A network of borehole-installed wide dynamic range seismographic stations - the Hayward Fault Network (HFN) - is being developed cooperatively with the US Geological Survey with support from USGS, EPRI, Caltrans, the University of California Campus/Laboratory Collaboration (CLC) program, LLNL, and LBNL (Figure 3.1 and Figure 3.2). The focus of the network and associated research is to improve working models for this very hazardous but poorly understood fault, and to integrate the data into the real-time monitoring and alert system being developed at the Berkeley Seismological Laboratory. The associated research centers on characterizing the seismicity along the Hayward fault zone via several methodologies: 1) re-analyzing the historical seismicity (this involves transcription of the data to computer readable form and the application of modern computer algorithms), 2) identifying repeating events (for estimating the subsurface slip rate), and 3) waveform inversion (for determining the seismic moment tensors). The chapter on Deep Bore Hole Instrumentation Along the San Francisco Bay Bridges, as discussed elsewhere in this annual report, is closely associated with the HFN.
The Hayward fault is somewhat anomalous in its behavior, being a fault zone which is both creeping yet presumably accumulating strain for M7-7.5 earthquakes at recurrence intervals of less than 200 years. New models for the plate boundary geometry in the San Francisco Bay region call for mid-crustal detachment and a central role for the Hayward fault in accumulating the deformation. The base of the seismicity, or brittle-ductile transition at 10-12 km, is more shallow at the Hayward Fault than to the east or west. Its low rate of occurrence of earthquakes (about 10 per year at M 2.5 and larger) operates to frustrate its study with conventional instrumentation. Historical seismicity rates, from various networks operating over the years, suggest that there is sufficient seismicity, perhaps an event per day at a detection threshold around magnitude 0.5 to 1.0 to allow special study of the fault zone processes. Such a detection sensitivity requires high-gain instrumentation and sophisticated noise mitigation techniques, possible in the East Bay Area only with borehole-installed seismometers.
The basic concept for the HFN is that, in an urban environment swamped with cultural background noise (traffic and industry vibrations) orders of magnitude above average levels in remote sites, any individual station running on-site detection software has a high probability of being desensitized at any time from the noise and thus will miss many of the small events that we need to record at all network stations for high-resolution study of Hayward Fault activity. It is totally impractical to telemeter all of the data at the necessarily high sample rates (500 sps, up to 4 components) to a central site for processing in real time. To circumvent this problem we employ two countermeasures: (1) the sensors are placed in boreholes as deep as possible (preferably 100 m or more) and in bedrock, for a significant reduction in surface noise, and (2) representative signals (a single 100 sps vertical component from each station) are processed centrally for the detection of legitimate microearthquakes, and recovery of the full data set for the event from the entire network is accomplished by command from the central site, in a fully automated system. The detector has been running for 18 months now in an offline processing mode successfully. The magnitude threshold is approximately 0.0 for small events along the Hayward Fault. With a false detection rate of about two-thirds, on average more than one event per day is recovered, satisfying the design goals of the network. As more stations in boreholes are added, the quantity and quality of the data will improve.
The network as envisioned will consist ultimately of 24 to 30 stations, 12-15 each north and south of the San Leandro seismic gap, managed respectively by UCB and USGS. Six-component borehole sensor packages designed and fabricated at LBNL's Geophysical Measurement Facility by Don Lippert and Ray Solbau, with 3 channels of acceleration (Wilcoxin 731A) and 3 of velocity (OyO HS-1 4.5 Hz geophones) are being installed in the entire network. THe HFN data are also encorporated in real-time into the the Rapid Earthquake Data Integration (REDI) Project.
The Bridge Safety Project of the California Transportation Department has made possible installation of sensor packages in 15 bedrock boreholes at five East Bay bridges cooperatively with L. Hutchings of LLNL. Three of the bridge sites are now telemetered to the BSL and, as new telemetry links are installed, either with frame relay links, spread-spectrum radios or cellular phones connections, they will be brought on-line and gathered into the centrally controlled network. Meanwhile, portable recorders are being used at the bridge sites. In a separate but closely-linked Caltrans-supported project, we are working towards the installation of permanent power and telemetry to eliminate monthly personnel-intensive visits to the remaining stations for battery and disk changes.
The HFN is possible because of progress over the past decade in instrumentation, telemetry, and computer hardware, allowing implementation of the centrally-controlled local network with very high sample rates (i.e., up to 100 sps). The Quanterra Q4120 instruments, designed for HFN, have four channels, three with special amplifiers for the accelerometers and continuous and triggered sampling to 1000 Hz. As for the BDSN sites, 56 kbits/sec frame relay digital telemetry is used.
During the past year a new HFN station was installed at St. Mary's College (SMCB) on December 5, 1997. The addition of SMCB raises the total number of HFN stations to seven and improves the coverage of the HFN on the NE side of the fault which is crucial for the analysis of small earthquakes which occur along the Hayward Fault Zone. The HFN station on Yerba Buena Island (YBIB) was damaged, probably by a proximity lightning strike, on 03/19/1998 and it was out of commission for three months while the equipment was being repaired. The vertical component geophone was damaged beyond repair so we are now recording and triggering off of the horizontal geophone that is oriented approximately transverse to the Hayward Fault.
During the past year, UCB has maintained and collected data from 7 Bay Bridge sites, three on the east span (at Piers E7 and E17 and on the east side of Yerba Buena Island) and 4 on the west span (at the anchorage piers at each end and at piers W2 and W5). The west span anchorage sites were installed early this year by UCB and LLNL personnel. They are presently very shallow, essentially surface, sensor emplacements. We have a standing (written) request in to Caltrans to drill holes for deep emplacements at these sites, should one of their drill crews become available. Both sites have high noise levels.
One of the HFN stations (YBIB), on Yerba Buena Island, will be a central hub for the Bay Bridge borehole network. In consultation with BSL engineers and Caltrans electicians, a detailed work plan has been developed, which will be carried out by UCB with Caltrans assistance in Sep and Oct. The infrastructure of boxes, power supplies, properly placed clock antennas, etc, will first be installed while the present event recorders are still in operation, followed by installation of the Quanterra dataloggers. In addition to the 7 sites mentioned above, an abandoned installation on the east end of the east span will be revived. The sites on each span of the bridge will form two separate sub-networks, each with one "concentrator" site communicating with UCB, and 3 satellite sites communicating with the concentrator site. To begin the effort, most of the upgrade for site W2 was carried out recently. Our equipment there was in the way of the bridge pier seismic retrofit, and was relocated by UCB and a Caltrans subcontractor. Appropriate splash-proof and corrosion-proof boxes were installed for the Quanterra and telemetry equipment. We have received the three Quanterra Q730 dataloggers and that are to be installed to record data from the four sites along the west span of the Bay Bridge. The signals from the Q730's at each pier will be telemetered to the Q4120 concentrator via spread-spectrum radios.
This year LLNL personnel installed a surface seismometer at the top of the YBIB borehole as a part of their site response studies. Subsequent to the outage period following the lightning strike, this installation has been revived and serviced by UCB personnel.
The Richmond/San Rafael Bridge site sustained one outage this winter due to salt water corrosion of signal connections, corrected by UCB.
Three sites on the Dumbarton Bridge are equipped with event recorders and are serviced at long intervals, with a lower priority than the Bay Bridge sites.