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 3.14 shows the location of the network, its relationship to the San Andreas fault, sites of significance from previous and ongoing experiments 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 27 July 2001 through 21 February 2009, 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 3.8), its high-frequency wide bandwidth recordings (0-100 Hz; 250 sps), and its sensitivity to very low amplitude seismic signals (e.g., recording signals from micro-earthquakes with magnitudes below magnitude $0.0$ $M_L$).

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), a long-term HRSN seismicity catalogue (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 relatively homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90$\%$ right-lateral strike-slip).

Figure 3.14: 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 nonvolcanic tremors in the Cholame, CA area (27 July 2001 through 21 February 2009), 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, the cumulative, often unexpected, results of UC Berkeley's HRSN research efforts (see: http://seismo.berkeley.edu/seismo/faq/parkfield_bib.html) 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.

The Parkfield area has also become an area of focus of the EarthScope Project (http://www.earthscope.org) through the deep borehole into the San Andreas Fault, the SAFOD experiment (http://www.earthscope.org/observatories/safod), 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

Installation of the HRSN deep (200-300m) borehole sensors initiated in late 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. Originally a 10 station network, completed in 1988, the HRSN was expanded to 13 borehole stations in late July 2001, and the original recording systems (see previous Berkeley Seismological Laboratory (BSL) Annual Reports) were upgraded to 24 bit acquisition (Quanterra 730s) and 56K frame relay telemetry to UCB. Properties of the sensors are summarized in Table 3.9.


Table 3.8: 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 is also given. 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 3.9: Instrumentation of the Parkfield HRSN. Most HRSN sites have L22 sensors and were originally digitized with a RefTek 24 system. The WESCOMP recording system failed in mid-1998 and after an approximate 3 year hiatus the network was upgraded and recording was replaced with a new 4-channel system. The new system, recording since July 27, 2001, uses a Quanterra 730 4-channel system. Three new stations were also added during the network upgrade period (bottom) with horizontal orientations that are approximately N45W and N45E. More accurate determinations of these orientations will be made as available field time permits.
Site Sensor Z H1 H2 RefTek 24 Quanterra 730            
EADB Mark Products L22 -90 170 260 01/1988 - 06/1998 03/2001 -            
FROB Mark Products L22 -90 338 248 01/1988 - 06/1998 03/2001 -            
GHIB Mark Products L22 90 failed unk 01/1988 - 06/1998 03/2001 -            
JCNB Mark Products L22 -90 0 270 01/1988 - 06/1998 03/2001 -            
JCSB Geospace HS1 90 300 210 01/1988 - 06/1998 03/2001 -            
MMNB Mark Products L22 -90 175 265 01/1988 - 06/1998 03/2001 -            
RMNB Mark Products L22 -90 310 40 01/1988 - 06/1998 03/2001 -            
SMNB Mark Products L22 -90 120 210 01/1988 - 06/1998 03/2001 -            
VARB Litton 1023 90 15 285 01/1988 - 06/1998 03/2001 -            
VCAB Mark Products L22 -90 200 290 01/1988 - 06/1998 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 -            


The 3 newest borehole stations (CCRB, LCCB, and SCYB) were added, 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 3.14 illustrates the location of the drill site, the new borehole sites, and locations of earthquakes recorded by the initial and upgraded/expanded HRSN.

These 3 new stations have a similar configuration to the original upgraded 10 station network and include an additional channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 3.8 and 3.9. All the HRSN data loggers employ FIR filters to extract data at 250 and 20 Hz (Table 3.10).


Table 3.10: Data streams currently being acquired at operational HRSN sites. Sensor type, channel name, sampling rate, sampling mode, and type of FIR filter are given. C indicates continuous; Ac acausal; Ca causal. ``?" indicates orthogonal, vertical, and 2 horizontal components.
Sensor Channel Rate (sps) Mode FIR
Geophone DP? 250.0 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 our facility on the California Department of Forestry's (CDF) property in Parkfield. 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. Prior to June, 2008, six of the sites transmitted directly to a router at the central recording site. The other seven sites transmitted 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. Due to disproportionately increasing landowner fees for access to the Gastro Peak site, we reduced our dependence on that site in the summer and fall of 2008 in cooperation with the USGS, and, as of this report, data from five of the stations previously telemetering through Gastro Peak have been re-routed through an alternative site at Hogs Canyon (HOGS).

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 event triggers with better timing accuracy and is also now recording continuous 20 and 250 sps data for all channels of the HRSN, which flow seamlessly into both the USGS automated earthquake detection system and into Berkeley's NCEDC for archiving and online access to the community. 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 (1987-2001) recorded SEGY format).

Another 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 data loggers 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 data loggers to modify or restart processes when necessary.

The network connectivity and seamless data flow to the NCEDC also provides near-real-time monitoring capabilities that are useful for rapid evaluation of significant events as well as the network's overall performance level. For example, shown in Figure 3.15 are P-wave seismograms of the teleseismic $M_{w}$ 7.6 earthquake in the Tonga region (Lat.: 23.050S; Lon.: 174.668W; depth 34 km) occurring on March 19, 2009 18:17:40 (UTC) recorded on the DP1 (vertical) channels of the 11 HRSN borehole stations in operation at the time. The seismic data from the quake was telemetered to Berkeley and available for analysis by the Northern California Seismic System (NCSS) real-time/automated processing stream within a few seconds of being recorded by the HRSN.

This is a good signal source for examining the relative responses of the BP borehole network station/components to seismic ground motion, and these and corresponding waveform plots for the horizontal (DP2 and DP3 channels) indicate that the following stations were not responding normally to seismic ground motions at the time of this event:
JCSB.BP.DP2 - spiking - no seismic response
JCSB.BP.DP3 - digitizer bit noise - no seismic response
LCCB.BP.DP1 - no seismic response
LCCB.BP.DP2 - no seismic response
LCCB.BP.DP3 - no seismic response
JCNB.BP.DP1 - no seismic response
JCNB.BP.DP2 - no seismic response
JCNB.BP.DP3 - no seismic response
MMNB.BP.DP1 - low frequency drift - no response
MMNB.BP.DP2 - low frequency drift - no response
In addition, the ground velocities inferred from the two horizontal components at RMNB and the DP2 horizontal at VCAB are significantly higher than the corresponding ground velocities inferred from the other operating BP network horizontal components. By rapidly generating such plots following large teleseismic events, quick assessment of the HRSN seismometer responses to real events is easily done and corrective measures implemented with relatively little delay.

Figure 3.15: Plot of P-wave seismograms of the teleseismic $M_{w}$ 7.6 earthquake in the Tonga region (Lat.: 23.050S; Lon.: 174.668W; depth 34 km) occurring on March 19, 2009 18:17:40 (UTC) recorded on the DP1 (vertical) channels of the 11 HRSN borehole stations in operation at the time. Here, vertical component geophone (velocity) data have been 0.1-0.5 Hz bandpass filtered.
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Data Flow

Initial Processing Scheme. Continuous data streams on all HRSN components are recorded at 20 and 250 sps on disk on the local HRSN computer at the CDF facility. These continuous data are transmitted in near-real-time to the Berkeley Seismological Laboratory (BSL) over a T1 link and then archived at the NCEDC. In addition, the near-real-time data are being transmitted over the T1 circuit to the USGS at Menlo Park, CA, where they are integrated into the Northern California Seismic System (NCSS) real-time/automated processing stream. This integration has also significantly increased the sensitivity of the NCSN catalog at lower magnitudes, effectively doubling the number of small earthquake detections in the critical SAFOD zone.

Shortly after being recorded to disk on the central site HRSN computer, event triggers for the individual station data are also determined, and a multi-station trigger association routine then processes the station triggers and generates a list of potential earthquakes. For each potential earthquake that is detected, a unique event identification number (compatible with the NCEDC classification scheme) is also assigned. Prior to the 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 and relatively distant and small amplitude nonvolcanic tremor 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.

Current Processing. Since the beginning of the network's data collection in 1987, and up until 2002, 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 s at the first arriving station were picked and located as part of the HRSN routine catalog. However, because of the network's extreme sensitivity to the large swarm of aftershocks from the San Simeon and M6 Parkfield earthquakes of September 2004 (e.g., 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) and because of ever declining funding levels, this approach has had to be abandoned.

The dramatic increase in event detections vastly exceeded 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 the telemetry and archiving of the 20 and 250 sps continuous data to continue uninterrupted. Subsequent funding limitations have precluded reactivation of the processing of triggered waveform data. Cataloging of the event detection times from the modified REDI real-time system algorithm is continuing, however, and the continuous waveform data is currently being telemetered directly to the BSL and USGS over the T1 link for near-real-time processing and archiving at the NCEDC, for access to the research community.

Funding to generate catalogs of local events from the 10s of thousands of aftershock detections has not been forthcoming, and, as a consequence, major changes in our approach to cataloging events have been implemented. The HRSN data is now integrated into NCSN automated event detection, picking, and catalog processing (with no analyst review). In addition, a high resolution procedure is now being developed to automatically detect, pick, locate, double-difference relocate, and determine magnitudes for similar and repeating events down to very low magnitudes (i.e., below magnitude -1.0$M_L$). These new schemes are discussed in more detail in the activities section below.

2008-2009 Activities

In addition to the routine operations and maintenance of the HRSN (California's first and longest operating borehole seismic network), HRSN project activities this year have been focused on: a) processing of ongoing similar and repeating very low magnitude seismicity and integrating this information into network SOH (state of health) monitoring, b) lowering operational (primarily landowner fee) and catalog production costs, c) monitoring non-volcanic tremor activity in the Parkfield-Cholame area, and d) SAFOD related activities.

Routine 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, resoldering, and replacing faulty pre-amp circuit cards; testing and replacement of failing batteries; and insulation and painting of battery and data logger housings to address problems with low power during cold weather. Remote monitoring of the network's health using the Berkeley Seismological Laboratory's SeisNetWatch software is 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 of very low amplitude seismic signals produced by microearthquakes (below magnitude 0.0$M_L$) and nonvolcanic tremors.

In the spring of 2008, signals from HRSN station JCNB began showing signs of deterioration. Shortly thereafter, data flow from this station stopped completely. Field investigation showed that the borehole sensor and cable had been grouted to within $\sim$34 feet of the surface and that a rodent had found itself trapped in the upper 100 foot void space and chewed through the cable, thus severing the connection to the deep borehole package. At this time, costs for reestablishing connection to the cable at depth have been prohibitive, and it is also likely that the grouted-in sensor has been compromised by fluids running down the exposed cable. Hence, plans are being made to substitute either a surface seismometer or a borehole sensor package within the open 34 foot section of the borehole to provide continued seismic coverage at the JCNB site. A long-idle sensor package has been identified as a possible replacement and it is now being assessed by BSL's engineering group to confirm functionality.

The network connectivity over the T1 circuit also allows remote monitoring of various measures of the state of health of the network in near-real-time, such as background noise levels. Shown in Figure 3.16 are power spectral density (PSD) plots of background noise for the 12 operational vertical components of the HRSN for a 50 second period beginning at 2:41 AM local time on day 9/07/2009 (Monday morning). By periodically generating such plots, we can rapidly evaluate, through comparison with previously generated plots, changes in the network's station response of seismic signals across the wide band high-frequency spectrum of the borehole HRSN sensors. Changes in the responses often indicate problems with the power, telemetry, or acquisition systems, or with changing conditions in the vicinity of station installations that are adversely affecting the quality of the recorded seismograms.

Once state of health issues are identified with the PSD analyses, further remote tests can be made to more specifically determine possible causes for the problem, and corrective measures can then be planned in advance of field deployment within a relatively short period of time.

Figure 3.16: Background noise Power Spectral Density (PSD) levels as a function of frequency for the 12, 250 sps vertical component channels (DP1) of the HRSN borehole stations in operation during the 50 second period analyzed, beginning 2:41 AM local time on day 9/07/2009 (Monday morning). The approximate 2 Hz minimum of the PSD levels occurs because of the 2 Hz sensors used at these sites. Below 2 Hz, noise levels rise rapidly, and the peak at 5 to 3 sec (.2 to .3 Hz) is characteristic of teleseismic noise observed throughout California. The PSD (dB) ranking (lowest to highest) at 3 Hz (intersection with vertical line) for the vertical channels is:
CCRB.BP.DP1 -176.92
LCCB.BP.DP1 -172.36
SCYB.BP.DP1 -131.14
FROB.BP.DP1 -126.88
SMNB.BP.DP1 -124.26
RMNB.BP.DP1 -124.00
GHIB.BP.DP1 -122.93
EADB.BP.DP1 -118.79
MMNB.BP.DP1 -116.03
VCAB.BP.DP1 -114.06
JCSB.BP.DP1 -113.11
VARB.BP.DP1 -108.92
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Similar Event Catalog

The increased microseismicity (1000's of events) resulting from the San Simeon M6.5 (SS) and Parkfield M6 (PF) events, the lack of funds available to process and catalog the increased number of micro-earthquakes, and the increased interest in using the micro-quakes in repeating earthquake and SAFOD research have required new thinking on how to detect and catalog microearthquakes recorded 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. This approach has been successful at detecting and locating a significantly greater number of micro-earthquakes over the previous NCSN detection and location rate (essentially doubling the number of events processed by the NCSN). However, the HRSN sensitized NCSN catalog is still only catching about 1/2 the number of local events previously cataloged by the HRSN using the old HRSN-centric processing approach. Furthermore, triggered waveforms for the additional small NCSN processed events are not typically made available; they are not reviewed by an analyst, nor do they generally have NCSN magnitude determinations associated with them.

These limitations severely hamper research efforts relying on similar and characteristically repeating micro-seismicity such as earthquake scaling studies, SAFOD related research, deep fault slip rate estimation, and the compilation of recurrence interval statistics for time-dependent earthquake forecast models. They also reduce, to some degree, the use of recurring micro-seismicity as a tool for monitoring the state-of-health (SOH) of either the HRSN or NCSN.

To help overcome these limitations this year, we have further developed and are in the process of implementing an automated similar event cataloging scheme based on pattern matching (match filter) scans using cross-correlation of the continuous HRSN data. The method uses a set 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 micro-events ranging over about 3 magnitude units to be fully cataloged using a single reference event, and it does a remarkably good job at discriminating and fully processing multiple superposed events.

Once a cluster of similar events has been processed, an additional level of resolution can then be achieved through the identification and classification of characteristically repeating microearthquakes (i.e., near identical earthquakes) occurring within the similar event family (Figure 3.17). The pattern scanning approach also ensures optimal completeness of repeating sequences owing to scans of the matching pattern through all available continuous data (critical for applications relying on recurrence interval information). For example, while the March 3 and April 28, 2006 events shown in Figure 3.17 were not picked up by the NCSN-HRSN integrated network, the pattern scanning approach we employ picked up both of these earthquakes.

It is immediately apparent from Figure 3.17 that on March 3, 2006, the DP1 channel was experiencing significant high amplitude step-decay spiking (due to pre-amp malfunction) and that on August 22, 2008, the signal amplitude was greatly attenuated (due to excess tension and separation of the signal cable wiring). Armed with this type of information, field engineers can quickly identify and address major problems. In addition to a visual assessment, the extreme similarity of the events lends itself to the application of differencing techniques in the time and frequency domains to automatically identify detailed SOH issues on all channels within a network.

Repeating sequences of this magnitude typically repeat every 1 to 2 years, and we are currently monitoring 25 of these sequences. Hence, on average, evaluations of this type can be made approximately every month on an automated basis. However, there are on the order of 200 such sequences known in the Parkfield area, leaving the possibility that automated SOH analyses could take place every 2 to 3 days.

For other networks recording continuously in the Parkfield area (e.g., NCSN, BDSN) it is also a relatively simple process to extend the SOH analysis using characteristic repeating events to their stations. Furthermore, numerous repeating event sequences are also know to exist in the San Francisco Bay and San Juan Bautista areas, where continuous recording takes place. Hence application of the repeating event SOH technique to these zones should also be feasible.

Figure 3.17: Ten most recent repeats of a characteristic sequence of repeating magnitude 0.9 (Mp, USGS preferred magnitude) microearthquakes recorded by vertical (DP1) channel of HRSN station MMNB. High-precision location and magnitude estimates of these events show they are extremely similar in waveform (typically 0.95 cross-correlation or better), nearly collocated (to within 5-10 m) and of essentially the same magnitude (+/- 0.13 Mp units). The dashed line labeled ``NEXT'' serves to illustrate that events in these types of sequences continue to repeat and that they can, therefore, be used for monitoring ongoing channel response relative to past performance.
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We are continuing to expand the number of pattern events and resulting multi-year scans to increase the frequency of sampling of similar and repeating event sequences for SOH purposes and for expanding the catalog of very small similar and repeating microearthquakes (down to Mp of -0.5). We are also adapting the codes to take advantage of faster computing now available.

Further development of the similar event processing 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.

Reducing Operational costs

In recent years increased scientific activity in the rural Parkfield area due to SAFOD has lead to an increased demand for site access and development on privately owned property and a corresponding increase in access fees charged by private land owners. As a result, land use fees paid by the HRSN project have increased dramatically from less than $\$1000$ annually prior to the SAFOD effort to over $\$14,000$. This represents over $15\%$ of the entire HRSN budget, with no corresponding increase in support from the project's funding agency. To compensate for the increased landowner costs, maintenance efforts had to be cut back, and, as a result, network performance suffered.

To help alleviate the problem, this year we implemented (through cooperation with the USGS) plans to minimize our dependence on access to private lands. This primarily involved establishing alternative telemetry paths for roughly half of the HRSN sites.

To date, telemetry paths for five HRSN sites (SMNB, CCRB, MMNB, VARB, and SCYB) have been redirected from the Gastro Peak relay site to an alternative relay site at Hogs Canyon (HOGS) through an agreement with the USGS. Telemetry of GHIB data has also been redirected from Gastro Peak through an alternative path. Plans to redirect telemetry of an additional site from Gastro Peak (LCCB) are being examined and field tested for viability. This year, the landowner also chose not to renew our access agreement for Gastro-Peak, saving us approx. $\$9800$ in annual fees. However, the owner has allowed us to continue operating one station (RMNB) located at the Gastro-Peak site free of charge for an unspecified period of time. Until alternative telemetry is implemented, the RMNB station is also serving as as a repeater for station LCCB.

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). The Cholame tremors occupy a critical location between the smaller Parkfield ($\sim$M6) and much larger Ft. Tejon ($\sim$M8) rupture zones of the SAF (Figure 3.14). Because the time-varying nature of tremor activity is believed to reflect time-varying deep deformation and presumably episodes of accelerated stressing of faults, because an anomalous increase in the rate of Cholame tremor activity preceded the 2004 Parkfield M6 by $\sim$21 days, and because periodic episodes and continued elevated tremor activity have continued since the 2004 Parkfield event in the Parkfield-Cholame and Monarch Peak areas (Nadeau and Guilhem, 2009), 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 in the region. Some recent results of the monitoring effort are described further in the ``Research'' section of this report.

Efforts in Support of SAFOD

An intensive and ongoing effort by the EarthScope component called SAFOD (San Andreas Fault Observatory at Depth) is underway to drill through, sample, and monitor the active San Andreas Fault at seismogenic depths and in very close proximity (within a few 10s of km or less) to 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 very small earthquake detections and continuous waveform data.

As of early September, 2007, SAFOD drilling had penetrated the fault near the HI repeating target sequence and collected core samples in the fault region that presumably creeps and surrounds the repeatedly rupturing HI patch. Unfortunately, due to complications during drilling, penetration and sampling of the fault patch involved in repeating rupture was not possible. Future efforts will be focused on long-term monitoring of the ongoing chemical, physical, seismological, and deformational properties in the zone (particularly any signals that might be associated with the next repeat of the SAFOD repeating sequences).

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

1) Integration and processing of the HRSN data streams with those from the NCSN in the Parkfield area continues, effectively doubling the number of small events available for monitoring seismicity in the target zone and for constraining relative locations of the ongoing seismic activity.

2) Telemetry of all HRSN channels (both 20 and 250 sps data streams) continues to flow directly from Parkfield, through the USGS Parkfield T1 and the NCEMC T1, to the USGS and the BSL for near-real-time processing, catalog processing, and data archiving on the web-based NCEDC. This also provides near immediate access of the HRSN data to the SAFOD community without the week- or month-long delay associated with the previous procedure of having to transport DLT tapes to Berkeley to upload and quality check the data.

3) We have also continued to apply our prototype similar event automated catalog approach to the primary, secondary, and tertiary SAFOD target zones as a continued effort to monitor the SAFOD target zone activity at very high relative location precision.

These efforts and the free access of HRSN waveform data to the SAFOD seismology group confirmed the latest repeat of the HI sequence on Aug 29 of 2008. Our monitoring efforts were also the first to report repeats of the SF and LA sequences occurring on December 19 and 20, 2008, respectively. Of particular interest were the SF and LA repeats which were recorded on the SAFOD main hole seismometer which had been installed in October.

Acknowledgments

Under Robert Nadeau's and Doug Dreger's general supervision, Rick Lellinger, Rich Clymer, Bob Uhrhammer, Taka'aki Taira, Doug Neuhauser, Peter Lombard, Bill Karavas, and John Friday all contribute to the operation of the HRSN. Bob Nadeau prepared this section with help from Bob Uhrhammer and Taka'aki Taira. During this reporting period, operation, maintenance, and data processing for the HRSN project was supported by the USGS, through grant 07HQAG0014.

References

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

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.

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 A. Guilhem, Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes, SCIENCE, 325, 191, 2009.

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