Northern Hayward Fault Network

Introduction

Complementary to the regional broadband network, a deployment of borehole-installed, wide-dynamic range seismographic stations is being established along the Hayward Fault and throughout the San Francisco Bay toll bridges network. This network is a cooperative development of the BSL and the USGS, with support from USGS, Caltrans, EPRI, the University of California Campus/Laboratory Collaboration (CLC) program, LLNL, and LBNL (Figure 4.1 and Table 4.1). Efforts at ongoing development of the network have also recently been enhanced by through coordinated efforts with the Mini-PBO project (Chapter 8, which is partially funded by NSF and by the member institutions of that project).

The purpose of the network is threefold: 1) to lower substantially the threshold of microearthquake detection, 2) to increase the recorded bandwidth for events along the Hayward fault, and 3) to obtain bedrock ground motion signals at the bridges from small earthquakes for investigating bridge responses to stronger ground motions. A lower detection threshold increases the resolution of the fault-zone seismic structure; allows seismologists to monitor the spatial and temporal evolution of seismicity at magnitudes down to $M \sim > -1.0$, where earthquake rates are many times higher than those captured by the surface sites of the NCSN; allows researchers to look for pathologies in seismicity patterns that may be indicative of the nucleation of large damaging earthquakes; and allows scientists to investigate fault and earthquake scaling, physics and processes in the Bay Area of California. This new data collection will also contribute to improved working models for the Hayward fault. The bedrock ground motion recordings are also being used to provide input for estimating the likely responses of the bridges to large, potentially damaging earthquakes. Combined with the improved Hayward fault models, source-specific response calculations can be made, as well.

The Hayward Fault Network (HFN) consists of two parts. The Northern Hayward Fault Network (NHFN) is operated by the BSL and currently consists of 25 stations, including those located on Bay Area bridges and at borehole sites of the Mini-PBO (MPBO) project. This network is considered part of the BDSN and uses the network code BK. The Southern Hayward Fault Network (SHFN) is operated by the USGS and currently consists of 5 stations. This network is considered part of the NCSN and uses the network code NC. This chapter is primarily focused on the NHFN and activities associated with the BSL operations.

Figure 4.1: Map showing the locations of the HFN stations operated by the BSL (NHFN - squares) and the USGS (SHFN - circles) and Mini-PBO stations (diamonds) in the San Francisco Bay Area. Operational sites are filled, while sites in progress are grey. Other instrumented boreholes are indicated as open symbols.
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NHFN Overview

The five MPBO sites have 3-component borehole geophone packages. All the remaining HFN sites have six-component borehole sensor packages. The packages were designed and fabricated at LBNL's Geophysical Measurement Facility by Don Lippert and Ray Solbau, with the exception of site SFAB. For the HFN sites three channels of acceleration are provided by Wilcoxon 731A piezoelectric accelerometers and three channels of velocity are provided by Oyo HS-1 4.5 Hz geophones. Velocity measurements for the MPBO sites are provided by Mark Products L-22 2 Hz geophones (Table 4.2). Sensors are generally installed at depths of about 100 m, but several sites have sensors emplaced at depths of over 200 m and the Dumbarton bridge sites have sensors at multiple depths (Table 4.1). During initial stages of the project, the NHFN sensors provided signals to on-site Quanterra Q730 and RefTek 72A-07 data loggers. In the current NHFN configuration on-line data logging is being done by on-site Quanterra Q4120 instrumentation. The SHFN sensors have been providing signals to Nanometrics HRD24 data loggers since initiation of data collection.

The 0.1-400 Hz Wilcoxon accelerometers have lower self-noise than the geophones above about 25-30 Hz, and remain on scale and linear to 0.5 g. In tests performed in the Byerly vault at UC Berkeley, the Wilcoxon is considerably quieter than the FBA-23 at all periods, and is almost as quiet as the STS-2 between 1 and 50 Hz.

Thirteen of the NHFN sites have Quanterra data loggers with continuous telemetry to the BSL. Similar to BDSN sites, these stations are capable of on-site recording and local storage of all data for more than one day and have batteries to provide backup power. Signals from these stations are digitized at a variety of data rates up to 500 Hz at 24-bit resolution (Table 4.3). In contrast to the BDSN implementation, the NHFN data loggers employ casual FIR filters at high data rates and acausal FIR filters at lower data rates. Because of limitations in telemetry bandwidth and disk storage, these 13 sites transmit triggered data at 500 sps, using the Murdock, Hutt, and Halbert (MHH) event detection algorithm (Murdock and Hutt, 1983), and continuous data at reduced rates (100, 20 and 1 sps) to the BSL.

The remaining 12 sites of the NHFN have in the past recorded data using RefTek data loggers. These sites do not have continuous telemetry for acquisition and required visits from BSL staff for data recovery. Collection of data from these sites has been discontinued, but efforts are underway to upgraded them with Quanterra Q4120 data loggers and continuous telemetry.

Signals from the 5 SHFN stations are digitized by Nanometrics data loggers at 100 sps and transmit continuous data to Menlo Park by radio. These digital data streams are processed by the Earthworm system with the NCSN data and waveforms are saved when the Earthworm detects an event.


Table 4.1: Stations of the Hayward Fault Network. Each HFN station is listed with its station code, network id, location, operational dates, and site description. The latitude and longitude (in degrees) are given in the WGS84 reference frame. The elevation of the well head (in meters) is relative to the WGS84 reference ellipsoid. The overburden is given in meters. The start dates indicate either the upgrade or installation time. The abbreviations are: BB - Bay Bridge; BR - Briones Reserve; CMS - Cal Memorial Stadium; CB - Carquinez Bridge; DB - Dumbarton Bridge; MPBO - mini-Plate Boundary Observatory RFS - Richmond Field Station; RSRB - Richmond-San Rafael Bridge; SF - San Francisco; SMB - San Mateo Bridge; SMC - St. Mary's College; and, YB - Yerba Buena. The * for stations indicates that the stations are not currently recording data. RSRB is shut down while Caltrans is retrofitting the Richmond-San Rafael bridge (as of April 19, 2001) and YBIB has been off-line since August 24, 2000 when power cables to the site where shut down. Other off-line stations are in the process of being upgraded as funding for equipment becomes available. The table also includes 2 MPBO stations which became operational in the last 2 years, and 3 MPBO borehole sensors that have recently been installed.
Code Net Latitude Longitude Elev (m) Over (m) Date Location
CRQB BK 38.05578 -122.22487 -25.0 38.4 1996/07 - current CB
HERB BK 38.01250 -122.26222 -25.0 217.9 2000/05 - current Hercules
BRIB BK 37.91886 -122.15179 219.7 108.8 1995/07 - current BR, Orinda
RFSB BK 37.91608 -122.33610 -27.3 91.4 1996/01 - current RFS, Richmond
CMSB BK 37.87195 -122.25168 94.7 167.6 1994/12 - current CMS, Berkeley
SMCB BK 37.83881 -122.11159 180.9 3.4 1997/12 - current SMC, Moraga
SVIN BK 38.03325 -122.52638   158.7 2003/08 - current MPBO, St. Vincent's school
OHLN BK 38.00742 -122.27371   196.7 2001/07 - current MPBO, Ohlone Park
MDHL BK 37.84227 -122.49374   160.6 in progress MPBO, Marin Headlands
SBRN BK 37.68562 -122.41127   157.5 2001/08 - current MPBO, San Bruno Mtn.
OXMT BK 37.498 -122.425   194.2 in progress MPBO, Ox Mtn.
BBEB BK 37.82167 -122.32867   150.0 2002/05 - current BB, Pier E23
E17B BK 37.82086 -122.33534   160.0 1995/08 - current * BB, Pier E17
E07B BK 37.81847 -122.34688   134.0 1996/02 - current * BB, Pier E7
YBIB BK 37.81420 -122.35923 -27.0 61.0 1997/12 - current * BB, Pier E2
YBAB BK 37.80940 -122.36450   3.0 1998/06 - current * BB, YB Anchorage
W05B BK 37.80100 -122.37370   36.3 1997/10 - current * BB, Pier W5
W02B BK 37.79120 -122.38525   57.6 2003/06 - current BB, Pier W2
SFAB BK 37.78610 -122.3893   0.0 1998/06 - current * BB, SF Anchorage
RSRB BK 37.93575 -122.44648 -48.0 109.0 1997/06 - current * RSRB, Pier 34
RB2B BK 37.93 -122.41   133.8 2003/07 - current * RSRB, Pier 58
SM1B BK 37.59403 -122.23242   298.0 not recorded SMB, Pier 343
DB3B BK 37.51295 -122.10857   1.5 1994/09 - 1994/11 DB, Pier 44
          62.5 1994/09 - 1994/09  
          157.9 1994/07 - current *  
DB2B BK 37.50687 -122.11566     1994/07 - current * DB, Pier 27
          189.2 1992/07 - 1992/11  
DB1B BK 37.49947 -122.12755   0.0 1994/07 - 1994/09 DB, Pier 1
          1.5 1994/09 - 1994/09  
          71.6 1994/09 - 1994/09  
          228.0 1993/08 - current *  
CCH1 NC 37.7432 -122.0967 226   1995/05 - current Chabot
CGP1 NC 37.6454 -122.0114 340   1995/03 - current Garin Park
CSU1 NC 37.6430 -121.9402 499   1995/10 - current Sunol
CYD1 NC 37.5629 -122.0967 -23   2002/09 - current Coyote
CMW1 NC 37.5403 -121.8876 343   1995/06 - current Mill Creek



Table 4.2: Instrumentation of the HFN as of 06/30/2002. Every HFN downhole package consists of co-located geophones and accelerometers, with the exception of MPBO sites. 6 HFN sites also have dilatometers (Dilat.) and the 5 MPBO sites have tensor strainmeters (Tensor.) 12 NHFN sites have Quanterra data loggers with continuous telemetry to the BSL. The remaining sites are being upgraded to Quanterra data loggers. The 5 SHFN sites have Nanometrics data loggers with radio telemetry to the USGS. The orientation of the sensors (vertical - Z, horizontals - H1 and H2) are indicated where known or identified as "to be determined" (TBD).
Site Geophone Accelerometer Z H1 h2 Data logger Notes Telem.
CRQB Oyo HS-1 Wilcoxon 731A -90 251 341 Q4120   FR
HERB Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120   FR
BRIB Oyo HS-1 Wilcoxon 731A -90 79 349 Q4120 Acc. failed, Dilat. FR
RFSB Oyo HS-1 Wilcoxon 731A -90 256 346 Q4120   FR
CMSB Oyo HS-1 Wilcoxon 731A -90 19 109 Q4120   FR
SMCB Oyo HS-1 Wilcoxon 731A -90 76 166 Q4120 Posthole FR
SVIN Mark L-22   -90 TBD TBD Q4120 Tensor. FR/Rad.
OHLN Mark L-22   -90 TBD TBD Q4120 Tensor. FR
MDHL Mark L-22   -90 TBD TBD None at present Tensor.  
SBRN Mark L-22   -90 TBD TBD Q4120 Tensor. FR
OXMT Mark L-22   -90 TBD TBD None at present Tensor.  
BBEB Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120 Acc. failed Radio
E17B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
E07B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
YBIB Oyo HS-1 Wilcoxon 731A -90 257 347 Q4120 Z geop. failed FR/Rad.
YBAB Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
W05B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
W02B Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120   Radio
SFAB None LLNL S-6000 TBD TBD TBD None at present Posthole  
RSRB Oyo HS-1 Wilcoxon 731A -90 50 140 Q4120 2 acc. failed FR
RB2B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present 1 acc. failed  
SM1B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
DB3B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present Acc. failed  
DB2B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
DB1B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present Acc. failed  
CCH1 Oyo HS-1 Wilcoxon 731A -90 TBD TBD Nanometrics HRD24 Dilat. Radio
CGP1 Oyo HS-1 Wilcoxon 731A -90 TBD TBD Nanometrics HRD24 Dilat. Radio
CSU1 Oyo HS-1 Wilcoxon 731A -90 TBD TBD Nanometrics HRD24 Dilat. Radio
CYD1 Oyo HS-1 Wilcoxon 731A -90 TBD TBD Nanometrics HRD24 Dilat. Radio
CMW1 Oyo HS-1 Wilcoxon 731A -90 TBD TBD Nanometrics HRD24 Dilat. Radio


Experience has shown that the MHH detector does not provide uniform triggering across the NHFN on the smallest events of interest. In order to insure the recovery of 500 sps data for these earthquakes, a central-site controller has recently been implemented at the BSL using the 500 sps vertical component geophone data for event detection. Originally the 100 sps vertical component geophone data was used for event detection but the bandwidth proved to be inadequate for detection of the smaller events where most of the seismic wave energy was at frequencies above 40 Hz. Triggers from this controller are being used to recover the 500 sps data from the NHFN data loggers.

Data from the NHFN and SHFN are archived at the NCEDC. At this time, the tools are not in place to archive the Hayward fault data together. The NHFN data are archived with the BDSN data, while the SHFN are archived with the NCSN data (Chapter 11). However, the new central-site controller will provide the capability to both include SHFN data in the event detection and extract SHFN waveforms for these events in the future.

As originally planned, the Hayward Fault Network was to consist of 24 to 30 stations, 12-15 each north and south of San Leandro, managed respectively by UCB and USGS. This is not happening quickly, although west of the fault, Caltrans has provided sites along the Bay bridges. This important contribution to the Hayward Fault Network has doubled the number of sites with instrumentation. At times, Caltrans provides holes of opportunity away from the bridges (e.g., HERB), so we have plans for additional stations that will bring the network geometry to a more effective state for imaging and real-time monitoring of the fault.

As a check on the calibration and an example of the capabilities of a borehole installed network, we compare the bandpass filtered (0.3-2 Hz) ground velocity data recorded at HERB, RFSB, BBEB, CMSB, BRIB, and SMCB for a M 6.9 deep focus teleseism that occurred in the vicinity of the Rat Islands in the Aleutian Islands chain at a depth of 685 km. in Figure 4.2.

Figure: Displayed are 30 seconds of 0.5-2.0 Hz BP filtered ground velocity data for a $M_{w}$ 6.9 deep focus teleseism which occurred 6/23/2003 at 12:12 UT at a depth of 685 km in the vicinity of the Rat Islands in the Aleutian Islands chain (51.44N,176.78E). The traces have been ordered by increasing distance (top to bottom). For reference, the great circle distance of the event from the NHFN is $\sim $ 44.2$^{\circ }$with an azimuth of $\sim $ 308$^{\circ }$. The NHFN waveforms are relative scaled. Absolute scaling of the plot has indicated that the transfer function gain for station BBEB may be too low, making the inferred filtered ground velocity too large for a true comparison of the ground velocity. By periodically analyzing the network-wide response to deep focus teleseisms, whose arrivals are of near vertical plane wave incidence of uniform amplitude, anomalous station response (indicating potential problems in the network) such as that seen for BBEB are easily identifiable and can be further investigated to ensure accurate station operation. The same teleseism may be seen in Figure 5.2, recorded on the HRSN.
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Table 4.3: Typical data streams acquired at each NHFN site, with channel name, sampling rate, sampling mode and FIR filter type. C indicates continuous; T triggered; Ca causal; and Ac acausal. The 100 sps channels (EP & HL) are only archived when the 500 sps channels are not available.
Sensor Channel Rate (sps) Mode FIR
Accelerometer CL? 500.0 T Ca
Accelerometer HL? 100.0 C Ca
Accelerometer BL? 20.0 C Ac
Accelerometer LL? 1.0 C Ac
Geophone DP? 500.0 T Ca
Geophone EP? 100.0 C Ca
Geophone BP? 20.0 C Ac
Geophone LP? 1.0 C Ac


2002-2003 Activities

In addition to routine maintenance, operations and data collection; activities of the NHFN project over the past year have also included numerous efforts at network expansion, quality assurance, performance enhancement and catalog development.

Station Maintenance

Shown in Figure 4.3 are power spectral density (PSD) distributions of background noise for a sample of 8 NHFN land and bridge site stations. In general, background noise levels of the borehole HFN stations is more variable and generally higher than that of the Parkfield HRSN borehole stations (Figure 5.3). This is due in large part to the significantly greater level of cultural noise in the Bay Area, and to the fact that noise reduction efforts on the much more recently installed NHFN stations are still underway. For example the two noisiest stations (i.e. BBEB and W02) are located on the Bay Bridge which is currently undergoing earthquake retrofit and east span reconstruction. These stations have also only recently come back on-line with upgraded infrastructure and instrumentation, so the full complement of noise reduction modifications have not yet been completed.

Figure: Plot showing the HFN.BK.DP1 background noise, PSD, for 8 of the NHFN stations. Plotted are the background low-noise PSD estimates. Ten minutes of .BK.DP1 data starting at 2003.225.0900 (2 AM PDT) were used in the analysis. Note that there is considerable variation in the general level and structure of the individual station background noise PSD estimates. Some of the stations show peaks at 60 Hz and its harmonics while others have a high average background level. The two bridge sites, BBEB and W02B are the noisiest while land site BRIB in Briones Regional Park (well away from the heavy cultural noise of the more populated region of the Bay Area) is the quietest. Two stations, CMSB and HERB show a peak in the 20-30 Hz range. The peak at CMSB is probably due to excitation of modes in the open bore hole and the peak at HERB is due to excitation of the local structure by the adjacent railway line and highways 4 and 80. The three stations in the middle of the group (RFSB, SMCB and CRQB) are responding to the local cultural noise. There are numerous ongoing experiments at the Richmond Field Station which are affecting the noise level at RFSB, CRQB is sited near a sewage treatment plant and the Carquinez bridge, and SMCB is currently only installed at post hole depth (3.5 m) on the St. Mary's campus.
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On average the MPBO NHFN sites are more consistent and quieter (Figure 8.6). This is due in large part to the greater depth of the MPBO sensors, the locations of MPBO stations in regions of generally less industrial and other cultural noise sources, and possibly to the absence of powered sensors (i.e. accelerometers) in their borehole sensor packages.

One of the most pervasive problems at NHFN stations equipped with the new Q4120 data loggers is power line noise (60 Hz and its harmonics at 120, 180, and 240 Hz). This noise reduces the sensitivity of the MHH detectors. Whenever a NHFN station is visited, the engineer at the site and a seismologist at the BSL work together to expedite the testing process, especially when attempting to identify and correct ground-loop faults which generally induce significant 60, 120, 180, and 240 Hz seismic signal contamination due to stray power line signal pickup, generally inductively coupled and aggravated by the presence of ground loops.

Below is a synopsis of maintenance efforts performed over the past year for several NHFN stations that gives some idea of the ongoing maintenance and performance enhancing measures that we are continuing to implement.

NHFN Station Maintenance Synopsis

BBEB: Installed upgraded power system in July. Installed Q4120 data logger and started data acquisition on September 10, 2002. Replaced coaxial cable and connector between Cylink radio and antenna to fix problem with poor data flow.

BRIB: Vault flooded in December during heavy rains owing to failure of sump pump. A portable electric generator and sump pump were used to pump out the water. Wood platforms were installed to raise the batteries off of the floor so that they will not become submerged if the vault floods again. The Rule 2000 Sump pump, Sure Bail switch, associated wiring and battery were repaired in the lab and reinstalled in the vault.

CMSB: Replaced batteries with two new C & D Technologies UPS 12-310 batteries. Replaced Q4120 data logger and FRAD. Rodents had chewed on the data logger case but they did not penetrate the case. Replaced defective rodent repeller near the FRAD and installed a second repeller near the data logger. Replaced preamp when it was discovered that channel 4 was bad. Upgraded Q4120 with installation of Q730PWR board. Experienced some problems during year with clock quality owing to poor antenna sky visibility.

CRQB: Upgraded Q4120 data logger with installation of Q730PWR board. Disconnect DAT to fix multiple boot up messages and questionable EP counts problem when booting up the Q4120 data logger. The DAT drive is not used so this is not a problem.

HERB: Swapped in a new preamp to fix a channel gain problem. Spent some time troubleshooting problem with 60 Hz and its harmonics contaminating geophone channels and running a series of experiments and discovered that the 120 Hz signal is a 100 kHz spike which repeats at a 120 Hz rate. Installed damping resistor when it was discovered to be missing. Also installed shunt capacitors to reduce the high frequency spike noise. Replaced power supply when it was discovered to have periods of imperfect regulation.

RFSB: Upgraded Q4120 data logger with installation of Q730PWR board and new software.

SMCB: Station was down from August 28 through October 29 owing to construction at Moore Hall which provided power and telemetry. Q4120 digitizers failed due to a blown fuse. While Q4120 was in lab for fuse replacement a Q730PWR board was added to give input power monitoring capability.

W02B: Installed hardware (data logger, etc.) in utility boxes bolted to the NW face of the pier, just above water level. Began data acquisition and telemetry on June 17th.

Geophone Calibrations

Comparisons of the inferred ground accelerations generated by local earthquakes, from co-sited HFN geophone and accelerometer pairs, shows that the waveforms generally are quite coherent in frequency and phase response but that their inferred ground accelerations differ significantly. At times the amplitudes differ by up to a factor of  2 while the times of the peak amplitudes are identical. This implies that the free period and damping of the geophones are well characterized and also that the generator constant is not accurate (assuming that the corresponding ground accelerations inferred from the accelerometers are accurate).

Generally speaking, the accelerometers, being an active device, are more accurate and also more stable that the geophones so it is reasonable to assume that the most likely reason for the difference is that the assumed generator constants for the geophones are not accurate. Rodgers et al. (1995) describe a way to absolutely calibrate the geophones in situ and to determine their generator constant, free period and fraction of critical damping. The only external parameter that is required is the value of the geophones inertial mass.

We have built a calibration test box which allows us to routinely perform the testing described by Rodgers et al. whenever site visits are made. The box drives the signal coil with a known current step and rapidly switches the signal coil between the current source and the data logger input. From this information, expected and actual sensor response characteristics can be compared and corrections applied. Also, changes in the sensor response over time can be evaluated so that adjustments can be made and pathologies arising in the sensors due to age can be identified. Once a geophone is absolutely calibrated, we can also check the response of the corresponding accelerometer.

We are now performing the initial calibration tests and response adjustments for all NHFN stations as sites are visited for routine maintenance. We also plan a scheduled re-tests of all sites to monitor for sensor responses changes through time.

Combined Catalog

We are building a HF-specific data archive from the existing waveform data that have been collected by the heterogeneous set of recording systems in operation along the Hayward fault (i.e. the NHFN, SHFN, NCSN, and BDSN continuous and triggered waveforms). Recently we have taken the NHFN triggers collected during operations between 1995.248 and 1998.365 (recorded on portable RefTek recorders) and origin times from the NCSN and BDSN catalogs for this time period and undertaken a massive association of event and trigger times. The purpose of the effort is to compile a relatively uniform catalog of seismic data to low magnitudes and extending back in time to the beginning of reliable HFN data collection. The process has reduced nearly a million individual time segments to 316 real events along the Hayward fault during the period-an increase in the number of events of a factor of about 2.5 to 3 over the NCSN catalog alone in the same area.

Event Detection

As noted in the Introduction, one of the purposes of the HFN is to lower the threshold of microearthquake detection. Towards this goal, we have been developing new algorithms: a pattern recognition approach to identify small events; a phase onset time detector with sub-sample timing resolution, and; a phase coherency method for single component identification of highly similar events.

Pattern Recognition

In order to improve the detection and analysis of small events (down to $M_{L} \sim $-1.0) some specialized algorithms are being developed. The Murdock-Hutt detection algorithms used by MultiSHEAR, which basically flags an event whenever the short-term average exceeds a longer-term average by some threshold ratio, is neither appropriate for nor capable of detecting the smallest seismic events. One solution is to use a pattern recognition approach to identify small events associated with the occurrence of an event which was flagged by the REDI system. Tests have indicated that the pattern recognition detection threshold is $M_{L} \sim $ -1.0 for events occurring within $\sim $10 km of a NHFN station. The basic idea is to use a quarter second of the initial P-wave waveform, say, as a master pattern to search for similar patterns that occur within $\pm$ one day, say, of the master event. Experimentally, up to six small CMSB recorded events, at the $M_{L} \sim $ -1.0 threshold and occurring within $\pm$ one day of a master pattern, have been identified.

The pattern recognition method is CPU intensive, however, and it will require a dedicated computer to handle the pattern recognition tasks. To expedite the auto-correlation processing of the master pattern, an integer arithmetic cross-correlation algorithm has been developed which speeds up the requisite processing by an order of magnitude.

Phase Onset Time Detection

The phase onset time detector makes use of the concept that the complex spectral phase data, over the bandwidth of interest (i.e., where the SNR is sufficiently high), will sum to a minimum at the onset of an impulsive P-wave. The algorithm searches for the minimum phase time via phase shifting in the complex frequency domain over the bandwidth where the SNR is above 30 dB, say, to identify the onset time of the seismic phase. The algorithm requires that the recorded waveforms be deconvolved to absolute ground displacement. This implicitly requires that any acausality in the anti-aliasing filtration chain, such as the FIR filters used in the BDSN Quanterra data loggers, be removed. The algorithm typically resolves P-wave onset times to one-fiftieth of the sample interval or better.

Phase Coherency

A spectral phase coherency algorithm was developed to facilitate high resolution quantification of the similarities and differences between highly similar Hayward fault events which occur months to years apart. The resolution of the complex spectral phase coherency methodology is an order of magnitude better that the cross correlation method which is commonly used to identify highly similar events with resolution of order a few meters. This method, originally developed using NHFN borehole data, is now being applied as well to data from another borehole network (the HRSN) to provide more rapid and objective identification of the large fraction ( approx. 40$\%$) of characteristically repeating microearthquakes that occur at Parkfield, CA.

New Installations

San Francisco-Oakland Bay Bridge

The infrastructure at seven stations along the San Francisco-Oakland Bay Bridge (SFAB, W02B, W05B, YBAB, E07B, E17B, and BBEB) was upgraded with the installation of weatherproof boxes, power, and telemetry in anticipation of installing Q4120 data loggers and telemetering the data back to Berkeley. BBEB was brought on-line in May of 2002, and W02B in June of 2003.

Land Sites

Agreements with Caltrans and St. Mary's college have been made to replace the post hole installation at St. Mary's college (SMCB) with a deep borehole installation. The hole is to be drilled by Caltrans as a hole of opportunity when the schedule of a Caltrans drilling crew has an opening. The site has been reviewed by UCB, Caltrans and St. Mary's college personnel, and we are now in the drilling queue. Depending on the geology at borehole depth, this site my either become a MPBO site (w/o accelerometers) or a standard land site installation including both geophones and accelerometers.

Caltrans has also provided funding for instrumentation of several other land sites which we will install as future Caltrans drill time becomes available. Currently we are considering sites for these additional holes-of-opportunity at Pt. Pinole, on Wildcat Mtn. in the north Bay.

Mini-PBO

The stations of the Mini-PBO project (Chapter 8) are equipped with borehole seismometers. As these stations have become operational, they augment HFN coverage (Figure 4.1). In the last year, SVIN and SBRN have added coverage to the north bay and east side of the south bay, respectively.

Acknowledgements

Thomas V. McEvilly, who passed away in February 2002, was instrumental in developing the Hayward Fault Network, and without his dedication and hard work the creation and continued operation of the NHFN would not have been possible.

Under Bob Nadeau's, Bob Uhrhammer's and Doug Dreger's general supervision, Rich Clymer, Wade Johnson, Doug Neuhauser, Bill Karavas, John Friday, and Dave Rapkin all contribute to the operation of the NHFN. Bob Nadeau, Bob Uhrhammer and Lind Gee contributed to the preparation of this chapter.

Partial support for the NHFN is provided by the USGS through the NEHRP external grant program. Expansion of the NHFN has been made possible through generous funding from Caltrans, with the assistance of Pat Hipley. Larry Hutchings of LLNL has been an important collaborator on the project.

References

Rogers, P.W., A.J. Martin, M.C. Robertson, M.M. Hsu, and D.B. Harris, Signal-Coil Calibration of Electromagnetic Seismometers, Bull. Seism. Soc. Am., 85(3), 845-850, 1995.

Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.

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