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 ) 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.
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.
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.
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
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
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, 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 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.
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.
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.
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.
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.
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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