The Berkeley Digital Seismic Network (BDSN) is a regional network of very broadband and strong motion seismic stations spanning northern California and linked to UC Berkeley through continuous telemetry (Figure 2.1 and Table 2.2). This network is designed to monitor regional seismic activity at the magnitude 3+ level as well as to provide high quality data for research projects in regional and global broadband seismology.
The network upgrade and expansion initiated in 1991 has continued, and it has grown from the original 3 broadband stations installed in 1986-87 (BKS, SAO, MHC) to 22 stations in 2001. Two stations were added in the past year (JCC, RFSB) and two stations were closed (PKD1, ARC).
We take particular pride in high quality installations, which involves often lengthy searches for appropriate sites away from sources of low-frequency noise as well as continuous improvements in installation procedures and careful monitoring of noise conditions at existing stations.
Future expansion of our network is contingent on the availability of funding and coordination with other institutions for the development of a denser state-of-the-art strong motion/broadband seismic network and joint earthquake notification system in this seismically hazardous region.
Twenty of the BDSN sites are equipped with 3 component broadband seismometers and strong-motion accelerometers, and a 24-bit digital data acquisition system (das) or data logger. Two additional sites (RFSB and SCCB) comprise a strong-motion accelerometer and a 24-bit digital data logger. Data from all BDSN stations are transmitted to UC Berkeley using continuous telemetry. In order to insure against data loss during utility disruptions, each site has a 3-day supply of battery power and is accessible via a dialup phone line. The combination of high-dynamic range sensors and digital data loggers ensures that the BDSN has the capability to record the full range of earthquake motion for source and structure studies. Table 2.3 lists the instrumentation at each site.
Most BDSN stations have Streckeisen three-component broadband sensors (Wielandt and Streckeisen, 1982; Wielandt and Steim, 1986). Guralp CMG-3T downhole broadband sensors contributed by LLNL are deployed in post-hole mode at BRIB and FARB. The strong-motion instruments are Kinemetrics FBA-23 or FBA-ES-T with 2 g dynamic range. The recording systems at all sites are either Q730, Q680, Q980 or Q4120 Quanterra data loggers, with 3, 6, 8, or 9 channel systems. The Quanterra data loggers employ FIR filters to extract data streams at a variety of sampling rates and these have been implemented as acausal filters in the BDSN. In general, the BDSN stations record continuous data at .01, 0.1, 1.0, and 20.0 samples per second and triggered data at either 80 or 100 samples per second using the Murdock, Hutt, and Halbert event detection algorithm (Murdock and Hutt, 1983) (Table 2.1). In addition to the 6-channels of seismic data, signals from thermometers and barometers are recorded at nearly every site (Figure 2.2).
In parallel with the upgrade of the broadband network, a grant from the CalREN (California Research and Education Network) Foundation enabled the BSL to convert data telemetry from analog leased lines to digital frame-relay connections. The frame-relay network uses digital phone circuits that can support 56 Kbit/s to 1.5 Mbit/s throughput. Since frame-relay is a packet-switched network, a site may use a single physical circuit to communicate with multiple remote sites through the use of "permanent virtual circuits". Frame Relay Access Devices (FRADs), which replace modems in a frame-relay network, can simultaneously support multiple interfaces such as RS-232 async ports, synchronous V.35 ports, and ethernet connections. In practical terms, the upgrade to frame relay communication provides faster data telemetry between the remote sites and the BSL, remote console control of the data loggers, additional services such as ftp and telnet to the data loggers, data transmission to multiple sites, and the ability to communicate and transmit data from multiple instruments such as GPS receivers and/or multiple data loggers at a single site. Today, 20 of the BDSN sites use frame-relay telemetry for all or part of their communications system.
In order to take advantage of the capabilities of the frame-relay telemetry, we have upgraded the Q980 data loggers with the installation of an Ethernet board (the Q4120 systems were delivered with Ethernet boards). 20 of the 22 BDSN sites now have Ethernet capability and 2 upgrades remain to be completed.
As described in Chapter 8, data from the BDSN are acquired centrally at the BSL. These data are used in the Rapid Earthquake Data Integration System as well as in routine earthquake analysis (Chapter 9). As part of routine quality control (Chapter 8), power spectral density analyses are performed weekly and Figure 2.3 shows a summary of the results for 2000-2001. The occurrence of a significant teleseism also provides the opportunity to review station health and calibration and Figure 2.4 displays the response of the BDSN to a M 8.4 earthquake in Peru (the largest earthquake that occurred during the past year).
BDSN data are archived at the Northern California Earthquake Data Center and this is described in detail in Chapter 10.
Given the remoteness of the off-campus stations, BDSN data acquisition equipment and systems have been designed, configured, and installed for both cost effectiveness and reliability. As a result, the need for regular station visits has been reduced. Most station visits are necessitated by some catastrophic failure. The 2000-2001 fiscal year was no exception.
The station BDM is in the East Bay Regional Park District Black Diamond Mines Park. Several times during the past year, BSL engineers were asked to disconnect and reroute cables due to construction within the park. Concurrently, although not clearly related, the power supply subsystem for the station has become intermittent. This resulted in the data logger shutting down or rebooting on many occasions. BSL engineers are building a complete replacement for the power subsystem including replacing batteries.
At the time the PKD vault was constructed, the local phone company was not able to provide a digital link to the site. This necessitated that data be transmitted from the vault to Carr Hill via spread-spectrum radio. At Carr Hill, the data are loaded onto a frame-relay circuit for subsequent transmission back to the BSL. Over the last few years, the BSL has experienced problems with the radio communication link due to interference with other radios. Initially, problems were observed primarily when the USGS downloaded data from their GPS network (several times a day). However, during FY 2000/2001, other groups acquiring data in the Parkfield area expanded their use of spread-spectrum radios. As radios were added in the same frequency band, our continuous telemetry data rate began to decrease dramatically. Beginning in July 2001, the BSL effectively lost contact with the station. The BSL is working with the local phone company in a renewed attempt to have digital data circuit installed at the PKD vault.
Station BRIB was the first effort of the BSL toward establishing a multi-parameter borehole site. This collaborative facility was a joint BDSN-NHFN-USGS site. Initially established in 1992, the BSL experienced problems with downhole cables touching the Guralp CMG-3T sensor. In 1996, the BSL stopped recording the borehole Guralp and installed a second Guralp on the floor of a nearby surface tank. This installation was extremely sensitive to tilts from water-level changes, requiring periodic visits to recenter the sensor.
In December of 2000, BSL staff removed the Guralp sensor package from the borehole and brought it back for evaluation. The centering mechanism on one component was found to have failed and was repaired by the manufacturer's representative. After testing in Byerly vault, the sensors were redeployed at Briones in a post-hole installation, within 5 meters of the borehole location.
The Guralp sensor which had been recording in the surface tank was then removed, tested, and and deployed at FARB, where the original CMG-40T failed.
The FARB station is located at South East Farallon Island approximately 30 kilometers west of the Golden Gate. Access to the Island is restricted by permit, and only by private boat or Coast Guard helicopter. For that reason, BSL chose to concurrently replace any and all cables, connectors, and antennas which were subject to the extremely corrosive marine environment.
Both the broadband and strong motion instruments at FARB are located in a stacked rock vault enclosure. In order to replace the seismometer, the rocks were removed, the seismometer replaced, and the rocks restacked. The connectors on the cables between the seismometers and the recorder/digitizer were replaced in order to eliminate corrosion and hopefully improve the signal to noise ratio.
Station KCC is located within a water diversion tunnel operated by Southern California Edison. Periodically, high pressure water is diverted to the tunnel. During the spring of 2000 such a diversion occurred over several weeks. The shaking from water flow was great enough to cause one of the STS-1 seismometers to move on its baseplate. BSL engineers visited the site in order to manually relevel the seismometer. Concurrently, all components were recentered.
At station SCCB, the strong-motion sensors were originally located in a concrete block building. During FY 2000/2001, the sensors were moved from the building to a free-field location approximately 100 meters away. Engineers from BSL trenched the cable from the building, relocated and extended cables to reach the distance.
At station MOD, the vertical STS-1 appeared to fail simultaneously with a drop in temperature and a winter storm. A BSL engineer visited the site in the following weeks, recentered the seismometer, and re-evacuated the bell jar covering the seismometer. At that time, the seismometer appeared to be working correctly. After several weeks however, the seismometer again appeared to fail. Previous experience with this type of intermittent performance pointed to a broken wire in the bulkhead connector on the seismometer baseplate. This problem is very difficult to repair in the field, so a replacement baseplate was installed during a second visit to the site. The removed baseplate was brought back to Berkeley, where one of the fine wires on the baseplate bulkhead connector was indeed found to have broken.
A new station near the community of Bayside, California was constructed, developed and put into operation. The Jacoby Creek (JCC) site was initially identified as a possible replacement for the station at Humboldt State College (ARC). The JCC site is located on property owned by the Barnum Timber Company, which was kind enough to grant the BSL a permit to occupy the private land at no cost for a period of five years.
The site was formerly a rock quarry and has not been operated in thirty years. The host material rock is known to be up to forty percent harder than the rock typically found in the Humboldt County. In our experience, hard, non-sedimentary rock outcrops are the best sites for installation of broadband seismometers. The mass and non-compliant nature of these materials minimize the tilting of the horizontal seismometers and provide excellent thermal mass which in turn helps minimize the diurnal effects.
At the Jacoby Creek site, a vault was constructed from a buried recycled steel ocean-going shipping container, similar to the vaults at PKD and HOPS. This construction method is favored by BSL where an existing tunnel, cave, or mine adit is not available. The construction uses materials which are relatively cheap, locally available and long lived. The site was also selected due the availability of phone and power.
The Jacoby Creek site features a set of Streckeisen STS-2 broadband sensor and FBA-23 accelerometers with a Q980 data logger and continuous frame-relay telemetry to the BSL. The battery and disk storage on site is sufficient to run the station for a minimum of three days in the event of a local outage. The first data from the station was recorded and archived on April 12, 2001.
The station ARC was closed on August 2, 2001 (see below). Owing to the 4 month overlap in the operation of ARC and JCC, several local, regional, and distant earthquakes were recorded by both stations. Figure 2.5 compares the ground velocity waveforms for a M 4.1 earthquake which occurred off the coast along the Mendocino Escarpment 80 km S41W of ARC and 78 km S46W of JCC.
At NHFN station RFSB, a surface strong motion instrument (Episensor) was added, along with a Q730 data logger, to complement the existing downhole sensors. As this site has expanded from its original design, the existing infrastructure was inadequate to enclose and secure the instruments. A new enclosure was prepared and installed with special emphasis on isolation of the two data acquisition systems and minimization of ground loops. The Episensor was installed atop a pedestal type pier dug 0.5 meters into the clay-rich soil. The sensor and cables are protected from the elements by a polyethlyene cover.
In the fall of 2000, the search for a site to extend BDSN north of the California-Oregon border began. Our experience shows that sites which are located on granitic rocks are quieter and have less ground tilting. Therefore the first step in locating a potential new site was to consult a geological map of the area. To extend the existing network, a new site in Oregon would ideally be located north of the midpoint between the existing BDSN sites at MOD and YBH. The deployment of this site is a collaboration among the BSL, the USGS NSN program and the IRIS/GSN program. It will be a joint site of the BDSN/NSN and GSN.
In the Medford (Oregon) area, we contacted officials at six ranger offices within two National Forests, in addition to the Bureau of Land Management office in Medford in hopes of finding an abandoned mining adit similar to YBH and MOD. While this area was historically an active mining region, the necessity that the site be located within reasonable (300 meters) of existing power and phone lines narrowed the possible sites from a list of hundreds to only a few.
Two field excursions were undertaken to locate and test sites. Several of the hopeful adit sites were flooded, several more were located but had collapsed since mining ended leaving only surface relief expressions,and several more were deemed too far from utilities before a suitable site was located 12 kilometers west of Shady Cove, Oregon.
At this time, permitting for the site HUMO is underway with the US Bureau of Land Management (Figure 2.1). Within the abandoned mine adit, approximately 80 meters from the entrance, the seismometer will be located in a side drift. Commercial phone and power are presently located approximately 250 meters from the entrance to the adit and can be extended to the site. By placing the seismographic equipment on public lands as opposed to private lands, the longevity of the site is assured.
Once permitting is completed, the front of the adit will be excavated and a steel culvert will be placed to line the entrance. The front of the adit will be secured with a lockable grate or door. All equipment - seismometer, data logger/recorder, clock, batteries, and telemetry subsystems - will be installed in an enclosure within the secure adit. It is expected that the construction and installation procedure will be completed in calendar year 2001.
Other site investigations are underway for expansion of the BDSN. With funding from the California OES (as part of the CISN), we hope to add two additional BDSN sites during FY 2001/2002. Possible candidates are the Alder Springs area (labelled GAS in Figure 2.1 and the Hat Creek (labelled HATC).
In the past year, we have been actively involved in the preparation phase towards the installation of a permanent broadband ocean bottom station (MOBB) in Monterey Bay, in collaboration with MBARI. This is a follow-up of the MOISE experiment conducted in the summer of 1997, in which a temporary broadband station was deployed using the ROV Ventana for 3 months in Monterey Bay (e.g. Romanowicz et al., 1998), in collaboration with MBARI and the IPG in Paris (France). The current plan is to install a Guralp CMG-1 seismometer system with a recording system developed by MBARI, as well as auxiliary sensors (current meter, DPG) close to the site of the MOISE deployment. BSL staff has been collaborating with MBARI colleagues on the design of the seismometer housing, software development for data acquisition, as well as methodology for burying the seismometer package under the seafloor, a necessary step to reduce the seismic background noise. The deployment is currently scheduled for November 2001.
The new station at JCC was established as a replacement for the station located at Humboldt State University (ARC). The ARC site originally began operation in 1948 with Wood-Anderson seismometers. Since that time a number of different instruments have occupied the site, most recently Streckeisen STS-2 and FBA-23 accelerometers. Since the original installation, the cultural noise at the Humbolt State site increased as the campus and surrounding area grew, while the instrumentation became more sensitive. These factors ultimately converged to necessitate a new site which is both more remote from cultural noise, and features hard rock suitable for broadband studies.
The JCC station was considered operational as of April 12, 2001. After several months of joint operation, the ARC station was decommissioned on August 2, 2001.
The station PKD1 began operation in October of 1991. The site was not selected on noise characteristics, but rather on the ease of installation. Seismometers were simply placed on a concrete slab near the USGS leased facility at Haliburton Ranch House (main site of the USGS Parkfield Prediction Experiment). The slab exhibited large tilt-induced noise fairly early on, which lead to the decision to search for a permanent site which was on non-sedimentary rock and the establishment of a station at Bear Valley ranch (PKD) in 1996. The STS-2 sensors were removed from PKD1 in 1997, but the site was maintained with an FBA-23. In the fall of 2000, all equipment at the Haliburton house was removed at the request of the landowner and the facility was leveled in January of 2001. The BSL closed the PKD1 site and relocated the the electromagnetic instrumentation that was also located at Haliburton as a joint effort of UC Riverside and BSL to Carr Hill, along with the BSL telecommunications equipment.
Under Barbara Romanowicz's general supervision, with Lind Gee's assistance and Bill Karavas as head technical guru, John Friday, Dave Rapkin, Doug Neuhauser and Bob Uhrhammer contribute to the operation of the BDSN. Bill Karavas, Bob Uhrhammer, and Lind Gee contributed to the preparation of this chapter.
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