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Northern Hayward Fault Network

Subsections

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 project 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 5.1 and Table 5.1).

The purpose of the network is twofold: to lower substantially the threshold of microearthquake detection and increase the recorded bandwidth for events along the Hayward fault; and to obtain bedrock ground motion signals at the bridges from small earthquakes for investigating bridge responses to stronger ground motions. A lower detection threshold will increase the resolution of fault-zone structural features and define spatio-temporal characteristics in the seismicity at $M \sim > -1.0$, where occurrence rates are dramatically higher than those captured by the surface sites of the NCSN. This new data collection will contribute to improved working models for the Hayward fault. The bedrock ground motion recordings are 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.

The Hayward Fault Network (HFN) consists of two parts. The Northern Hayward Fault Network (NHFN) is operated by the BSL and currently consists of 20 stations, including those located on the Bay bridges. 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 5.1: 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.
\begin{figure*}\begin{center}
\epsfig{file=hfn_map.ps, width=15cm}\end{center}\end{figure*}

NHFN Overview

All sites of the HFN have six-component borehole sensor packages which were designed and fabricated at LBNL's Geophysical Measurement Facility by Don Lippert and Ray Solbau, with the exception of site SFAB. 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 (Table 5.2). Sensors are installed at depths of 100-300 m and provide signals to the on-site data loggers (Quanterra Q4120 and Q730, Nanometrics HRD24, or RefTek 72A-07 systems).

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.

Eight 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 5.3). 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 7 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 require visits from BSL staff for data recovery. Seven of these sites located on the Bay Bridge are scheduled to be upgraded with Quanterra data loggers and continuous telemetry in the fall of 2002 (see Figure 11.2 in Chapter 11). The Bay Bridge component of the NHFN has been delayed during the past year, primarily due to the major effort required to upgrade the HRSN (Chapter 6).

Signals from the 5 SHFN stations are digitized by Nanometrics data loggers at 200 sps and transmit continuous data to Menlo Park by radio. The digital data streams are processed by the Earthworm system with the NCSN data and waveforms are saved when the Earthworm detects an event.


Table 5.1: 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 date indicates 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 YBIB and RSRB indicates that the stations are not currently operational at this time. 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. The table also includes 2 MPBO stations which became operational in the last year.
Code Net Latitude Longitude Elev (m) Over (m) Date Location
BRIB BK 37.91886 -122.15179 219.7 108.8 1995/07 - current BR, Orinda
CMSB BK 37.87195 -122.25168 94.7 167.6 1994/12 - current CMS, Berkeley
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
RFSB BK 37.91608 -122.33610 -27.3 91.4 1996/01 - current RFS, Richmond
RSRB BK 37.93575 -122.44648 -48.0 109 1997/06 - current * RSRB, Pier 34
SMCB BK 37.83881 -122.11159 180.9 3.4 1997/12 - current SMC, Moraga
YBIB BK 37.81420 -122.35923 -27.0 61 1997/12 - current * BB, Pier E2
OHLN BK 38.00742 -122.27371     2001/07 - current MPBO, Ohlone Park
SBRN BK 37.68562 -122.41127     2001/08 - current MPBO, San Bruno Mtn.
SFAB BK 37.78610 -122.3893   0.0 1998/06 - current BB, SF Anchorage
W02B BK 37.79120 -122.38525   57.6 1996/04 - current BB, Pier W2
W05B BK 37.80100 -122.37370   36.3 1997/10 - current BB, Pier W5
YBAB BK 37.80940 -122.36450   3.0 1998/06 - current BB, YB Anchorage
E07B BK 37.81847 -122.34688   134.0 1996/02 - current BB, Pier E7
E17B BK 37.82086 -122.33534   160.0 1995/08 - current BB, Pier E17
BBEB BK 37.82167 -122.32867   150 1994/03 - 1995/10 BB, Pier E23
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  
DB2B BK 37.50687 -122.11566     1994/07 - current DB, Pier 27
          189.2 1992/07 - 1992/11  
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  
SM1B BK 37.59403 -122.23242   298.0 not recorded SMB, Pier 343
RB2B BK 37.93372 -122.41313   44 1997/06 - current RSRB, Pier 58
CCH1 NC 37.7432 -122.0967 226   1995/05 - current Chabot
CGP1 NC 37.6454 -122.0114 340   1995/03 - current Garin Park
CMW1 NC 37.5403 -121.8876 343   1995/06 - current Mill Creek
CSU1 NC 37.6430 -121.9402 499   1995/10 - current Sunol
CYD1 NC 37.5629 -122.0967 -23   2002/09 - current Coyote



Table 5.2: Instrumentation of the HFN as of 06/30/2002. Every HFN downhole package consists of co-located geophones and accelerometers, with the exception of OHLN and SFAB. 6 HFN sites also have dilatometers (Dilat.) and the 2 MPBO sites have tensor strainmeters (Tensor.) 7 NHFN sites have Quanterra data loggers with continuous telemetry to the BSL. The remaining sites use RefTek data loggers for on-site recording. 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 Telemetry
BRIB Oyo HS-1 Wilcoxon 731A -90 79 349 Q4120 Acc. failed, Dilat. FR
CMSB Oyo HS-1 Wilcoxon 731A -90 19 109 Q4120   FR
CRQB Oyo HS-1 Wilcoxon 731A -90 251 341 Q4120   FR
HERB Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120   FR
RFSB Oyo HS-1 Wilcoxon 731A -90 256 346 Q4120   FR
RSRB Oyo HS-1 Wilcoxon 731A -90 50 140 Q4120 2 acc. failed FR
SMCB Oyo HS-1 Wilcoxon 731A -90 76 166 Q4120 Posthole FR
YBIB Oyo HS-1 Wilcoxon 731A -90 257 347 Q4120 Z geop. failed Radio
OHLN Mark L-22   -90 TBD TBD Q4120 Tensor. FR
SBRN Mark L-22   -90 TBD TBD Q4120 Tensor. FR
SFAB None LLNL S-6000 TBD TBD TBD RefTek 72A-07 Posthole  
W02B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
W05B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
YBAB Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
E07B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
E17B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
BBEB Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present Acc. failed  
DB1B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07 Acc. failed  
DB2B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
DB3B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07 Acc. failed  
SM1B Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present    
RB2B Oyo HS-1 Wilcoxon 731A -90 TBD TBD RefTek 72A-07    
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
CMW1 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


As part of the USGS and BSL collaboration on the HFN, data from the NHFN and SHFN sites with continuous telemetry are shared in near real-time. NHFN data are transmitted to the USGS and SHFN data are transmitted to the BSL.

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 will be 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 13). 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 displacements, as inferred from the vertical component accelerometer and from the vertical component geophone data streams recorded at BRIB, CMSB, CRQB, HERB, OHLN, SBRN (the newest MPBO station, sited on San Bruno Mtn, San Francisco Peninsula), and RFSB, for a M 7.7 deep focus earthquake that occurred in the Fiji Islands at a depth of 580 km. in Figure 5.2.

Figure 5.2: Ground displacement waveforms, inferred from accelerometer and velocity sensors at six borehole stations (4 NHFN and 2 MPBO) for the 19 August 2002 deep focus Fiji Islands teleseism (11:01 UT, -21.70, -179.51, 580 km deep, M 7.7). The waveforms have been 0.3-2 Hz bandpass filtered and deconvolved to ground displacement and ordered by epicentral distance for comparison. The highly similar waveforms indicate that the instruments are operating normally and that the transfer functions are correct.
\begin{figure*}\begin{center}
\epsfig{file=hfn_waveforms.eps, width=15cm}\end{center}\end{figure*}


Table 5.3: 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


2001-2002 Activities

During this year, two stations of the NHFN continued to be not operational. YBIB was shut down when power was cut off in August 2000 and RSRB was taken offline in April 2001 during the retrofit project on the Richmond-San Rafael Bridge. YBIB is anticipated to return after solar panels are installed in late 2002/early 2003. No estimate of the return of RSRB is currently available.

Station Maintenance

The most pervasive problem at NHFN stations equipped with 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.

CMSB

Replaced batteries. Repaired signal cable where it was chewed by rodents. Q4120 is drawing 2.9 amps. The preamp draws about 200 ma which is correct. This indicates higher than normal current (Q4120 current should be $\sim $2.3 amps) into the Q4120 and gradual failure of one of the circuits.

HERB

Q4120 installed on 18 September, 2001. Geophone channel experiencing large 60 Hz and harmonics signals apparently due to the presence of ground loops. Investigation of this signal contamination continues.

RFSB

The Q4120 serial ports failed due to faulty capacitors.

SMCB

Experienced numerous intermittent telemetry problems during year. Rebuilt power system and installed new FRAD, power supply and batteries in a second Hoffman box. Q4120 failed due to a blown hard wired fuse on circuit board. Replaced fuse and reinstalled Q4120.

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 four 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, and; a spectrogram method for characterizing the frequency-time power distribution of the observed seismic waveforms.

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 (ie, 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

The spectral phase coherency algorithm was developed to facilitate high resolution quantification of the similarities and differences between highly similar Hayward fault events which occur occur months to years apart. Figure 5.3 shows an example of three highly similar $M_{L}$$\sim $1.3 events. The 0.997 complex spectral phase coherency between the waveforms for events a (1998.202.132956) and c (2000.170.171607) implies that the centroids of these two events are not more than $\sim $20 cm apart spatially. Extrapolation of magnitude versus fault rupture area, empirically derived using M $\sim $4-7 earthquakes, yields an expected rupture radius of $\sim $20 m for a M 1.3 event. However, the 0.997 phase coherency implies that the source rupture time histories can not differ by more than $\sim $ 6 $\mu$sec which, in turn, implies that either the rupture spatial-temporal histories of the two sources are virtually identical over a radius of $\sim $20 m or that the sources have a high stress drop and a rupture radius of order a few meters at most. Of these two possibilities, the latter is considered the most likely.

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.

Spectrogram Analysis

Figure 5.4 shows an example of a spectrogram derived from the CMSB Z-component ground acceleration recording of a M 1.3 local earthquake (1998.202.132956). Most of the power is in the first few seconds and coincides with the P-wave and S-wave arrivals and their immediate coda. There is little energy above $\sim $110 Hz and after $\sim $5 seconds for this M 1.3 event and $\sim $10 km propagation path. Spectrograms can be used as a tool to help in the characterization of seismic sources and propagation paths.

Figure 5.3: Example of using complex spectral phase coherency as a discriminant for analyzing a trio of highly similar earthquakes which occurred on the Hayward fault approximately 6 km northwest of Berkeley. The borehole station CMSB (197 m depth) raw Z-component acceleration data for the three earthquakes are shown in parts (a), (b) and (c). Note that the M 1.3 waveforms in (a) and (c) are visually virtually identical while the M 1.7 waveforms in (b) differs in detail and it has a 11 msec shorter S-P interval than either (a) or (c). The Phase coherency between (a) and (c), shown as the solid line in (d) (calculated using 10 seconds of the Z-component waveforms starting $\sim $ 1.38 seconds prior to the P-wave onset, i. e. using the entire waveform including through the S-wave coda), is 0.997 in the 2-80 Hz frequency band. The inference is that the (a) and (c) centroid locations differ by not more than 20 cm (assuming that the near source scatterers are isotropically distributed). The dashed line in (d) is the phase coherency between the (b) and (c) waveforms and the dips in the coherency at $\sim $15 Hz and $\sim $30 Hz can be interpreted as destructive interference caused by differences in their centroid locations of order 100 m (compatible with the $\sim $60 m along ray path differences in their S-P times). The solid and dashed lines in (e) are the (c) waveform signal and noise amplitude spectra, respectively. The SNR is 40+ dB in the $\sim $10-70 Hz band and above unity in the $\sim $1.5-110 Hz band and the change in slope above $\sim $65 Hz is interpreted as the corner frequency of the M 1.3 earthquake source.
\begin{figure*}\begin{center}
\epsfig{file=bob02_hfn_2eq.eps, width=15cm, clip=}\end{center}\end{figure*}

Figure: Spectrogram of the CMSB Z-component ground accelerations for the M 1.3 earthquake (Inset (a) in Figure 5.3). Plotted is relative power (dB) as a function of time and frequency.
\begin{figure}\begin{center}
\epsfig{file=bob02_hfn_spect.eps, width=8.5cm}\end{center}\end{figure}

New Installations

San Francisco-Oakland Bay Bridge Stations

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 Q730 data loggers and telemetering the data back to Berkeley in the fall of 2002.

Mini-PBO

The stations of the Mini-PBO project (Chapter 9) are equipped with borehole seismometers. As these stations have become operational, they augment HFN coverage (Figure 5.1). In the last year, OHLN at Ohlone Park, Hercules, has added to the coverage in the vicinity of San Pablo Bay and provides an interesting comparison with the NHFN station HERB.

Acknowledgements

Thomas V. McEvilly passed away in February 2002 (Chapter 2). Tom 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 and Doug Dreger's general supervision, Rich Clymer, Wade Johnson, Doug Neuhauser, Bob Uhrhammer, Bill Karavas, John Friday, and Dave Rapkin all contribute to the operation of the NHFN. 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

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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