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 3.13 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 2006 through December 2007, 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 of micro-earthquakes with magnitudes 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), 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: 3.13 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 double-difference relocations of nonvolcanic tremors in the Cholame, CA area (January 2006 through December 2007), 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://www.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 (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

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. 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. Since July 25, 2001, 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 -          


The 3 newest borehole stations 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 3.13 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 newest SAFOD 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; 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 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 have been in the process of reducing our dependence on that site, and, as of this report, data from three of the stations previously telemetering through Gastro Peak have been routed through an alternative site and Hogs Canyon (HOGS) (See ``2007-2008 Activities,'' this section).

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 3.14 are P-wave seismograms of the deep focus $M_{w}$ 7.7 earthquake in the Sea of Okhotsk (Lat.: 53.8920; Lon.: 152.8840) occurring on July 5, 2008 02:12:04 (UTC) (6688 km from Parkfield, CA; depth 636 km) recorded on the DP1 (vertical) channels of the 12 HRSN borehole stations in operation at the time. No casualties were reported from this event. 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. All station waveforms in the plots are ordered by distance.

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
GHIB.BP.DP1 - digitizer bit noise - no seismic response
LCCB.BP.DP1 - poor response
LCCB.BP.DP1 - poor 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.

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 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 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.

Figure 3.14: Plot of P-wave seismograms of the deep focus $M_{w}$ 7.7 earthquake in the Sea of Okhotsk (Lat.: 53.8920; Lon.: 152.8840; 6688 km from Parkfield, CA; depth 636 km) occurring on July 5, 2008 02:12:04 (UTC) recorded on the DP1 (vertical) channels of the 12 HRSN borehole stations in operation at the time. All station waveforms in the plots are ordered by distance from the earthquake. Here, vertical component geophone (velocity) data have been 0.1-0.5 Hz bandpass filtered.
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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. Cataloging of the event detection times from the modified REDI real-time system algorithm is also continuing, 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 -1.0Ml). These new schemes are discussed in more detail in the activities section below.

2007-2008 Activities

In addition to the routine operations and maintenance of the HRSN (California's first and longest operating borehole seismic network), research into: a) how to process ongoing similar and repeating seismicity to very low magnitudes, b) ongoing non-volcanic tremors in the Parkfield-Cholame area, c) SAFOD related activities, and d) various approaches to lowering operational (primarily landowner fee) costs 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, resoldering, and replacement of 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 0.0Ml) and nonvolcanic tremors.

Also in the spring of this year, the central site processing computer at our CDF site failed. We had previously obtained funds to purchase a back-up computer at the site, and these funds were used to purchase a new computer which is now installed and operating properly. During the failure, data was telemetered directly over the T1 line, resulting in relatively little data loss. Now, with the computer processing reestablished, backup support in case of telemetry failures is once again in place.

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$100 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 100 foot section of the borehole to provide continued seismic coverage at the JCNB site.

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 3.15 are power spectral density (PSD) plots of background noise for vertical components of the HRSN for a 30 minute period beginning at 2 AM local time on day 7/31/2008 (Thursday morning). By periodically generating such plots, we can rapidly evaluate the network's recording of seismic signals across the wide 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 effecting the quality of the recorded seismograms.

Notable in Figure 3.15 are the relatively low PSD levels and overall consistency for most of the HRSN stations. One exception is the relatively high PSD for station LCCB's DP1 channel, which at the time of the PSD analysis was experiencing serious spiking and elevated noise levels across the entire spectrum. Also notable on the DP1 channels for stations GHIB and SCYB are 60 Hz noise peaks, which are indicative of ground loop problems. Noise peaks for station RMNB can also be seen at 15 Hz and 30 and 60 Hz harmonics. These spectral peaks are not always present but occur for about 4 hours at night every other day or so when the Southern California Gas company's generator kicks in to supplement the charging of batteries for their otherwise solar powered installation located about 30 m from the RMNB site.

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.

Reducing Operational costs

The 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 have had to be cut back, and network performance has suffered.

To help alleviate the problem, this year we have begun implementing plans to minimize our dependence on access to private lands. This has primarily involved establishing alternative telemetry paths for HRSN sites with a minimum of additional effort and equipment. Central to this effort has been reaching cooperative agreements with other agencies involved in research in the area (i.e., the USGS and UNAVCO).

To date, telemetry paths for three HRSN sites (SCYB, CCRB, and SMNB) have been redirected from the Gastro Peak relay site to an alternative relay site at Hogs Canyon (HOGS) through an agreement with the USGS. Plans to redirect telemetry of an additional 4 sites from Gastro Peak (MMNB,VARB,LCCB, and GHIB) through Mine Mountain are now being field tested, and, if proven sound, negotiations with UNAVCO and the Mine Mountain landowner will be undertaken and infrastructure for the alternative paths will be installed.

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 3.13). 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. Some recent results of the monitoring effort are described further in the ``Research'' section of this report.

Figure 3.15: Background noise Power Spectral Density (PSD) levels for the 250 sps vertical component channels (DP1) of the HRSN borehole stations as a function of frequency. The data are from 2 AM Local time on 7/31/2008 (Thursday morning). The vertical (DP1) channels for stations CCRB and MMNB were out during this period. 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 5 to 3 sec (.2 to .3 Hz) is characteristic of teleseismic noise observed throughout California. The PSD (dB) ranking at 3 Hz (intersection with vertical line) for the vertical channels in operation at the time of the analysis was:
SCYB.BP.DP1 -171.30859
FROB.BP.DP1 -167.66554
SMNB.BP.DP1 -165.58936
EADB.BP.DP1 -164.69122
JCSB.BP.DP1 -162.62303
RMNB.BP.DP1 -162.25391
GHIB.BP.DP1 -161.32861
VCAB.BP.DP1 -157.98297
VARB.BP.DP1 -156.12631
LCCB.BP.DP1 -144.19135
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Similar Event Catalog

The increased microseismicity rates resulting from the San Simeon M6.5 and Parkfield M6 events and the increased interest in even smaller events in the SAFOD target zone 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 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 (approximately double that of the NCSN network alone). However, 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 ongoing 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 monitoring seismicity in the SAFOD target region).

To help overcome these limitations, we are continuing our efforts to develop and implement our 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 straightforward process to identify characteristically repeating microearthquake sequences within the cluster (frequently a single similar event ``cluster'' will contain several sequences of repeating events).

This high level of precision and low magnitude completeness has already proven useful to SAFOD for helping to delineate and constrain the active fault structure in the target zone (see, ''Efforts in Support of SAFOD'', below). It has also proven vital this year for helping to resolve a long-standing debate in the seismologic community regarding the stress-drop scaling issues by providing pairs of nearly collocated events with similar waveforms but significantly differing magnitudes for use in kinematic slip inversions using an eGf approach (Dreger et al., 2007).

This year, the automated cataloging procedure for similar events is continuing to be refined to capture even smaller events and events over a larger area, as well as for increased processing speed. Eventually, a composite catalog of similar event groups from throughout the HRSN coverage zone is planned.

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.

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) 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 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.

During the final push to penetrate the repeating SAFOD target last Fall, our SAFOD similar event detections and catalogs were also 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 in order to provide the detailed information that was needed by drill crews for the final targeting of the HI target penetration and coring.

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, Rick Lellinger, Rich Clymer, Bob Uhrhammer, Doug Neuhauser, Peter Lombard, Bill Karavas, John Friday and Don Lippert all contribute to the operation of the HRSN. Bob Nadeau prepared this section. 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.

Dreger, D., R.M. Nadeau, and A. Morrish, Repeating Earthquake Finite-Source Models: Strong Asperities Revealed on the San Andreas Fault, Geophys. Res. Lett., revised version submitted, 2007.

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.

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