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 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 spatio-temporal characteristics in the seismicity at , 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.
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 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. 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 as the STS-2 between 1 and 50 Hz.
Seven 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 3.3). In contrast to the BDSN implementation, the NHFN data loggers employ casual FIR filters at high data rates and acasual 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 13 sites of the NHFN record 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 2001 (see Figures 8.2 and 8.3 in Chapter 8). The Bay Bridge component of the NHFN has been delayed in the past year, primarily due to the major effort required to upgrade the HRSN (Chapter 4).
Signals from 4 of the 5 SHFN stations are digitized by Nanometrics data loggers 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 data logger 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 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 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.
Similar to BDSN sites, NHFN Quanterra data loggers 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. Last 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 NHFN equipment. The test instrument is designed for in situ testing of the sensors (impulse voltage to geophones), the preamp (gain), the data logger (sensitivity), and also to aid in determining the noise floor and noise characteristics of the data logger and the preamp using resistance terminations. 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.
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.
As a check on the calibration and an example of the capabilities of borehole installed network, we compare the high-pass (HP) filtered (2 Hz) ground displacements, as inferred from the vertical component accelerometer 500 Hz (CL1) and from the vertical component geophone 500 Hz (DP1) data streams recorded at BRIB, CMSB, RFSB, and SMCB, for a M 3.0 earthquake that occurred in Pacifica, 34 km SW of UCB, in Figure 3.4.
An example of the seismic background noise PSD, inferred from the accelerometer and from the geophone sensors at CMSB are shown in Figure 3.5. The broad background noise peak, centered at 10 Hz is attributed to organ pipe mode resonances of the steel casing and the water column in the open borehole.
Figure 3.6 shows characteristically repeating and highly similar events that have been identified in the region. The NHFN borehole installed sensors are designed with the goal, in part, of obtaining temporally stable waveform recordings of microearthquakes, with a magnitude threshold lower than is observable with surface installed stations, to facilitate the search for highly similar and repeating earthquakes.
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 MHH detectors. The data loggers were upgraded to MSHEAR 001122 during February 2001 to enable near-real-time central site triggering. During the past year, the following stations required site visits:
BRIB: Installed new preamp with channel mapping changed for compatibility with MSHEAR. Repaired power cable which was accidentally cut in two places.
CMSB: Repaired faulty power supply and replaced batteries.
RFSB: Reconfigured equipment and installed hardware into new equipment enclosure. Spent considerable time systematically reducing 60 Hz (and harmonics) signals on seismic channels and eliminating ground loops. Removed downhole sensor package in June 2001 and brought back to lab for repair and installation of ferrite beads for noise suppression and reinstalled it two days later. Added 4 bags of sand to securely couple the sensor package in the bottom of the borehole and eliminate resonance of the suspension cable.
RSRB: Changed configuration to record 3 geophones and 1 accelerometer on August 1, 2000 because two of the accelerometer channels were noisy and unresponsive to seismic signals. Shut down on April 19, 2001 because Caltrans is retrofitting the bridge.
SMCB: Investigated site geology as it is a candidate site for a Mini-PBO borehole.
YBIB: Replaced battery. Station went down on August 24, 2000 when San Francisco engineers shut down the power lines owing to their unsafe condition and lack of funds to repair them. It is still down and awaiting installation of a new power line.
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 data loggers 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 (as shown in the 1999-2000 Annual Report). 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.
In the past year, a frame-relay connection to HERB was established. A Quanterra Q4120 data logger is scheduled for installation, but has been waiting for Caltrans to provide "24-hour" power (during the past year, AC power was available only during normal working hours. The installation of power was recently completed and the Q4120 is scheduled for deployment in September 2001.
The 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.
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. Bob Uhrhammer and Lind Gee contributed to the preparation of this chapter.
Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.