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 2000. Two stations were added in the past year (MOD and SCCB) and one station was closed (MIN).
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 a broadband seismometer, a strong-motion accelerometer, and a 24-bit digital datalogger. Two additional sites (PKD1 and SCCB) consist of a strong-motion accelerometer and a 24-bit digital datalogger. 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 dataloggers 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 use Streckeisen three-component broadband sensors (Wielandt and Streckeisen, 1982; Wielandt and Steim, 1986). A Guralp CMG-3T downhole broadband sensor contributed by LLNL is deployed at BRIB and a Guralp CMG-40T is installed at 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 dataloggers, with 3, 6, 8, or 9 channel systems. The Quanterra dataloggers employ FIR filters to extract data streams at a variety of sampling rates. 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 (MHH) 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 (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 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 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 dataloggers 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 1999-2000. 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 Mw 6.8 in the Sakhalin Islands.
BDSN data are archived at the Northern California Earthquake Data Center and this is described in detail in Chapter 10.
Given the remoteness of 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 1999-200 fiscal year was no exception.
The year 2000 transition required both hardware and software upgrades to the Quanterra dataloggers used by the BSL. During 1999 we continued to replace our remaining GOES satellite clocks in the BDSN, which output only 2 digit years, with new GPS clocks that were verified to be year 2000 compliant. 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.
Mice, attracted by the warm electronics, destroyed power equipment at SAO and FARB, as well as telemetry equipment on the Farallon Islands. Despite installed lightning and surge protection devices, nearby lightning strikes affected telemetry at SAO, WDC, YBH and YBIB stations.
Felled trees cut utility power at several stations during the winter. Based on our standard installation, stations should continue to operate for up to 3 days after utility failure. Beyond that point, a special low voltage cutoff switch turns off the battery power to all equipment. When utility power is restored, the low voltage switch will restore battery power to the instruments, thereby enabling them to restart cleanly without the need for a site visit.
At FARB, telemetry is achieved via unlicensed spread spectrum radio. During this year, our telemetry was affected by the installation of a nearby transmitter by a government contractor. The new radio interfered with ours, resulting in no data over the FARB link. A trip was made to FARB, and the BSL radios were replaced in an effort to solve the problem, before the new transmitter was discovered. The problem was solved ultimately by realigning antennas and changing frequencies.
A similar problem arose at PKD this year. The USGS installed a radio-based system for accessing their GPS network at Parkfield. The system turns on at several times a day and queries the receivers. During the period the radio is on, communications from PKD to Carr Hill are lost. Despite efforts to move the radio apart, this continues to be an ongoing problem. Because of the Quanterras storage capability, we are not losing data during these outages (generally 15-20 min).
Changes in the NSN satellite system required visits to CMB, SAO, and WDC in order to reorient the VSATs.
During the year, we experienced the usual minor problems associated with the frame-relay connections. At several times, failure of the "logical layer" in frame-relay led to telemetry problems for some stations outside of LATA 1 as did the failure of major PacBell circuits. The most significant problem this year was the failure of the BDSN T1 circuit for approximately 6 hours on Jan 27, 2000. This circuit brings the incoming data from the BDSN, HFN, and BARD networks to the BSL. The failure was due to a problem in the network interface box, which was replaced.
A digitizer board failed at BDM, possibly due to a lightning strike, and the Quanterra was removed and sent back to Massachusetts for refurbishing. This failure coincided with work on the telecommunications system at Black Diamond Mines Park Headquarters and the station was offline for over 2 months.
Corrosion caused problems with the external clock antenna at FARB.
Sensor problems arose at CMB, KCC, and FARB. The problem with the CMB NS STS-1 was identified after an inspection of the records following a large teleseism. The line driver has been replaced, but the problem still persists. It is believed that this problem began in August of 1999. At KCC, SCE opened a valve in the tunnel to release water from Mammoth Pool (as they did several winters ago). This led to high noise levels from mid May to mid Aug, 2000. When the valve was closed, the NS STS-1 did not recover and will need to be replaced. FARB has shown high noise levels which may indicate a problem with the Guralp seismometers.
During the 1999-2000 fiscal year, two new stations were added to BDSN - MOD (Modoc County) and SCCB (Santa Clara County). A third new site, JCC, is under development at the time of this writing.
The search for a new location in the northeast corner of California was begun in 1997. Modoc County is characterized by its remoteness, geological diversity, and population of less than 30,000. The county is largely high plateau ((1600 meters elevation) consisting of ancient basalt lava flows, interbedded with volcanic ash three to ten meters in thickness. Regionally, depth to ground water may be as little as two meters. Cattle ranching and hay growing are the principal businesses in the county. Historically, there have been small attempts at mining in the region.
With the assistance of the U.S. Forest Service, BSL staff searched for abandoned mining adits on government property located within 500 meters from telephone and power utilities. Unfortunately, because of the county's low population, these utilities exist almost entirely along paved roads. Ultimately, a suitable abandoned adit was found on private property.
Initially, the landowner was resistant to the idea of locating University equipment on his property. However, after more than one year of discussions, the landowner agreed to allow a seismographic installation upon review and recommendation of his attorney. Coincidentally, the landowner's attorney was a graduate of the University of California, Berkeley! From that point, a ten-year agreement to occupy the site was quickly reached, and construction and development proceeded.
The seismometers at the new site were located approximately 3 meters inside a rock adit with 3-4 meters of overburden. At the entrance to the adit, the BSL staff constructed a 2- by 3-meter concrete atrium, with an insulated security door to the outside. The datalogger, ancillary ac/dc power supply, batteries, GPS clock, geodetic GPS of the BARD network, and telemetry equipment were located in the atrium structure. A rigid foam panel blocked the entrance between the atrium and the rock adit, minimizing air exchange.
The rock adit was specially prepared prior to installation of the seismometers. Care was taken to bring down loose overhead rocks lest they fall in the future. With the assistance of a California Department of Forestry fire crew, the inside of the adit was excavated by hand down to hard rock. A concrete pad was then poured for leveling the seismometers. Care was taken to vibrate the concrete pad in order to minimize air entrapment within the concrete, since air bubbles trapped in the hardened concrete are believed to be a source of noise to the seismometers as barometric cells pass. Finally, prior to installation of the seismometers, a welded steel cage was constructed over the pier to brace the overhead rock and prevent small rocks, which may fall from the ceiling over time, from damaging the seismometers below.
One vertical and two horizontal STS-1 seismometers were installed upon stainless steel warpless baseplates. Additionally, a 2g strong-motion Episensor from Kinemetrics plus digitally recorded thermistor and barometers were installed. Data telemetry from the site was especially challenging, given the remoteness of the site. In co-operation with the NSN, a VSAT satellite telemetry link brings the seismic and geodetic GPS data to both Berkeley and Golden. A dial-in modem provides back-up data retrieval capability.
As completed, the station MOD represents an example of BSL's efforts to work jointly with the larger community. The U.S. Forest Service, National Seismic Network, California Department of Forestry, and UC Alumni were of assistance in developing this extremely remote station.
An additional station of the BDSN was added at the Santa Clara County communications facility in south San Jose. This site is centrally located within the Santa Clara valley on a serpentine and Franciscan outcrop. Emergency response dispatchers and personnel are at the site around the clock. Power at the site is backed up by uninterruptible power supplies and diesel generators. This is also the location of the BARD station LUTZ. Collocation of the seismic equipment with the existing GPS has the advantage of sharing a single telemetry link to the BSL.
Construction at the site was limited to trenching from an existing concrete radio and microwave vault to a rock outcrop situated 100 meters from the building. By locating the seismometer outside the concrete building, free-field interference was minimized. A concrete pad was poured to level the seismometer at that location. A double shielded cable was used to connect the seismometer to the datalogger, in order to minimize RF pickup from the nearby emergency transmission equipment. Ferrite beads were added to the cable to further reject unwanted RF which could couple into the cable.
In July of 1999, we obtained a permit for the Jacoby Creek site from Barnum Timber. This site will replace the station ARC, which is located at Founders Hall in Humboldt State University. The new site, which previously was used for a post-event portable instrument deployment, is in an abandoned quarry. In addition to hard and dense surface outcrop, the site is attractive due to its proximity to both commercial phone and power lines. BSL has a 5 year agreement with the landowner.
At this writing, construction and development of the site is underway. An instrument vault, similar to those constructed at HOPS and PKD, is planned.
The USGS Menlo Park upgraded three short-period stations in the Cape Mendocino area to broadband systems during the summer and fall of 1999. BSL staff member John Friday was involved in the installation of Cahto Peak and Mt. Pierce, providing expertise on the issues related to thermal stability and noise reduction. Initially, the USGS Menlo Park and the BSL planned to have Cahto Peak and Mt. Pierce as joint BDSN/NCSN sites with Quanterra dataloggers and dual telemetry feeds. However, in May of 2000, the USGS decided to replace their microwave telemetry in this area with a satellite-based system. As as result, the BSL is considering other sites for the 2 Quanterra dataloggers.
During 1999-2000, the search for new locations for future stations continued. Because of the long-term nature of our research, only sites which offer a potential presence greater than ten years are considered. Distances to utilities (phone and commercial power), geology, and, of course, the network spacing are considered when exploring sites for new stations.
Sites on the University of California properties at UC Santa Cruz and Blodgett Forest Field Station were evaluated. At Santa Cruz, a portable data recorder and an STS-2 seismometer were located in an old quarry on the campus. Data was recorded continuously for approximately one week. Although the surface expression was very hard rock, data analysis indicated high noise-probably from vehicle traffic on the campus-within the seismic band.
Blodgett Forest Field Station is located near Auburn, California, west of Lake Tahoe and approximately midway between the existing stations at CMB and ORV. The geology of the area is massive granite. As a forestry research station, the property is heavily forested with tall pine trees. Installed weather instruments are reported to have recorded seasonal treetop winds approaching 100 kph. Together, the wind and trees could cause significant background noise. An evaluation of the background noise, using a portable instrument set as above, will need to be performed to test the site completely.
In June 2000, BSL discussed the possibility of adding a station at the Lake Tahoe Community College. The teaching professor at the college has expressed an interest in the research of the BSL and we are exploring how we may work together. A site visit plus portable instrument recording and evaluation of the background noise are pending. Also, in the past few months, BSL staff have been investigating possible sites between HOPS and JCC, as part of the CREST project.
The BDSN station MIN, sited at the Lassen Volcanic National Park Headquarters at Mineral, California, was closed on December 8, 1999 (after the new BDSN broadband station at MOD was installed). The decision was made to close MIN because it exhibited a very high cultural broadband noise level, owing to its siting on Pliocene volcanic debris and its proximity (100 m) to a road with heavy logging truck traffic, and also because it was relatively close to two other BDSN broadband stations (WDC: 83 km N72W and ORV: 88 km S6E). For over 50 years, data from MIN played an important role in the analysis of earthquakes in northern California and vicinity because it was one of only two stations in the region with Wood-Anderson instruments (the other being ARC, located 217 km W of MIN on the Humboldt State University campus in Arcata). Over the last 50 years, these seismograms were crucial for the determination of ML. However, once several broadband BDSN stations became operational in northern California and ML could be robustly determined using Wood-Anderson records synthesized from broadband seismic recordings (Uhrhammer et al., 1996), the importance of maintaining a BDSN station at Mineral waned.
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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Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.
Uhrhammer, R. A., S. J. Loper, and B. Romanowicz, Determination of Local Magnitude Using BDSN Broadband Records, Bull. Seism. Soc. Am., 86, 1314-1330, 1996.
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Wielandt, E., and G. Streckeisen, The leaf spring seismometer: design and performance, Bull. Seis. Soc. Am., 72, 2349-2367, 1982.