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). This network is designed to monitor regional seismic activity at the magnitude M 3 level as well as to provide high quality data for various 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 21 stations in 1999. Two stations were added in the past year (BDM and POTR).
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.
Each of the BDSN sites is equipped with a broadband seismometer, a strong-motion accelerometer, and a 24-bit digital datalogger and is recorded at UC Berkeley using continuous telemetry (Figures 2.2-2.3). 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 dataloggers ensures that the BDSN has the capability to record the full range of earthquake motion for source and structure studies. Figure 2.4 shows a synopsis of low frequency background nosie over the last year at BDSN stations. The high level of noise at station FARB reflects a combination of limited bandwidth sensor (CMG 40) and the island environment. Figure 2.5 shows an example of recording of the P wavetrain from a large deep teleseismic across the BDSN network.
As noted above, all BDSN sites are equipped with three-component broadband sensors and three-component strong motion accelerometers. A Guralp CMG-3T downhole broadband sensor contributed by LLNL is deployed at BRIB and a Guralp CMG-40T is installed at FARB. All other BDSN stations use Streckeisen sensors (Wielandt and Streckeisen, 1982; Wielandt and Steim, 1986). 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 Q935 or Q4120 Quanterra dataloggers, with 6, 8, or 9 channel systems. The Quanterras employ FIR filters to extract data streams at a variety of sampling rates (Figure 2.6). 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 (Table 2.1) using the Murdock, Hutt, and Halbert event detection algorithm (Murdock and Hutt, 1983). In addition to the 6-channels of seismic data, signals from thermometers and barometers are recorded at nearly every site.
In parallel with the growth 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 dataloggers, additional services such as ftp and telnet to the dataloggers, data transmission to multiple sites, and the ability to communicate and transmit data from multiple instruments such as GPS receivers and/or multiple dataloggers at a single site. Today, 20 of the BDSN sites use frame-relay telemetry for part of their communications system.
In order to take advantage of the capabilities of the frame-relay telemetry, we have upgraded the Q935 dataloggers with the installation of an Ethernet board (the Q4120 systems were delivered with Ethernet boards). 19 of the 21 BDSN sites now have Ethernet capability and 2 upgrades remain to be completed.
Central site data acquisition is performed by two computer systems located at the BSL using the comserv software. A third system acquires data from the NSN VSAT (BDSN stations CMB, SAO, and WDC and TERRASCOPE station ISA) (Figure 2.7). Comserv clients feed data into shared memory regions for REDI processing, to the seismic alarm process, to the DAC480 system, and to the Momento Mori Web page.
BDSN data are routinely monitored for state-of-health. An automated power spectral density analysis is computed weekly to characterize the seismic noise level recorded by each broadband seismometer. The algorithm sends the results via email to the engineering and some research staff members and it also generates a bargraph output which compares all the BDSN broadband stations by components. A summary of the results for 1998-1999 is displayed in Figure 2.4. The occurrence of a significant teleseism also provides the opportunity to review station performance and calibration. Figure 2.5 displays the response of the BDSN to a deep teleseism.
BDSN data are archived at the Northern California Earthquake Data Center and this is described in detail in the chapter on Archive and Distribution.
During the summer of 1998, a new broadband station was added at the Black Diamond Mines, East Bay Regional Park, south of Antioch. The Park was the site of extensive underground lignite mining during the 1800's. The clean sand intra-burden was mined as late as post World War II for glass making. The East Bay Regional Park District conducts educational and historical tours in two areas of the underground sand mine workings. Under permit from the Park District, the addition of the seismographic equipment in an area of the mine not open to public tours, is in keeping with the educational and research goals of the Park.
The instruments are located approximately 150 meters from the daylight entrance to the mine. Approximately 100 meters of sandstone, shale, and coal overburden cover all the instruments. The overburden is sufficient to limit thermal variations. Additionally, the seismometers are covered with 7 centimeters of foil-faced, closed-cell foam. Differential GPS was used to precisely locate a reference outside the mine, and traditional surveying methods then located the instrument offset from the GPS determined coordinates.
Station BDM features a Q4120 data logger, STS-2 broadband seismometers, FBA-ES-T accelerometers and 56 Kbit/s continuous telemetry to Berkeley. A GPS clock provides reference timing. Low loss co-axial cable (<2 dB per 100 feet) was used to minimize the attenuation of the signal, and corresponding loss of external clock source.
In the fall of 1998, a new broadband station was added in the Potrero Hills area of the Sacramento River Delta near Fairfield. The new site, POTR, is located with the facility of OEA Aerospace Inc. The facility is on and along a sandstone ridge, the highest and most dense rock in the delta area. The seismic instruments are located within a 1960's era Nike missile silo that is currently used to store records. The massive concrete silo features walls greater than one-meter thick and is buried three meters beneath the surface. This structure provides very good thermal stability, which is further enhanced by 10 centimeters of foil-faced, closed-cell foam covering the seismometers.
A GPS receiver was also co-located at the Potrero Hills- OEA Aerospace site, as part of the BARD network, with the station name of POTB. The GPS system is thus able to utilize the same power and telemetry systems as the seismic instruments.
Station BDM features a Q4120 data logger, STS-2 broadband seismometers, FBA-23 accelerometers and 56k baud continuous telemetry to Berkeley. A GPS clock provides external reference timing.
During 1998-99 the search for new locations to potentially add stations to the BDSN continued. Because of the on-going nature of our research, only sites that offer long-term (> ten years) presence are considered. Proximity to utilities (phone and commercial power), geology, and station distribution are considered in prioritizing the areas to explore for new stations. Two sites were identified as potentially interesting only to be discounted after subsequent noise testing found them to have large background resonances or ground tilts. At the time of this writing two new sites have been permitted for BDSN station development.
The new site MOD (Modoc County California) is located in the northeast corner of the state, 10 kilometers south of the California/Oregon border, and 20 kilometers west of the California/Nevada border. Due to the extreme remoteness of the area, no broadband or short period instruments have been located within 100 km of this site.
The instruments are being located in an underground adit with approximately 5 meters of overburden for thermal stability. Because the adit is located on private property, discussion and negotiation with the landowner took nearly two years to successfully conclude. The final agreement assures that the BSL will occupy the site for at least the next ten years.
The second new site, in Humboldt County, is expected to replace the current site ARC. The new site is located on quarry property owned by a timber company. The site was previously used for a post-event portable instrument experiment. In addition to hard and dense surface outcrop, the site is attractive due to its proximity to both commercial phone and power. Again, the BSL has sought a long-term presence with the landowner.
In addition to the two new sites (Modoc and Humboldt Counties) the BSL engineers are working with the USGS to develop subsystems, characterize instruments, and establish installation procedures for the CREST (Cooperative Reporting of EarthquakeS and Tsunamis) Project in northern California. In particular, the BSL is collaborating with the USGS on two sites for joint operation. KMP (Mt. Pierce) and KCP (Cahto Peak) will be collaborative USGS/BSL installations with data transmission to both Berkeley and Menlo Park. Unlike other CREST sites, these stations will use Quanterra dataloggers (typical CREST sites use RefTek systems).
Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.
Wielandt, E., and J. Steim, A digital very broad band seismograph, Ann. Geophys., 4, 227-232, 1986.
Wielandt, E., and G. Streckeisen, The leaf spring seismometer: design and performance, Bull. Seis. Soc. Am., 72, 2349-2367, 1982.