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 being developed cooperatively by the BSL and the USGS, with support from USGS, Caltrans, EPRI, the University of California Campus/Laboratory Collaboration (CLC) program, LLNL, and LBNL (Figure 3.1 and Table 3.1).
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. Lower detection threshold will increase the resolution of fault-zone structural features and define spatial-temporal characteristics in the seismicity at M > 0.0, 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.
Highlights of the past year include the installation of a new site at the Caltrans Hercules maintenance yard, the implementation of the central site triggering algorithm (critical to the detection of small events), and the final configuration of the Bay Bridge monitoring hubs (planned for installation in the fall of 2000).
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 3.2). Sensors are installed at depths of 100-300 m and provide signals to the on-site dataloggers (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. Figure 3.2 compares the noise level of the Wilcoxon accelerometer with other sensors used in the BDSN. 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 at the STS-1 and STS-2 between 1 and 50 Hz.
Seven of the NHFN sites have Quanterra dataloggers 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 (Table 3.3) at 24-bit resolution. Because of limitations in telemetry bandwidth and disk storage, these 7 sites transmit triggered data at 500 sps (based on a MHH detector) and continuous data at reduced rates (100, 20 and 1 sps) to the BSL.
The remaining 13 sites of the NHFN record data using RefTek dataloggers. 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 dataloggers and continuous telemetry in the fall of 2000 (see Figures 8.2 and 8.3 in Chapter 8).
Signals from 4 of the 5 SHFN stations are digitized by Nanometrics dataloggers at a mixture of 100 and 200 sps (CGP1 and CCH1 200; CMW1 and CSU1 100) 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. One site of the SHFN does not have telemetry and no datalogger is on-site at this time. The USGS hopes to resolve the telemetry problem in the coming year.
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 100 sps data for event detection. Triggers from this controller will be used to recover the 500 sps data from the NHFN dataloggers.
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 10). 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.
The year 2000 transition required software upgrades to the Quanterra dataloggers used by the BSL. Several components of Quanterra's UltraSHEAR acquisition software and the underlying OS/9 operating system were not year 2000 compliant, and required upgrades. These software issues are described more completely in Chapter 8.
Similar to BDSN sites, NHFN Quanterra dataloggers are capable of continuous on-site recording and local storage at full resolution for more than one day and have batteries to provide backup power. On-site detectors can monitor critical conditions and preset changes in operating mode can be invoked. The UltraSHEAR communication software developed for the NHFN allowed control and change of parameters from the central site. In the past year, this was replaced with MultiSHEAR, which provides for multi-station nodes using up to three Q730 satellite acquisition platforms around a Q4120, with a single 4-station telemetry link to the central site (see Chapter 8).
A test instrument (Figure 3.3) and associated termination plugs and cabling was designed and built to facilitate calibration and background noise testing of the sensors, preamp, and datalogger at the NHFN stations. The test instrument is designed for in situ testing of the sensors (impulse voltage to geophones), the preamp (gain), the datalogger (sensitivity), and also to aid in determining the noise floor and noise characteristics of the datalogger and the preamp using resistance terminations. Whenever a NHFN station is visited, the engineer at the station and a seismologist in the lab 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.
The geophones can be pulsed in situ with a calibration signal to check their response characteristics. However, the accelerometers can not be tested in situ and we must evaluate their response to seismic signals and their background noise PSD level to determine whether or not they are operating within their specifications.
At each NHFN station there is one component of ground motion that is recorded by both a geophone and an accelerometer. As a check on the calibration we compare the absolute ground accelerations, as inferred from the two sensors oriented in the same direction, recorded at four NHFN stations for the largest earthquake that occurred within 10 km of CMSB during the past year in Figure 3.4. Note that waveform pairs are nearly identical at all stations except RFSB. The difference at RFSB is due to a mislabeled data stream and the geophone being recorded is actually the P2 horizontal component. The data streams from RFSB have been problematic in that RFSB is the only site that has the combination of an unknown noise source that exhibits an unusual 5.8 Hz comb spectra as well as high 60, 120, 180, and 240 Hz power related peaks. We are continuing to troubleshoot the equipment at this station.
The NHFN stations utilize two different dataloggers, Quanterra Q4120 dataloggers for the telemetered NHFN stations and RefTek 72A-07 dataloggers for the locally recorded Bay Bridge stations. Both dataloggers have 24-bit integer resolution. A third type of datalogger, a Quanterra Q730, will be installed soon at six sites on the San Francisco-Oakland Bay Bridge.
The most pervasive problem at the Q4120 equipped stations is power line noise (60 Hz and its harmonics at 120, 180, and 240 Hz). This noise reduces the sensitivity of the Hutt-Murdock detectors. During the past year, the following stations required site visits:
BRIB: Upgraded disk to 4 Gb.
CMSB: The FRAD AC power supply was replaced with a DC-DC converter so that the telemetry would remain operational during a power outage. The clock antenna cable was cut and was replaced.
RFSB: Replaced the V100 FRAD, with a V320 FRAD that has an Ethernet port, in August 1999. At the same time a noise problem was traced to a faulty DC-DC converter on the A/D board and when it was replaced the noise problem disappeared. In May 2000, a 5.8 Hz pulse noise signal was present on all channels. Considerable time was spent tracking down the source of the noise and we were able to reduce its effect but not eliminate it by changing the grounding and DC-DC converter configurations. We now consider that the best approach to curing the noise problems is to rebuild the station hardware installation since RFSB was the first NHFN station to be installed and the hardware configuration has changed more than at any other NHFN station.
RSRB: Upgraded disk to 4 Gb. Respliced the signal cable in effort to reduce the noise. Recordings of local earthquakes show that two of the accelerometers have failed so the preamp and wiring were modified to record 3 geophone and 1 accelerometer channels.
YBIB: Installed Cylink radio, mast, and antenna, FRAD, and Ethernet cable and also replaced the power supply. Considerable telemetry problems were experienced which were eventually cured by moving the antenna on top of McCone Hall to the south end of the building to minimize signal adsorption by the eucalyptus grove on the west side of campus.
During the past year the RefTek equipped stations were visited approximately once every two months on average for maintenance and servicing which consisted primarily of replacing the accelerometer batteries, checking the overall condition of the equipment, and swapping the data disk. No hardware failures occurred during the year.
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 dataloggers and telemetering the data back to Berkeley.
The newest NHFN station (HERB) is sited at the Caltrans Hercules maintenance yard. The site is 5 km NE of the Hayward Fault and it is the farthest north of the NHFN on-land stations. Starting on April 17, 2000, a Caltrans crew drilled a 5-5/8 inch borehole to a depth of 218 m in five days (the deepest of any of the NHFN stations). The hole was mud-logged during the drilling and the primary material is siltstone and shale, with units of sandstone and clay. It is a very competent hole and no casing was required. Caltrans Borehole Geophysical Logging Services also provided geophysical testing using a P-S logger, a Caliper, a Gamma logger, and a resistance loggger and the results are shown in Figure 3.5. The instrument sonde is a ``standard HFN'' package. After the sonde was installed the hole was back-filled with cement slurry with approximately the same density as the surrounding material. A 10m hole was drilled 5 m north of the main hole and cased with PVC for the future installation of a surface instrument. We are currently awaiting the installation of the frame0relay telemetry circuit and plan to move the Quanterra Q4120 datalogger and associated hardware from CRQB (a very noisy station that is sited in a shallow borehole near the west abutment of the Carquinez Bridge) to HERB in the near future so that it can began operation.
The expansion of the NHFN has been made possible through generous funding from Caltrans and with the assistance of Pat Hipley. Larry Hutchings of LLNL has been an important collaborator on the project.
Under Tom McEvilly's general supervision, Rich Clymer, Bob Nadeau, Bob Uhrhammer, Wade Johnson, Bill Karavas, John Friday, Dave Rapkin, and Doug Neuhauser contribute to the operation of the NHFN. Tom McEvilly, Bob Uhrhammer, and Lind Gee contributed to the preparation of this chapter.