Complementary to the regional surface broadband and short-period networks, the Hayward Fault Network (HFN) (Figure 3.10 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, and LBNL. The project's objectives included an initial characterization period followed by 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 developmental and operational stages. 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 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 NHFN sites in 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, 13 of the stations are currently operational, with telemetered data streams flowing 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. Nine of the 30 stations are non-backbone stations that have not been upgraded to continuous telemetry. Though collection of data from these sites has been discontinued, their borehole sensor packages are still in place (having been grouted in), and efforts to find funding for upgrade of these sites with Quanterra Q4120, Q730, or Q330 data loggers and continuous telemetry continue. Two previously active backbone sites (BBEB and SMCB) have been taken out of service permanently. BBEB was taken out of service when its sensor cable was severed by contractors during seismic retrofit work on the east span of the Bay Bridge in August of 2007. The site now operates only as a telemetry repeater site. SMCB (a shallow post-hole site) was taken out of service after it was upgraded to a deep borehole installation in 2007. The upgraded deep borehole site (one of the 30 NHFN stations listed) is now named SM2B.
The remaining 6 sites (PETB, RB2B, E07B, W05B, CMAB, and PINB) are in the process of being added to the NHFN backbone. Four of the sites have been drilled and instrumented and are awaiting installation of their infrastructures, electronics, and telemetry (PETB, RB2B, E07B, and W05B). As of the writing of this report, drilling (provided by Caltrans) has begun on an additional site (CMAB) located at the Cal Maritime Academy. This site is intended to replace a particularly noisy backbone station at the south end of the Carquinez bridge (CRQB). With support for drilling and the purchase of a sensor package from Caltrans, the plan is to transfer the surface infrastructure and recording equipment at CRQB to the Cal Maritime site once drilling has been completed.
After complex negotiations involving (among others) the East Bay Regional
Parks District and UNAVCO, permission was given to create an additional site (PINB)
at Pt. Pinole Regional Park. However, it has now been recognized that installation
of a deep borehole at this site is potentially not feasible 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 currently in the process of evaluating
the situation further to decide whether or not the PINB installation will need to
be abandoned in favor of an alternative future site.
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 13 operational 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 8 (excluding VALB) 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 sec. snippets. 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 3 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 4 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 3-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 dropouts, 3) manual power recycle and testing of hung Quanterra data loggers, 4) replacement of blown fuses or other problems relating to dead channels identified through remote monitoring at the BSL, 5) repair of 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.
As an example, this year maintenance visits were necessary at several of
the MPBO stations. At OHLN, BSL and USGS instruments are collocated.
Power is provided by the local school district. Several times during
the past year, power to the seismic site failed mysteriously and was
restored after BSL personnel contacted the school. In May of 2009, BSL
engineers contacted the school district to replace the AC circuit
breaker. Since it was replaced, power has not failed again. All of the
back up batteries that were originally installed in 2001 were also
replaced. At the station SBRN, all of the batteries were replaced, and
additional batteries were installed to increase the reserve capacity.
At SVIN, the internal disk drive of the data logger failed and was replaced.
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 the 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.
Shown in Figure 3.12 is an example display of NHFN geophone channels for a local M3.2 event (13 September 2009, M3.2 near Brentwood, CA) occurring 40 km east of the center of the NHFN at 14.1 km depth. It is immediately apparent from this simple display that the vertical components at stations OHLN, RFSB, SBRN, SVIN, and W02B were either insensitive to or entirely unresponsive to this event, indicating an immediate need for attention by field personnel. Upon closer inspection, the presence of a 60 Hz buzz exists at stations SBRN and HERB, indicating that the grounding schemes for these channels is in need of modification. At any given station, 60 Hz related noise sources can change over periods of weeks to months, requiring continued vigilance and adaptability of the grounding scheme in order to maintain the desired high sensitivity to low amplitude seismic signals. recorded on the DP1 (vertical) channels of the 13 NHFN borehole stations in operation at the time. Here, vertical component geophone (velocity) data have been 0.1-0.5 Hz bandpass filtered, and the highest available sampling rate for a given component is plotted.
Figure 3.13 shows a plot of 0.1-0.5 Hz bandpass filtered ground velocity P-wave seismograms from the teleseismic 7.6 earthquake in the Tonga region (Lat.: 23.050S; Lon.: 174.668W; depth 34 km) occurring on March 19, 2009 18:17:40 (UTC) as recorded by all operational channels (geophones and accelerometers) of the NHFN borehole stations. On this date and for this frequency band overall network performance appears significantly better than that observed for the local event shown in Figure 3.12. This serves 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 are easily done and corrective measures implemented with relatively little delay.
Generally speaking, the accelerometers, being an active device, are more accurate and also more stable than 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 inaccurate. 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 geophone's inertial mass.
We have built a calibration test box which allows us to routinely perform the testing described by Rodgers et al. during site visits. 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 also check the response of the corresponding accelerometer.
Currently, we are also in the process of preparing a competitive proposal to Caltrans to continue to expand the NHFN with additional borehole installations and to upgrade several NHFN sites with strong-motion surface sensors to provide up-hole down-hole data for fundamental research on amplification effects in the upper 1-200 meters.
As of the writing of this report, permission and citing has been completed and drilling (provided by Caltrans) has begun on an additional site (CMAB) located at the Cal Maritime Academy. This site is intended to replace a particularly noisy backbone station at the south end of the Carquinez bridge (CRQB). With support for drilling and the purchase of a sensor package from Caltrans, the plan is to transfer the surface infrastructure and recording equipment at CRQB to the Cal Maritime site once drilling has been completed.
This 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 currently in the process of evaluating the situation further to decide whether or not the PINB installation will need to be abandoned in favor of an alternative future site (possibly at the Wildcat location).
Under Bob Nadeau's and Doug Dreger's general supervision, Rich Clymer, Doug Neuhauser, Bob Uhrhammer, Bill Karavas, 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 and Bob Uhrhammer.
Support for the NHFN is provided by the USGS through the NEHRP grant program (grant no. 07HQAG0014) and by Caltrans through grant no. 59A0578. 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 years past.
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 pp., 1983.
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