts by providing low noise and high sensitivity seismic waveforms from active and passive sources, and by providing a backbone of earthquake and tremor detection and continuous waveform data from the numerous microearthquakes and tremors that are occurring in the general vicinity of SAFOD.

At this stage SAFOD drilling has penetrated the fault with a sub-horizontal hole slightly beneath the SAFOD target sequences, and current efforts have been focused on obtaining final estimates of the targets relative location to the existing hole to accuracies of meters if possible. This high degree of accuracy is required in order to target accurately three multi-lateral side cores for sampling and monitoring within the final target zone.

HRSN Activities this year have contributed in three principal ways to these and longer-term SAFOD monitoring efforts:

1) In collaboration with the USGS, we have integrated the 7 vertical HRSN channels telemetered from Parkfield into the NCSN triggering scheme (described above) to increase the sensitivity of NCSN detection in the SAFOD area. This has effectively doubled the number of small events the target location working group has for constraining the relative location of the target sequences.

2) Again in collaboration with the USGS, we have nearly completed a telemetry upgrade that will allow all 38 channels of the HRSN data (both 20 sps and 250 sps data streams) to flow directly from Parkfield, through the USGS Menlo Park processing center, and also to the BSL for near-real-time processing and archiving on the web based NCEDC. This will provide near immediate access of the HRSN data to the community without the week's to month's delay associated with having to transport DLT tapes to Berkeley, upload, and quality check the data.

3) We have also applied our prototype similar event automated catalog approach to the primary and two secondary SAFOD target zones and were able to provide the SAFOD event location working group with rapid and precise double-difference and relative magnitude catalogs of 82 similar events in the zone immediately surrounding target region occurring between 2001 day 178 and 2006 day 218 (August 6 of this year).

Figure 41.6 shows the double difference locations and estimated rupture dimensions (based on Nadeau and Johnson, 1998) of 67 of these events that were derived using one event from the SAFOD primary target sequence as the reference. Other primary target events are shown in green, and events from a secondary target located $\sim $ 40 m to the southeast are shown in blue. Several other suspected repeating sequences can be seen as tight clusters of similarly sized events. We are in the process of confirming these events as characteristically repeating sequences members.

The SAFOD similar event catalogs are now being used by the working group to extract data from the corresponding PASO array, Pilot Hole, NCSN and mainhole data sets for integration with the HRSN data to provide as much and as detailed information as possible in the final push at locating the target sequence for the lateral side core drilling.

Acknowledgments

Thomas V. McEvilly, who passed away in February 2002, was the PI on the HRSN project for many years. Without his dedication, continued operation of the HRSN would not have been possible. Under Bob Nadeau's and Doug Dreger's general supervision, Rich Clymer, Bob Uhrhammer, Doug Neuhauser, Don Lippert, Bill Karavas, John Friday, Rick Lellinger and Pete Lombard all contribute to the operation of the HRSN. Bob Nadeau prepared this chapter. During this reporting period, operation, maintenance, and data processing for the HRSN project was supported by the USGS, through grant 05HQGR0080.

References

Bakun, W. H., and A. G. Lindh, The Parkfield, California, prediction experiment, Earthq. Predict. Res., 3, 285-304, 1985.

Daley, T.M. and T.V. McEvilly, Shear wave anisotropy in the Parkfield Varian Well VSP, Bull. Seism. Soc. Am., 80, 857-869, 1990.

Hickman, S., M.D. Zoback and W. Ellsworth, Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth, Geophys. Res. Lett., 31, L12S01, doi:10.1029/2004GL020688, 2004.

Karageorgi, E., R. Clymer and T.V. McEvilly, Seismological studies at Parkfield. IV: Variations in controlled-source waveform parameters and their correlation with seismic activity, 1987-1994, Bull. Seismol. Soc. Am., 87, 39-49, 1997.

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.

Nadeau, R.M. and D. Dolenc, Nonvolcanic Tremors Deep Beneath the San Andreas Fault, SCIENCE, 307, 389, 2005.

Nadeau, R. M., and L. R. Johnson, Seismological Studies at Parkfield VI: Moment Release Rates and Estimates of Source Parameters for Small Repeating Earthquakes, Bull. Seismol. Soc. Amer., 88, 790-814, 1998.

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