Parkfield Borehole Network

Introduction

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

Figure 5.1 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 1987-1998.5, routine locations of seismicity since August 2002, and the epicenter of the 1966 M6 earthquake that motivated the PPE. The HRSN records exceptionally high-quality data, owing to its 13 closely spaced three-component borehole sensors (generally emplaced in the extremely low attenuation and background noise environment at 200 to 300 m depth (Table 5.1)), its high-frequency wide bandwidth recordings (0-100 Hz), and its low magnitude detection threshold (below magnitude -1.0).

Several aspects of the Parkfield region make it ideal for the study of small earthquakes and their relationship to tectonic processes. These include the fact that the network spans the expected nucleation region of a repeating magnitude 6 event and 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, and a homogeneous mode of seismic energy release as indicated by the earthquake source mechanisms (over 90$\%$ right-lateral strike-slip).

Figure 5.1: Map showing the San Andreas Fault trace, the location of the original 10 Parkfield HRSN stations (filled diamonds) and the 3 new sites (open diamonds) installed to enhance coverage around the SAFOD facility. Station GHIB (Gold Hill, not shown) is located on the San Andreas Fault about 8 km to the Southeast of station EADB. The location of the BDSN broadband station PKD is also shown (filled square). The location of the SAFOD pilot hole and main drill site are shown by the filled star. Location of the 2 alternative M2 repeating earthquake targets (70 meters apart) are shown as concentric circles. Because of the SAFOD experiment, the 4 km by 4 km dashed box surrounding the SAFOD zone is a region of particular interest to BSL researchers. The epicenter of the 1966 M6 Parkfield main shock is located at the large open circle. Routine locations of earthquakes recorded by the expanded and upgraded 13 station HRSN are shown as open black circles. Locations of events recorded by the earlier vintage 10 station HRSN, relocated using an advanced 3-D double-differencing algorithm applied to a cubic splines interpolated 3-D velocity model (Michelini and McEvilly, 1991), are shown as gray points. The locations of the 8 source points for the Vibroseis wave propagation monitoring experiment (Karageorgi et al., 1992, 1997) are represented by small black triangles.

In a series of journal articles and Ph.D. theses, we have presented the cumulative, often unexpected, results of U.C. 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.

More recently, the Parkfield area has become an area of focus of the Earthscope Project (http://www.earthscope.org) through the San Andreas Fault Observatory at Depth (SAFOD) experiment (http://www.icdp-online.de/sites/sanandreas/news/news1.html). 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 downhole measurements in order to study the physical and chemical conditions under which earthquakes occur and to exhume rock and fluid 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 early 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 from about $M_{L}$ 1.5 to $M_{L}$ about 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, 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 down to M = -1, 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 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 5.2.

Table 5.1: 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 5.2: 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 -

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 5.1 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 5.1 and 5.2. All HRSN Q730 data loggers employ FIR filters to extract data at 250 and 20 Hz (Table 5.3).

Table 5.3: 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, and is providing remote access and control of the system. It is also providing data with better timing accuracy and longer records, which are to eventually flow seamlessly into NCEDC. The new system also solves the problems of timing resolution, dynamic range, and missed detections, in addition to providing the added advantage of conventional data flow (the old system recorded SEGY format).

Because of limitations in bandwidth, a modified version of the REDI system (Chapter 9) is used to detect events in the HRSN data, extract waveform triggers, and transmit the waveform segments to the BSL. However, the December 22, 2003 San Simeon earthquake and its aftershocks sent the HRSN into nearly continuous triggering. As a result, BSL staff disabled the transmission of triggered data.

At present, all continuous 20 sps data streams and 7 vertical component channels at 250 sps are telemetered to the BSL and archived on the NCEDC in near-real-time. 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 then are processed for archiving at the NCEDC.

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 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 also allows remote monitoring of the background noise levels being recorded by the HRSN stations. For example shown in Figure 5.2 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.

2003-2004 Activities

Over the past year, activities associated with the operation of the HRSN primarily involved five components: 1) routine operations and maintenance of the network, 2) enhancement of the network's performance for detection and recording of very low amplitude seismic signals, 3) repair of the severed 48 pair cable at the VARB site, 4) collaborative integration and analysis of HRSN and SAFOD's temporary deployments for refining the structure and target location estimates in the SAFOD drill path, 5) data processing and analysis of the pre-San Simeon, and post-San Simeon data.

Operations and Maintenance

In addition to the routine maintenance tasks required to keep the HRSN in operation, various refinements and adjustments to the networks infrastructure and operational parameters have been needed this year to enhance the HRSN's performance and to correct for pathologies that continue to manifest themselves in the recently upgraded and expanded system.

Figure: Background noise PSD plot for the seven continuously telemetered BP.DP1 data streams from Parkfield. The data are 20 minute samples starting at 2003.225.0900 (2 AM PDT). The plots show the background noise PSD as a function of frequency for the highest available sampling rate (250 sps) vertical component data which are continuously telemetered to Berkeley. Note the relatively low PSD levels and the overall consistency for all the HRSN stations. By comparison, the PSD curves among the borehole Northern Hayward Fault Network land and bridge stations are much more variable and show a generally higher background noise level (Figure 4.3). On the other hand, PSD curves for the MPBO stations of the NHFN are much more consistent with the HRSN PSD's (Figure 7.4). The differences among the various station PSD's can, in large part, be explained by the relative cultural noise levels at the various stations, by the depth of the borehole sensors, and by whether the boreholes remain open holes (noisier) or have been filled with cement. The 2 Hz minimum in the PSD plots for the HRSN sensor results from 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. In the 2 to 5 Hz range, VCAB and JCNB have historically shown higher background noise which is believed to result from excitation modes in the local structure. A small 60 Hz blip can be seen in the SCYB curve due to its close proximity to a power-line.

Smaller scale maintenance issues addressed this year 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, and the testing and replacement of failing batteries. Larger efforts included the implementation of periodic emergency generator tests, replacement of the central site air conditioning unit, a major insulation, painting and power enhancement effort at our Gastro Peak repeater site to address problems with outages and low power during cold weather, and a switch to an alternative sensor on the VARB station deep string due to the failure of one of the 1877' deep sensor components.

Enhancing HRSN Performance

Over the past year significant efforts were made to identify and reduce noise problems arising from the new and expanded data acquisition system. Detection, monitoring, and high-resolution recording of earthquakes down to the smallest possible magnitudes with the highest possible signal-to-noise (especially in the region of the proposed SAFOD drilling) is a major objective of the HRSN data collection effort. Consequently, elimination of all sources of unnaturally occurring system noise is a primary goal. The minimization of data loss due to station outages and data-dropouts is also critical to this objective.

The HRSN data acquisition involves integration of a number of distinct components at each station (i.e., sensor, pre-amp, solar panels, solar regulator, batteries, Freewave radio, antenna, lightening arresters, and associated cabling, connectors and grounds) and radio telemetry apparatus between the seismic stations, telemetry relay stations, and the central processing site on the CDF site in Parkfield.

This complex integration of station and communication components combined with a variety of associated concerns (e.g., ground loops, cable resistances, radio feedback into recording equipment at stations, radio interference between stations, marginal line of site paths, cloud cover and solar power, the integration of older (pre-upgrade) hardware components with new components, old component deterioration and failures, and malfunctioning and unexpected performance characteristics of newer components) all make identification of specific causes of network generated (i.e. artificial) noise difficult to identify.

Exhaustive and iterative testing of HRSN performance has identified two primary causes for observed artificial noise remaining in the system (i.e. solar regulator spiking and pre-amp self-noise generation). Over the past year we have designed and have implemented fixes for these problems.

Solar Regulators

Regularly occurring spikes occurring during the daylight hours were observed in the continuous data streams and found to be due to the solar regulators. We have tested a variety of solar regulator designs and have identified the Prostar 30 as having the optimal cost-benefit. We have purchased and installed several of these devices at several of the HRSN sites with the ultimate goal of installing the Prostar's at all the HRSN stations as time and funding permit.

Pre-amplifier Noise

We found that a significant source of artificial noise was coming from the station pre-amplifiers. In the upgraded system, pre-amps from the older network were used. During integration of the older pre-amps with the increased dynamic range capabilities of the 24-bit Quanterra system, gain settings of the pre-amps were reduced from x10,000 to x80 in order to match signal sensitivity of the new system with the older one. While these lower pre-amp gain levels are still within the operational design of the pre-amps, they are no longer in their optimal range and a significant contribution of preamp's self-generated noise is present in the recorded seismograms. Initially, this was not expected to be a significant problem. However, we have subsequently found that even the small increase in pre-amp noise that results from the pre-amp gain reduction significantly impacts the sensitivity of the network for detecting and recording the smallest locatable events.

Figure 5.3 shows the pre-amp noise reduction effect observed on background noise signals at three vertical components of the HRSN when gains are raised from x80 to x1,000. Considerable signal hash is seen at gain levels of x80 (top waveform in each station pair), and significantly reduced when gains are increased to x1,000 (lower waveforms). Since we are also interested in recording large earthquakes on-scale, simply increasing gain levels on all stations is not the preferred solution, since doing so causes the recording system to saturate at much lower magnitudes. Instead we are attempting to redesign the pre amps using modern components to reduce the noise levels at the lower gain levels. However our attempts at redesign have not yet yielded satisfactory results.

Since a primary objective of the HRSN is to monitor the evolving patterns of the numerous small earthquakes that occur at very low magnitudes, and since this objective also complements the scientific objectives of the recently funded SAFOD experiment, the pre-amp noise problem was a priority maintenance item. We have opted, therefore, to raise the gain levels for the near-term on all the station pre-amps from x80 to x1,000. By early October of 2003, these gain changes were implemented at all 13 HRSN stations. Plans are to continue investigating pre-amp redesigns until a suitable alternative is found at which time we will install the new pre-amps and lower the pre-amp gain back to x80-allowing both the increased detection of small events and the on-scale recording of events up to about magnitude 4 to 4.5.

Figure 5.3: Preamp noise reduction test. Shown are 30 seconds of vertical background signal recorded at stations EADB, FROB and JCNB on day 229 of 2003 at 0700 UTC (top of station pairs, recorded at x80 gain and scaled up by 1000/80 for comparison to the x1000 preamp gain levels) and 0700 UTC on day 233 (bottom of station pairs, recorded at x1000 preamp gain). Note the substantial reduction in background noise, due primarily to the lower preamp generated noise at higher preamp gain.

Repairs at VARB

In August 2003, we observed problems with the recording of data at VARB, located at the Varian well site. After contacting the USGS field technician at Parkfield we learned that the sensor cable to the seismic string down the Varian well had been severed as part of a cleanup effort at the site. This activity not only cut the connection to the 1877' deep sensor, but also severed connection to the full seismic string of functional seismometers and accelerometers that existed down the deep VARB borehole.

During joint planning of the cleanup effort, we made it clear that it was important to retain connectivity to the seismic sensors at VARB, and the importance of this fact was acknowledged by the USGS personnel managing the effort. Subsequent plans for the cleanup specified coordination of the effort with field personnel from UC Berkeley. Unfortunately, BSL staff were not notified of the cleanup date, nor the specifics of the cleanup plan (which apparently included severing the deep strings cable at the well head and of course we would have loudly objected to).

Because VARB has the deepest HRSN borehole sensor and is centrally located within the network, it is a critical site for the HRSN. In addition at that time, we were in the process of preparing VARB and all the HRSN stations to make high-gain recordings of the controlled source shots from the SAFOD related 50 km line experiment to aid in characterizing the velocity and Fault Zone Guided Wave propagation structure in the region around SAFOD. Needless to say, the loss of VARB severely hampered these efforts. The situation was further complicated by the details of how the Varian well cable disconnect was made.

It was important for a number of scientific reasons to reconnect our VARB acquisition system to a sensor of known depth. However, there are 48 pairs of wires in the severed seismic string cable, and unfortunately, the mapping of these wires to their corresponding sensors was not documented when the cable was severed. In addition because the cable was severed at the well head, some 100' of trenching and cable was needed to reconnect the sensor and recording installation.

The estimated cost for the additional man-hours, parts, travel and lodging needed to do the necessary repairs at VARB was several thousand dollars. With emergency funding from NEHRP and assitance from the USGS field technician at Parkfield, we put VARB back on-line and made the high-gain HRSN adjustments in time for the 50 km line experiment shots.

Figure 5.4: Shown are the number of triggers per hour for a period between 1 month prior to and 6 months following the San Simeon earthquake, and an "eyeball" fit of the decay curve (dashed curve). The cumulative number of HRSN triggers in the first 5 months following San Simeon exceeds 70,000 and trigger levels continued to be over 200 triggers per day through June of 2004. Extrapolation of the decay curve fit indicates that daily trigger levels will not return to near pre-San Simeon levels untell well into 2005 (assuming no other large earthquakes in the area occur).

SAFOD Collaboration

An intensive and ongoing effort by the SAFOD target event location working group is underway with its goals being: 1) the characterization of the detailed velocity and seismicity structure in the crustal volume containing the SAFOD main hole and 2) to determine the most accurate estimates of the absolute locations of SAFOD's target events. As part of this effort, a series of coincident active and passive source seismic experiments was performed during Oct. - Nov. of 2003. The HRSN data played a key role in this effort by providing complementary seismic waveform data from the active sources and by providing a backbone of earthquake detection, waveform, and location data from the numerous microearthquakes that occurred within the SAFOD local zone during the Oct. 15 through Nov. 11 experimental period (256 events within 5 km of the SAFOD target(s)).

In a special section of Geophysical Research Letters from May of 2004, several papers make significant use of the HRSN data for characterizing the SAFOD area and illustrate the role that the HRSN data have played in the SAFOD effort over the past year (e.g., Oye et al., 2004; Roecker et al., 2004; Thurber et al., 2004; Nadeau et al., 2004).

Data Analysis and Results

Monitoring the evolution of microseismicity, particularly in the SAFOD drilling and target zone, is a primary objective of the HRSN project. In addition, the continued analysis of the HRSN data for determining detailed seismic structure, for the study of similar and characteristic microearthquake systematics, for estimation of deep fault slip rate evolution, and for various studies of fault zone and earthquake physics is also of great interest to seismologists. Before advanced studies of the Parkfield microseismicity can take place, however, initial processing, analysis and routine cataloging of the earthquake data must be done. An integral part of this process is quality control of the processed data, including a final check of the routine catalog results. The numerous aftershocks from the M6.5 San Simeon earthquake (Figure 5.4) have seriously complicated the tasks of initial processing, analysis and location of the routine event catalog. And, a significant revision of the "traditional" processing scheme we have used since 1987 will be required to deal with the post-San Simeon data. We have requested funding for this effort in a proposal to EarthScope, and if granted our intent is to more fully develop, test and implement the new procedures for processing of these data.

Most of our efforts during the 2000-2002 were spent on implementing the emergency upgrade and SAFOD expansion of the HRSN, and routine processing of the data collected during that period was deferred until after upgrade and installation efforts were completed. In 2003 we began in earnest the task of routine processing of the ongoing data that was being collected. Our initial focus was on refining and developing our processing procedures to make the task more efficient and to ensure quality control of the processed catalogs. We also began working back in time to fill in the gap that developed during the deferment period. Because routine processing of the post-San Simeon data is effectively impossible at this time, because of the overwhelming number of aftershocks, we have suspended our efforts at processing the ongoing data and focused our efforts at filling in the complete gap of unprocessed data (i.e., back to March of 2001). Outlined below in the "Pre-San Simeon Processing" subsection are the procedures and issues related to this effort. In the subsequent subsection we illustrate and discuss briefly the issues that need to be addressed in order to process the post-San Simeon event data.

Pre-San Simeon Processing

Initial Processing. 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 to the BSL at 250 sps over the frame relay-circuit 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 data upload to the NCEDC archive in some cases.

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.

An ongoing effort has been the development of a new earthquake detection scheme, with the goal of routinely detecting SAFOD area events to magnitudes below -1.0. A first cut version of the new scheme has been implemented and is currently detecting real earthquakes at an increased rate-nearly 3 times the number of earthquakes detected before the upgrade. In order to facilitate the processing and archiving of the increased number of potential earthquakes ($\sim$ 350 per month), the BSL has recently developed a Graphical User Interface (GUI). The GUI is integrated with the NCEDC dbms and allows review of the waveforms from every potential event. Initial analysis of the data using the GUI involves review of the waveforms and classification of the event as an earthquake or non-earthquake. The GUI also allows the analyst to log potential network problems that become apparent from the seismograms. The HRSN analyst then classifies the earthquakes as either a local, distant-local, regional, or teleseismic event and then systematically hand picks the P- and S-phases for the local and distant local events (for the period Sept. 2002 - Aug. 2003 the number of picked events was $\sim$ 2000).

Picking of the numerous microearthquake events is no mean task. On average about 7 P-phases and 4 S-phases are picked for each event, putting the total number of annual phase picks for the HRSN data on the order of to 22,000. We have experimented with algorithms that make initial auto-picks of the phase arrivals, but have so far found picking by hand to be significantly more accurate and has the added advantage of allowing the analyst to assess the state of health of each station-component. In all our tests, autopicks have also invariably resulted in some missed events and catalog locations that are significantly more scattered and with higher residuals than locations done with purely hand-picked data.

A peculiarity of processing very small earthquake data, is that multiple events commonly occur within a few seconds of one another. The close timing of these events does not allow the local triggering algorithm to recover from one event before another occurs. As a result, the central site processor often does not trigger uniquely for each event. In such cases only one, 30 sec waveform gather and one earthquake identifier will be created for all the events. These multiple earthquake records (MER) account for only 3 to 5$\%$ of the total seismicity recorded by the HRSN. However, there are times when this rate rises to over 10$\%$. In order to assign each event in an MER a unique event identifier for the NCEDC dbms and to make picking and automated processing of these events more manageable an additional feature of the GUI was developed that allows the analyst to "clone" MER into separate gathers for each event.

Quality Control. Once false triggers have been removed and picks for the local and distant local events have been completed, quality control on the picks is made to ensure that all picks have phase and weights assigned, that extraneous characters have been removed from the pick files, that double station-phase picks have not inadvertently been made, and that no repicks of the same event had been accidentally made during any cloning that was performed. Initial locations are then performed and phase residuals analyzed in order to determine whether severe pick outliers must be removed or adjusted. Unstable location solutions based on events with few picks are also assessed to see if the addition of marginal phases will improve the stability of the location determination. After any required pick adjustments have been made, the events are then relocated, and combined with error information to allow ranking of the confidence of location quality.

These procedures have all been put in place and tested for the new HRSN configuration. Currently we have located 13 months of local earthquakes recorded by the new HRSN (over 2200 events) and are moving backwards in time to pick and locate the earlier data collected since March of 2001. We currently have enough data and are confident enough with the procedures to begin organizing the locations for formal inclusion into the NCEDC dbms and dissemination to the community. These efforts are now underway. We are also in the early stages of establishing a scalar seismic moment catalog for the new HRSN events that is also to be included in the NCEDC dbms.

Catalog Assessment. We continue to examine the earthquake data in search of possible earthquake precursors. This includes quality control and evaluation of the routine earthquake catalog locations and analyses of the spatial and temporal distribution of the microseismicity in relation to the occurrence of larger earthquakes in the area and heightened alert levels declared as part of the Parkfield Prediction Experiment. The new data and event detection scheme allows complete event detection down to $\sim$ magnitude 0.0. As a result, the rate of earthquake detection by the HRSN exceeds that of the NCSN by about a factor of 5 in the 30 km stretch of the SAF centered at the location of the 1966 M6 Parkfield event (Figure 5.1). The additional rate of HRSN event detection significantly increases both the spatial and temporal resolution of the changing seismicity patterns and provide unique additional information on the earthquake pathology at very low magnitudes.

Post-San Simeon Data

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. Since the beginning of the network's data collection in 1987, 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.

Following the occurrence of the M6.5 San Simeon earthquake on December 22, of 2003, the long-standing data handling procedure outlined in the previous section was no longer viable due to the enormous rate of San Simeon aftershock detections (Figures 5.5 and 5.6). In the first 5 months following the mainshock, over 70,000 event detections were made by the HRSN system (compared to a yearly average detection rate of 6000 prior to San Simeon), and spot checks of the continuous 20 sps data revealed that the overwhelming majority of these detections resulted from seismic signals generated by San Simeon's aftershocks.

Data from the California Integrated Seismic Network (CISN) show that there were $\sim$ 1,150 San Simeon aftershocks with magnitudes $>$ 1.8 occurring in the week following the mainshock. During this same period, the number of event detections from the HRSN was $\sim$ 10,500 (compared to an average weekly for the year prior to San Simeon of 115 detections/per week). This suggests that the HRSN is detecting San Simeon aftershocks well below magnitude 1, despite the network's $\sim$ 50 km distance from the mainshock (Figures 5.5 and 5.6).

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 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 250 sps waveform data is currently being periodically uploaded from the DLT tapes onto the NCEDC for access to the research research community. Research funding has also been requested from NSF-EarthScope to develop and apply new techniques to process these continuous data with the aim of identifying the Parkfield local events from among the San Simeon aftershocks and of compiling waveform and location catalogs for the local earthquakes.

Figure 5.5: Earthquake locations of 5847 events in the San Simeon and Parkfield areas of California occurring between December 1, 2003 and April 1, 2004, inclusive (small black circles). Locations are from the Northern California Seismic Network catalog. Black triangles are the stations of the borehole High Resolution Seismic Network (HRSN). The M6.5 San Simeon event of December 22, 2003 and its aftershocks occurred in a region approximately 40 to 50 km southwest of the HRSN. Prior to the mainshock, seismicity in this region was predominantly quiescent.

Figure 5.6: Unfiltered seismogram of the San Simeon mainshock and immediate aftershocks. Shown is a two hour snapshot of continuous 250 sps data recorded by the vertical component of the borehole HRSN station SMNB (located $\sim$ 50 km to the northeast of the mainshock). Approximately 15 minutes of background pre-event recording is shown before the mainshock. Following the mainshock seismic signal levels are elevated well above the background level almost continuously and signals from multiple aftershocks (and possibly Parkfield local events) generally overlap. The very low background noise recordings of the borehole HRSN stations and the networks high detection sensitivity (designed for detecting very low magnitude Parkfield local events) causes the network to trigger on 10's of thousands of distant and small San Simeon aftershocks (many below magnitude 1), this despite the HRSN's distance of $\sim$ 50 km from the sequence.

Acknowledgements

Thomas V. McEvilly, who passed away in February 2002, was the PI on the HRSN project for many years. Without his dedication the creation and 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, 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 grants: 03HQGR0065 and 04HQGR0085. NSF also provided support during the 2000-2003 period for the SAFOD expansion of the HRSN through grant EAR-9814605.

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