Subsections


Northern Hayward Fault Network



Introduction

Complementary to the regional surface broadband and short-period networks, the Hayward Fault Network (HFN) (Figure 3.12 and Table 3.5) is a deployment of borehole-installed, wide-dynamic range seismographic stations along the Hayward Fault and throughout the San Francisco Bay toll bridges system. Development of the HFN initiated through a cooperative effort between the BSL (Berkeley Seismological Laboratory) and the USGS, with support from the USGS, Caltrans, EPRI, the University of California Campus/Laboratory Collaboration (CLC) program, LLNL (Lawrence Livermore National Laboratory), and LBNL (Lawrence Berkeley National Laboratory). The project's objectives included an initial characterization period followed by a longer-term monitoring effort using a backbone of stations from among the initial characterization set. Subsequent funding from Caltrans, however, has allowed for continued expansion of the backbone station set for additional coverage in critical locations.

The HFN consists of two components. The Northern Hayward Fault Network (NHFN), operated by the BSL, consists of 30 stations in various stages of development and operation. These include stations located on Bay Area bridges, at free-field locations, and now at sites of the Mini-PBO (mPBO) project (installed with support from NSF and the member institutions of the mPBO project). The NHFN 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. The purpose of the HFN is fourfold: 1) to contribute operational data to California real-time seismic monitoring for response applications and the collection of basic data for long-term hazards mitigation, 2) to increase substantially the sensitivity of seismic data to low amplitude seismic signals, 3) to increase the recorded bandwidth for seismic events along the Hayward fault, and 4) to obtain deep bedrock ground motion signals at the bridges from more frequent, smaller earthquakes.

In addition to the NHFN's contribution to real-time seismic monitoring in California, the mix of deep NHFN sites at near- and far- field sites and the high-sensitivity (high signal-to-noise), high-frequency broadband data recorded by the NHFN also contributes significantly to a variety of scientific objectives, including: a) investigating bridge responses to stronger ground motions from real earthquakes; b) obtaining a significantly lower detection threshold for microearthquakes and possible non-volcanic tremor signals; c) increasing the resolution of the fault-zone seismic structure (e.g., in the vicinity of the Rodgers Creek/Hayward Fault step over); d) improving monitoring of spatial and temporal evolution of seismicity (to magnitudes below $M \sim 0.0$) that may signal behavior indicative of the nucleation of large, damaging earthquakes; e) investigating earthquake scaling, physics, and related fault processes; f) improving working models for the Hayward fault; and g) using these models to make source-specific response calculations for estimating strong ground shaking throughout the Bay Area.

Below, we focus primarily on activities associated with BSL operations of the NHFN component of the HFN.

Figure 3.12: Map showing the locations of the HFN stations operated by the BSL (NHFN - squares and diamonds) and the USGS (SHFN - circles). Currently and previously active NHFN monitoring sites (i.e., those with data archived at the NCEDC) are filled blue/black. Sites CRQB and SMCB have been decommissioned in favor of replacement sites (CMAB and SM2B respectively) with higher quality data. Data prior to the termination of recording at sites RSRB and BBEB (resulting from retrofit work on the Richmond-San Rafael and Bay Bridges) is also available at the NCEDC. Sites in progress are yellow/grey. Other instrumented but currently non-operational boreholes are indicated as open symbols. PINB is non-instrumented open symbol site under consideration. Currently, station BBEB operates only as a telemetry repeater site because access to the borehole was cut off during seismic retrofit work on the eastern span of the Bay Bridge.
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Table 3.5: 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 (depth of sensor package below surface) 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. Installation of PINB is pending, due to environmental issues. The * in the Date column indicates stations recording data only during the premonitoring period (manually retrieved data now at LLNL). Temporary deployment data are not archived on the NCEDC, and the stations have been mothballed and are currently offline. The borehole sensors are still in place, however, and funds have been requested in a proposal to Caltrans to bring three of these sites back online (which three has yet to be determined). Note that due to Bay Bridge retrofit work, station BBEB now operates only as a telemetry relay station and no longer records seismic activity.
Code Net Latitude Longitude Elev (m) Over (m) Date Location
VALB BK 38.1215 -122.2753 -24 155.8 2005/11 - current Napa River Bridge
PETB BK 38.1189 -122.5011 -30 113 2010/09 - current Petaluma River Bridge
CMAB BK 38.06885 -122.22909 6 148 2009/12 - current Cal Maritime Academy
CRQB BK 38.05578 -122.22487 -25 38.4 1996/07 - 2010/05 CB
HERB BK 38.01250 -122.26222 -25 217.9 2000/05 - current Hercules
PINB* BK 38.0113 -122.3653 tbd tbd not recorded Point Pinole
BRIB BK 37.91886 -122.15179 219.7 108.8 1995/06 - 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 - 2007/06 SMC, Moraga
SM2B BK 37.8387 -122.1102 200 150.9 2007/06 - current SMC, Moraga
SVIN BK 38.03325 -122.52638 -21 158.7 2003/08 - current mPBO, St. Vincent's school
OHLN BK 38.00742 -122.27371 -0 196.7 2001/07 - current mPBO, Ohlone Park
MHDL BK 37.84227 -122.49374 94 160.6 2006/05 - current mPBO, Marin Headlands
SBRN BK 37.68562 -122.41127 4 157.5 2001/08 - current mPBO, San Bruno Mtn.
OXMT BK 37.4994 -122.4243 209 194.2 2003/12 - current mPBO, Ox Mtn.
BBEB BK 37.82167 -122.32867 -31 150.0 2002/05 - 2007/08 BB, Pier E23
E17B BK 37.82086 -122.33534   160.0 1995/08 - current * BB, Pier E17
E07B BK 37.81847 -122.34688 tbd 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 tbd 36.3 1997/10 - current * BB, Pier W5
W02B BK 37.79120 -122.38525 -45 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 - 2001/04 RSRB, Pier 34
RB2B BK 37.93 -122.41 -18 133 2009/11 - 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 3.6: Instrumentation of the HFN as of 06/30/2010. Every HFN downhole package consists of collocated 3-component geophones and accelerometers, with the exception of mPBO sites which have only 3-component geophones and are also collecting tensor strainmeter data. Six HFN sites (5 of the SHFN and 1 of the NHFN) also have dilatometers (Dilat.). Currently, 15 NHFN sites have Quanterra data loggers with continuous telemetry to the BSL. The remaining backbone sites are still being developed with support from Caltrans. 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). VPN is Virtual Private Network.
Site Geophone Accelerometer Z H1 h2 data logger Notes Telem.
VALB Oyo HS-1 Wilcoxon 731A TBD TBD TBD Q330   FR
PETB Oyo HS-1 Wilcoxon 731A TBD TBD TBD Q300   FR/Rad.
CMAB Oyo HS-1 Wilcoxon 731A TBD TBD TBD Q4120   Rad./VPN
CRQB Oyo HS-1 Wilcoxon 731A -90 251 341 None at Present   FR
HERB Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120   FR
PINB Oyo HS-1 Wilcoxon 731A TBD TBD TBD TBD   TBD
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 None at present Posthole FR
SM2B Oyo HS-1 Wilcoxon 731A TBD TBD TBD Q4120   FR
SVIN Mark L-22   -90 298 28 Q4120 Tensor. FR/Rad.
OHLN Mark L-22   -90 313 43 Q4120 Tensor. FR
MHDL Mark L-22   -90 TBD TBD Q4120 Tensor. FR
SBRN Mark L-22   -90 347 77 Q4120 Tensor. FR
OXMT Mark L-22   -90 163 253 Q4120 Tensor. FR
BBEB Oyo HS-1 Wilcoxon 731A -90 TBD TBD None at present 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 None at present 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 None at present 2 acc. failed FR
RB2B Oyo HS-1 Wilcoxon 731A -90 TBD TBD Q4120 1 acc. failed FR
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


NHFN Overview

The initial characterization period of HFN development ended in 1997. During that period, the NHFN sensors provided signals to on-site, stand-alone Quanterra Q730 and RefTek 72A-07 data loggers, and manual retrieval and download of data tapes was required. Also in that year, the long-term monitoring phase of the project began, involving the installation of 24-bit data acquisition and communication platforms and data telemetry to the BSL archives to create a backbone of initial NHFN stations.

Over the years, Caltrans has provided additional support for the upgrade of two non-backbone sites to backbone operational status and for the addition of several new sites to the monitoring backbone. These expansion efforts are ongoing. Also since February 1 of 2007, the 5 stations of the mPBO project have been folded into the NHFN.

Of the 30 stations considered part of the NHFN history, 10 are non-backbone stations that have not been upgraded to continuous telemetry. Though collection of monitoring data from these sites has never taken place, their borehole sensor packages are still downhole (having been grouted in), and 8 of these sites were mothballed for possible reactivation' in the future. Reactivation of two of the mothballed sites is currently in progress (W05B and E07B), and efforts to fund reactivation/upgrade of the other mothballed sites with Quanterra or Basalt data loggers and continuous telemetry are ongoing. Sixteen of the 30 stations are operational, with 15 of the sites telemetering recorded data streams that flow continuously into the BSL's BDSN processing stream with subsequent archival in the Northern California Earthquake Data Center (NCEDC) archive. These include the 5 mPBO sites. One additional site, BBEB, had previously recorded data as an active backbone site, but in August of 2007 its sensor cable was severed during retrofit work on the east span of the Bay Bridge. This site now operates only as a telemetry repeater site.

Three additional previously active backbone sites (RSRB, SMCB, and CRQB) are no longer in service. RSRB was taken off-line during the retrofit of the Richmond-San Rafael Bridge, with the expectation that it would be reactivated upon completion of the retrofit work. Unfortunately, during the retrofit, the sensor cable to the site was inadvertently dropped into the bay by contractors and was not recoverable. Both stations SMCB (a shallow post-hole installation) and and CRQB (a shallow and very noisy installation) were replaced with nearby higher quality installations at SM2B and CMAB, respectively.

Installation of one planned new borehole site (PINB) at Pt. Pinole Regional Park is being reconsidered after unexpected environmental issues were recognized relating to the sites historical use as a dynamite manufacturing facility and the possible release of deep seated chemical contaminants from the planned drilling of the borehole.


Table 3.7: Typical data streams acquired at NHFN sites, with channel name, sampling rate, sampling mode, and FIR filter type. C indicates continuous, T triggered, Ca causal, and Ac acausal. Typically, the DP1 continuous channel is archived and the remaining high sample rate data (i.e., CL and DP channels) are archived as triggered snippets. Prior to September 2004, however, only triggered data was archived for all high sample rate channels. Of the stations currently recording data, CMAB, HERB, BRIB, RFSB, CMSB, SM2B, W02B, and RB2B record at maximum sample rates of 500 Hz; VALB and PETB at maximum 200 Hz and mPBO sites (SVIN, OHLN, MHDL, SBRN, OXMT) at maximum 100 Hz.
Sensor Channel Rate (sps) Mode FIR
Accelerometer CL? 500.0 T Ca
Accelerometer HL? 200.0 C Ca
Accelerometer BL? 20.0 C Ac
Accelerometer LL? 1.0 C Ac
Geophone DP? 500.0 T,C Ca
Geophone EP? 200.0 C Ca
Geophone EP? 100.0 C Ca
Geophone BP? 20.0 C Ac
Geophone LP? 1.0 C Ac


Installation/Instrumentation: The NHFN Sensor packages are generally installed at depths ranging between 100 and 200 m, the non-backbone, non-operational Dumbarton bridge sites being exceptions with sensors at multiple depths (Table 3.5).

The five former mPBO sites that are now part of the NHFN have 3-component borehole geophone packages. Velocity measurements for the mPBO sites are provided by Mark Products L-22 2 Hz geophones (Table 3.6). All the remaining backbone and non-backbone NHFN sites have six-component borehole sensor packages. The six-component packages were designed and fabricated at LBNL's Geophysical Measurement Facility and have three channels of acceleration, provided by Wilcoxon 731A piezoelectric accelerometers, and three channels of velocity, provided by Oyo HS-1 4.5 Hz geophones.

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.

All 15 recording NHFN backbone sites have Quanterra data loggers with continuous telemetry to the BSL. Signals from these stations are digitized at a variety of data rates up to 500 Hz at 24-bit resolution (Table 3.7). The data loggers employ causal FIR filters at high data rates and acausal FIR filters at lower data rates.

Data Rates and Channels: Because of limitations in telemetry bandwidth and disk storage, 7 of the 10 (excluding CMAB, VALB and PETB) six-component NHFN stations transmit maximum 500 Hz data, one channel of geophone data continuously (i.e., their vertical geophone channels), and an additional 3 channels of triggered data in 90 second snippets. VALB transmits maximum 200 Hz data with one continuous geophone channel and three triggered channels. PETB transmits maximum 200 Hz data continuously on all six channels (three geophone, three accelerometer), and CMAB transmits maximum 500 Hz data continuously on all six channels. A Murdock, Hutt, and Halbert (MHH) event detection algorithm (Murdock and Hutt, 1983) is operated independently at each station on 500 sps data for trigger determinations. Because the accelerometer data is generally quieter, the three triggered channels are taken from the Wilcoxon accelerometers when possible. However, there is a tendency for these powered sensors to fail, and, in such cases, geophone channels are substituted for the failed accelerometers. Station VALB also transmits data from only four channels; however, all channels are transmitted continuously at a maximum of 200 Hz sampling. Continuous data for all channels at reduced rates (20 and 1 sps) are also transmitted to and archived at the BSL. The five mPBO originated sites transmit their three-component continuous geophone data streams, which are also archived at BSL, at 100, 20, and 1 sps.

Integration with the NCSS, SeisNetWatch, and SeismiQuery: The NHFN is primarily a research network that complements regional surface networks by providing downhole recordings of very low amplitude seismic signals (e.g., from micro-earthquakes or non-volcanic tremor) at high gain and low noise. Nonetheless, we have now also completed the integration of data flow from all operating NHFN stations into the Northern California Seismic System (NCSS) real-time/automated processing stream for response applications and collection of basic data for long-term hazards mitigation. The NCSS is a joint USGS (Menlo Park) and Berkeley Seismological Laboratory (BSL) entity with earthquake reporting responsibility for Northern California, and data from networks operated by both institutions are processed jointly to fulfill this responsibility.

Through this integration, the NHFN picks, waveforms, and NCSS event locations and magnitudes are automatically entered into a database where they are immediately available to the public through the NCEDC and its DART (Data Available in Real Time) buffer. The capability for monitoring state of health information for all NHFN stations using SeisNetWatch has also now been added, and up-to-date dataless SEED formatted metadata is now made available by the NCEDC with the SeismiQuery software tool.

Figure 3.13: Plot showing typically observed background noise PSD for the NHFN borehole stations (including the mPBO in gray lines) as a function of frequency. The data are for a 1000 second period on February 1, 2010 beginning at 02:00 (AM) local time on a Monday morning. Note that there is considerable variation in the general level and structure of the individual station background noise PSD estimates. The signals from three of the stations (RFSB, SM2B, and VALB) have 60 Hz noise (sometimes accompanied by 120 and 180 Hz harmonics), which is indicative of the presence of ground loops that need to be addressed. The PSD ranking (lowest to highest) of the 14 stations in operation at the time (PETB was not yet recording data) at 3 Hz (near minimum PSD for most NHFN stations) is:
CMSB.BK.DP1 -159.616
SM2B.BK.DP1 -149.143
SVIN.BK.EP1 -147.990
BRIB.BK.DP1 -147.706
OXMT.BK.EP1 -147.052
OHLN.BK.EP1 -147.004
MHDL.BK.EP1 -140.832
SBRN.BK.EP1 -137.196
RFSB.BK.DP1 -136.228
HERB.BK.DP1 -130.961
CMAB.BK.DP1 -118.527
VALB.BK.EP1 -109.230
CRQB.BK.DP1 -104.792
W02B.BK.DP1 -81.926
\begin{figure}\centering\setlength{\belowcaptionskip}{3pt}\epsfig{file=nhfn10_PSD.eps, width=8.5cm}\end{figure}

Figure 3.14: Plot of ground accelerations recorded on the geophones (black lines) and accelerometers (gray lines) of the 12 NHFN borehole stations operational at the time of a recent Bay Area earthquake (28 June 2010, M 3.25 offshore of San Francisco, CA). The traces are filtered with a 1-8 Hz bandpass filter, scaled by their maximum values, and ordered from bottom to top by distance from the epicenter
\begin{figure}\centering\setlength{\abovecaptionskip}{0pt}\setlength{\belowcaptionskip}{0pt}\epsfig{file=nhfn10_lcl.eps, width=7cm}\end{figure}

Figure 3.15: Plot of P-wave seismograms of the teleseismic $M_w$ 8.8 earthquake in the offshore Maule, Chile (Lat.: 35.909S; Lon.: 72.733W; Depth: 35 km) occurring on February 27, 2010 at 03:34:14 (UTC), recorded on the DP1 (vertical) channels of the 10 NHFN borehole stations in operation at the time. Here, vertical component geophone (velocity) data have been 0.1-0.5 Hz bandpass filtered.
\begin{figure}\centering\setlength{\belowcaptionskip}{3pt}\epsfig{file=nhfn10_teles.eps, width=7.5cm}\end{figure}

Station Maintenance

Ongoing network maintenance involves regular inspection of the collected seismic waveform data and spectra of nearby seismic events, and also of noise samples. Other common problems include changes to background noise levels due to ground loops and failing preamps, as well as power and telemetry issues. Troubleshooting and remediation of problems often benefit from a coordinated effort, with a technician at the BSL examining seismic waveforms and spectra while the field technicians are still on site. BSL technicians and researchers regularly review data and assist in troubleshooting.

The NHFN station hardware has proven to be relatively reliable. Nonetheless, numerous maintenance and performance enhancement measures are still carried out. In particular, when a new station is added to the backbone, extensive testing and correction for sources of instrumental noise (e.g., grounding related issues) and telemetry through-put are carried out to optimize the sensitivity of the station. Examples of maintenance and enhancement measures that are typically performed include: 1) testing of radio links to ascertain reasons for unusually large numbers of dropped packets, 2) troubleshooting sporadic problems with numerous frame relay telemetry dropouts, 3) manual power recycle and testing of hung Quanterra data loggers, 4) replacing blown fuses or other problems relating to dead channels identified through remote monitoring at the BSL, 5) repairing frame relay and power supply problems when they arise, and 6) correcting problems that arise due to various causes, such as weather or cultural activity.

Quality Control

Power Spectral Density Analyses: One commonly used quality check on the performance of the borehole installed network includes assessment of the power spectral density (PSD) distributions of background noise. Figure 3.13 shows PSD of background noise for vertical geophone components of the 14 NHFN stations operating at the time.

By periodically generating such plots, we can rapidly evaluate the network's recording of seismic signals across the wide high-frequency spectrum of the borehole NHFN sensors. Changes in the responses often indicate problems with the power, telemetry, or acquisition systems or with changing conditions in the vicinity of station installations that are adversely affecting the quality of the recorded seismograms. In general, background noise levels of the borehole NHFN stations are more variable and generally higher than those of the Parkfield HRSN borehole stations (see Parkfield Borehole Network section). This is due in large part to the significantly greater cultural noise in the Bay Area and the siting of several near-field NHFN sites in proximity to bridges.

On average, the mPBO component of the NHFN sites is more consistent and somewhat quieter. This is due in large part to the greater average depth of the mPBO sensors, the locations of mPBO stations in regions with 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 Q4120 data loggers is power line noise (60 Hz and its harmonics at 120 and 180 Hz). This noise reduces the sensitivity of the MHH detectors and can corrupt research based on full waveform analyses. When NHFN stations are visited, the engineer at the site and a seismologist at the BSL frequently work together to identify and correct ground-loop problems, which often generate 60, 120, and 180 Hz contamination from inductively coupled power line signals.

Real Event Displays: Another method for rapid assessment of network performance is to generate and evaluate the seismograms from moderate local and large teleseismic earthquakes recorded by the NHFN stations. This is an essential component of NHFN operations because the seismic data from local, regional, and teleseismic events is telemetered directly to the BSL and made available to the Northern California Seismic System (NCSS) real-time/automated processing stream within a few seconds of being recorded by the NHFN for seismic response applications.

Shown in Figure 3.14 is an example display of NHFN geophone and accelerometer channels for a recent local Bay Area earthquake (28 June 2010, $M_{L}$ 3.25 offshore of San Francisco, CA). It is immediately apparent from this simple display that the some components of stations OHLN and SVIN were in need of attention by field personnel.

Figure 3.15 shows seismograms of the recent teleseismic $M_{w}$ 8.8 earthquake of February 27, 2010 at 03:34:14 (UTC) occurring offshore of Maule, Chile (Lat.: 35.909S; Lon.: 72.733W; Depth: 35 km) On this date and for this frequency band (0.1-0.5 Hz) network performance appears good for the 10 stations in operation at the time; however, an additional 4 sites did not record this event, for various reasons, and had to be visited by field personnel. Figures 3.14 and 3.15 serve to illustrate the value of routine evaluation of both local (higher frequency) and teleseismic (lower frequency) events when monitoring the state of health of the NHFN.

Owing to their near similar source-receiver paths, signals from teleseismic events also serve as a good source for examining the relative responses of the BK borehole network station/components to seismic ground motion, after correction for differences in instrument response among the stations. By rapidly generating such plots (particularly with correction for instrument response) following large teleseismic events, quick assessment of the NHFN seismometer responses to real events is easily done and corrective measures implemented with relatively little delay.

2009-2010 Activities

As in every year, routine maintenance, operations, quality control, and data collection have played an important part in our activities. In this year, we are fortunate to have received funds and government furnished equipment (GFE) data loggers to update equipment and improve station infrastructure from an American Recovery and Reinvestment Act award from the USGS. The equipment will be used to upgrade data loggers at 9 stations, including the mPBO stations. As the GFE data loggers did not arrive at the BSL until Summer 2010, we did not embark on any equipment upgrades at the NHFN sites in this reporting interval. Some maintenance activities, however, were funded by the award.

Other NHFN project activities have included: a) efforts to obtain additional funds for future upgrade and expansion of the network, b) leveraging NHFN activities through partnerships with various institutions outside of BSL, c) network adaptations to compensate for changing conditions associated with retrofit work on Bay Area bridges, and d) new station additions and network expansion efforts.

Additional Funding

Operation of this Bay Area borehole network is funded by the ANSS and through a partnership with the California Department of Transportation (Caltrans). ANSS (Advanced National Seismic System) provides operations and maintenance (O&M) support for a fixed subset of 9 operational stations that were initiated as part of previous projects in which the USGS was a participant. Caltrans provides support for development and O&M for an additional 10 stations that have been or are in the process of being added to the network with Caltrans partnership grants. Caltrans also continues to provide additional support for upgrade and expansion when possible.

In June of this year, our team held 2 meetings at Berkeley with our Caltrans contact and made a presentation at Caltrans in Sacramento to argue against O&M funding reductions and for further upgrade and expansion of the NHFN. These efforts resulted in a request by Caltrans for a proposal to install surface instruments at up to 6 of our borehole installations and to reactivate 3 currently mothballed NHFN sites. We submitted our proposal in September of 2010 and are awaiting a decision from Caltrans.

Partnerships

The NHFN is heavily leveraged through partnerships with various institutions, and we have continued to nurture and expand these relationships. Over the past year, we have continued our collaborative partnerships with the USGS, Caltrans, St. Mary's College, and the Cal Maritime Academy. In addition, we have coordinated with a Lawrence Berkeley National Laboratory (LBNL) project to help develop to ensure complementary placement of borehole installations at LBNL with our existing NHFN stations.

New Installations

Since reorganization of engineering support for the NHFN project this past year, significant progress has been made on activation of NHFN stations. This year, three new sites have been brought fully online (CMAB, PETB and RB2B). Two additional instrumented sites are awaiting completion of the retrofit of the Bay Bridge before being completed and brought online (E07B and W05B).

Last year, complex negotiations involving (among others) the East Bay Regional Parks District and UNAVCO were finally completed, giving us permission to create borehole site (PINB) at Pt. Pinole Regional Park. However, it has now been recognized that installation of a deep borehole at this site is potentially problematic due to environmental issues (in the past, the park had been a dynamite manufacturing facility, leaving the possibility that liberation of chemical contaminants may occur from extraction of borehole materials during drilling). We are continuing to evaluate the viability of installation at this site, given these circumstances.

Acknowledgments

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 and Doug Dreger's general supervision, Bill Karavas, Doug Neuhauser, Bob Uhrhammer, John Friday, Taka'aki Taira, and Rick Lellinger all contribute to the operation of the NHFN. Bob Nadeau prepared this section with help from Taka'aki Taira.

Support for the NHFN is provided by the USGS through the NEHRP grant program (grant numbers 07HQAG0014 and G10AC00093) and by Caltrans through grant number 65A0366. The ARRA Award to support maintance and equipment upgrades at the NHFN stations is USGS grant number G09AC00487. Pat Hipley of Caltrans has been instrumental in the effort to continue to upgrade and expand the network. Larry Hutchings and William Foxall of LLNL have also been important collaborators on the project in past years.

References

Rodgers, 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 pages, 1983.

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