The Bay Area Regional Deformation (BARD) network of continuously operating Global Positioning System (GPS) receivers monitors crustal deformation in the San Francisco Bay area (``Bay Area") and northern California (Murray et al., 1998a). It is a cooperative effort of the BSL, the USGS, and several other academic, commercial, and governmental institutions. Started by the USGS in 1991 with 2 stations spanning the Hayward fault (King et al., 1995), BARD now includes 67 permanent stations (Figure 8.1) and will expand to about 75 stations by July 2003. The principal goals of the BARD network are: 1) to determine the distribution of deformation in northern California across the wide Pacific-North America plate boundary from the Sierras to the Farallon Islands; 2) to estimate three-dimensional interseismic strain accumulation along the San Andreas fault (SAF) system in the Bay Area to assess seismic hazards; 3) to monitor hazardous faults and volcanoes for emergency response management; and 4) to provide infrastructure for geodetic data management and processing in northern California in support of related efforts within the BARD Consortium and with surveying, meteorological, and other interested communities.
BARD currently includes 67 continuously operating stations, 34 in the Bay Area and northern California (Table 8.1), 15 near Parkfield, along the central San Andreas fault, and 18 near the Long Valley caldera near Mammoth (Table 8.2). The BSL maintains 21 stations (including 2 with equipment provided by Lawrence Livermore National Laboratory (LLNL) and UC Santa Cruz). Other stations are maintained by the USGS (Menlo Park and Cascade Volcano Observatory), LLNL, Stanford University, UC Davis, UC Santa Cruz, and East Bay Municipal Utilities District, the City of Modesto, the National Geodetic Survey, and the Jet Propulsion Laboratory. Many of these stations are part of larger networks devoted to real-time navigation, orbit determination, and crustal deformation.
Between 1993 and 1996, the BSL acquired 5 Ashtech Z-12 receivers from UC Berkeley and private (EPRI) funding, which together with 2 USGS receivers, formed the nucleus of the initial BARD network. Since 1996, the BSL has acquired additional Ashtech Z-12 receivers with Dorne-Margolin design choke ring antennas: 13 in 1996 from a combination of federal (NSF), state (CLC), and private (EPRI) funding, 4 in 2000 from USGS funding, and 7 in 2001 from NSF funding. Most of these receivers have been installed to enhance continuous strain measurements in the Bay Area and to consolidate the regional geodetic network. The network includes several profiles between the Farallon Islands and the Sierra Nevada in order to better characterize the larger scale deformation field in northern California (Figure 8.1). Six more of the BSL receivers will be installed next year, 2 along the southern Hayward fault, and 4 as part of the NSF-funded mini-PBO project establishing collocated GPS/seismometer/borehole strainmeter observatories in the Bay Area (see Chapter 9).
In 1996, researchers from the BSL, the USGS, Stanford University, LLNL, UC Davis, and UC Santa Cruz formed a consortium of institutions studying tectonic deformation in the San Francisco Bay area and northern California. Members of the BARD consortium agreed to pool existing resources and coordinate development of new ones in order to advance an integrated strategy for improving the temporal and spatial resolution of the strain field. This strategy includes the continued development of the network of continuous GPS receivers, the development and maintenance of a pool of GPS receivers for survey-mode operations that may be deployed in semi-permanent mode in the Bay Area when not otherwise in use, archiving of all data at the NCEDC, and development of a coordinated data analysis facility that will process permanent, semi-permanent, and survey data.
Today, raw and Rinex data files from the BSL stations and the other stations run by BARD collaborators are archived at the BSL/USGS Northern California Earthquake Data Center data archive maintained at the BSL (Romanowicz et al., 1994). The data are checked to verify their integrity, quality, completeness, and conformance to the RINEX standard, and are then made accessible, usually within 2 hours of collection, to all BARD participants and other members of the GPS community through Internet, both by anonymous ftp and by the World Wide Web (http://quake.geo.berkeley.edu/bard/).
Data and ancillary information about BARD stations are also made compatible with standards set by the International GPS Service (IGS), which administers the global tracking network used to estimate precise orbits and has been instrumental in coordinating the efforts of other regional tracking networks. The NCEDC also retrieves data from other GPS archives, such as at SIO, JPL, and NGS, in order to provide a complete archive of all high-precision continuous GPS measurements collected in northern California.
Many of the BARD sites are classified as CORS stations by the NGS, which are used as reference stations by the surveying community. All continuous stations operating in July 1998 and May 2000 were included in a statewide adjustments of WGS84 coordinates for this purpose. Members of the BARD project regularly discuss these and other common issues with the surveying community at meetings of the Northern California GPS Users Group and the California Spatial Reference Center.
In the remainder of this section, we describe the standard BARD station and some of the BARD-related activities the BSL has performed over the last year, including maintenance to existing stations, installation of a new station and an experimental single-frequency receiver profile, improvement in processing methods, and analysis of the data to estimate deformation signals monitored by the network.
A BSL continuous GPS station uses a low-multipath choke-ring antenna mounted to a reinforced concrete pillar approximately 0.5 meter above local ground level. The reinforcing steel bars of the pillar are drilled and cemented into rock outcrop to improve long-term monument stability. It uses a low-loss antenna cable to minimize signal degradation on the longer cable setups that normally would require signal amplification. Low-voltage cutoff devices are installed to improve receiver performance following power outages. The Ashtech Z-12 receiver is programmed to record data once every 30 seconds, observing up to 12 satellites simultaneously at elevations down to the horizon.
Tests performed by UNAVCO on low antenna mounts revealed that estimates of tropospheric water vapor from the GPS data are strongly correlated with signal multipath errors, which can degrade the precision of the vertical position estimates. Most of the BSL GPS stations use monuments that elevate the antennas 0.5-1.0 m above the ground surface, which helps to minimize the correlations between multipath and tropospheric parameters.
The stations are equipped with SCIGN-designed hemispherical domes. Domes cover the antennas to provide security and protection from the weather and other natural phenomenon. The SCIGN dome is designed for the Dorne-Margolin antennas and minimizes differential radio propagation delays by being hemispherical about the phase center and uniform in thickness at the 0.1 mm level. It is also very resistant to damage and, in its tall form combined with the SCIGN-designed antenna adapter, can completely cover the dome and cable connections for added protection. All new stations use the adapters and tall domes. Some of the older stations in well protected areas use the short domes.
Data from all BSL-maintained stations are collected at 30-second intervals and transmitted continuously over serial connections (Table 8.1). Station TIBB uses a direct radio link to Berkeley, and MODB uses VSAT satellite telemetry. The 18 stations use frame relay technology, either alone or in combination with radio telemetry. Twelve GPS stations are collocated with broadband seismometers and Quanterra data collectors (Table 4.2). With the support of IRIS we developed software that converts continuous GPS data to MiniSEED opaque blockettes, which can be stored and retrieved from the Quanterra data loggers (Perin et al., 1998). The MiniSEED approach provides more robust data recovery from onsite backup on the Quanterra disks following telemetry outages. Our comparisons also show the loss of individual records is fewer when using the Quanterra MiniSEED rather than direct serial method due to the superior short-term data buffer in the Quanterra. Data from the 12 collocated stations plus SUTB are retrieved in this manner.
During July 2001-June 2002, we performed maintenance on existing BARD stations, installed a new station, and prepared for new stations near the Hayward fault, on the San Francisco peninsula, and north Bay area regions.
In May 2002, forced entry in the building housing the GPS equipment at SAOB resulted in theft of GPS receiver and damage to building and telemetry system. We reinforced the plywood building walls with a layer of wire mesh followed by a surface layer of plywood secured with screws and liquid adhesive. Inside the building, the GPS receiver and short-haul modems where replaced and stored within a double locked large metal ``Hoffman" box.
Also in May 2002, the receiver and Freewave radio at Sutter Buttes (SUTB) were replaced due to a data outage following an electrical storm and possible lightning strike. The site is located on top of the South Butte, 2000 feet above the Central Valley.
The BSL staff is evaluating the performance of the UNAVCO-designed L1 system in an urban setting. This single-frequency receiver is relatively inexpensive but is less accurate than dual-frequency receiver systems that can completely eliminate first-order ionospheric effects. Hence we expect the L1 system to be most useful for short baseline measurements where ionospheric effects tend to cancel due to similar propagation paths. The systems are self-contained, using solar power and integrated radio modems. During 1999, the BSL borrowed 2 receivers and a master radio from UNAVCO to perform the evaluation, but persistent hardware and software problems limited progress on this project. UNAVCO subsequently resolved many of the problems and in summer 2000, we received new, improved equipment and software for 4 systems and a master radio.
During 2000 and 2001, we completed permitting at 4 sites on a 10-km profile extending normal to the Hayward fault between the UC Berkeley campus and the permanent BRIB site (Fig. 8.2). This profile, complemented by BRIB and EBMD to the west of the fault, will be most sensitive to variations in locking at 2-8 km depth. We expect that these systems will provide useful constraints on relative displacements near the Hayward fault in 3-5 years, and should help to resolve variations in creeping and locked portions of the fault (e.g., Bürgmann et al, 2000).
Three sites are located on East Bay Municipal Utilities District (EBMUD) property. The station BDAM is located just east of the Briones Dam and a few km west of the Briones (BRIB) continuous BARD station. Wildcat (WLDC) is located near the San Pablo Reservoir, and VOLM is located on the ridge of the East Bay Hills close to Volmer Peak. The fourth site, Grizzly Flat (GRIZ), is located on East Bay Regional Park property just west of Grizzly Peak. Finding suitable stations with line-of-sight telemetry across the East Bay Hills proved challenging. Data from WLDC must pass through all the other stations, with its relay path being (in order) BDAM, VOLM, GRIZ, a repeater on the UC Berkeley Space Sciences Building, and then finally the master radio on the roof of McCone Hall where the BSL is located on campus.
In April 2002, we installed the L1 Profile with assistance from two engineers from UNAVCO. We used a large gas powered hand drill to bore a 2" diameter, 18" deep hole into bedrock at the BDAM and VOLM sites, and cemented in a galvanized pipe using expansive grout. GRIZ and WLDC are located in areas where we could not easily access bedrock. GRIZ is located on top of a small plateau covered in volcanic deposits that have weathered to clay. Bedrock was observed nearby uphill of the WLDC site, but could not be used due to telemetry constraints. At both of these sites a subsurface concrete pier was constructed, laced with chicken wire to reduce cement fracturing during drying, and anchored by steel rebar pounded into the ground at several angles.
The electronics, including gps/radio unit, battery and solar power manager, are securely stored within a medium-sized Hoffman box, or locking metal enclosure, attached to pipe. Whenever possible access to the inside of box is necessary to remove bolts attaching the box itself. GPS antenna is mounted on a fiberglass rod attached to top of pipe. All loose cables are zip-tied in place and all stainless steel bolts are epoxied to discourage theft. A typical site, with a Yagi antenna for communications, is shown in Figure 8.3.
Since April, we have been assessing the data quality and processing the data to estimate daily site positions. Problems with telemetry outages at WLDC during the early morning, pre-dawn hours, were found to be due to a faulty battery, and were corrected when we installed new batteries at all the sites. GRIZ currently is experiencing intermittent data outages which were not solved by the new battery or by replacing the receiver/radio unit. We are currently investigating possible problems with the solar power regulator. We are also in the process of obtaining 2 additional systems from UNAVCO that will be installed on the roofs of the Space Sciences and McCone Hall buildings, which will make the profile cross the Hayward fault and allow direct measurement of surface creep in this region.
We are developing techniques to process the data using the GAMIT/GLOBK analysis package. We corrected software provided by UNAVCO to synchronize the phase, pseudorange, and clock offset observables, which allows the data to be cleaned in an automatic fashion. Preliminary results suggest that repeatabilities of 1-2 mm in daily horizontal positions on the shortest (several km) baselines can be achieved (Figure 8.4), but these degrade to 3-4 mm on the longer (10 km) baselines. We are investigating ways to simultaneously process the dual-frequency data from nearby BARD stations (e.g., BRIB, OHLN), with the single-frequency L1 data to improve these results. Currently data from second frequency on the BARD stations is not used, which degrades the definition of the local reference frame and repeatability of the baselines.
The data from the BARD sites generally are of high quality and measure relative horizontal positions at the 2-4 mm level. The 24-hour RINEX data files are processed daily with an automated system using high-precision IGS orbits. Final IGS orbits, available within 7-10 days of the end of a GPS week, are used for final solutions. Preliminary solutions for network integrity checks and rapid fault monitoring are also estimated from Predicted IGS orbits (available on the same day) and from Rapid IGS orbits (available within 1 day). Data from 5 primary IGS fiducial sites located in North America and Hawaii are included in the solutions to help define a global reference frame. Average station coordinates are estimated from 24 hours of observations using the GAMIT software developed at MIT and SIO, and the solutions are output with weakly constrained station coordinates and satellite state vectors.
Processing of data from the BARD and other nearby networks is split into 7 geographical subregions: the Bay Area, northern California, Long Valley caldera, Parkfield, southern and northern Pacific Northwest, and the Basin and Range Province. Each subnet includes the 5 IGS stations and 3 stations in common with another subnet to help tie the subnets together. The weakly constrained solutions are combined using the GLOBK software developed at MIT, which uses Kalman filter techniques and allows tight constraints to be imposed a posteriori. This helps to ensure a self-consistent reference frame for the final combined solution. The subnet solutions for each day are combined assuming a common orbit to estimate weakly constrained coordinate-only solutions. These daily coordinate-only solutions are then combined with tight coordinate constraints to estimate day-to-day coordinate repeatabilities, temporal variations, and site velocities.
The estimated relative baseline determinations typically have 2-4 mm WRMS scatter about a linear fit to changes in north and east components and the 10-20 mm WRMS scatter in the vertical component. Average velocities for the longest running BARD stations during 1993-2000 are shown in Figure 8.5, with 95% confidence regions. We have allowed random-walk variations in the site positions in order to more accurate characterization of the long-term stability of the site monuments and day-to-day correlations in position. The velocities are relative to stable North America, as defined by the IGS fiducial stations, which we assume have relative motions given by Kogan et al., (2000).
Most of the Sierra Nevada sites (CMBB, QUIN, and ORVB), as well as SUTB in the Central Valley, show little relative motion, indicating that the northern Sierra Nevada-Central Valley is tectonically stable. The motion of these sites relative to North America differs from the inferred motion of the western Basin and Range Province, suggesting 3 mm/yr right-lateral shear across the Walker Lane-Mt. Shasta seismicity trend. Deformation in the Pacific Northwest is generally consistent with interseismic strain accumulation along the Cascadia megathrust, the interface between the Juan de Fuca and North America plates, particularly in Washington where the velocity vectors are nearly parallel to the oblique convergence direction. Greater arc-parallel motion in Oregon and northern California may be due to the influence of the SAF system to the south and clockwise rotation of the southern Oregon forearc (Savage et al., 2000).
Deformation along the coast in central California is dominated by the active SAF system, which accommodates about 35 mm/yr of right-lateral shear. The Farallon Island site (FARB) off the coast of San Francisco is moving at nearly the rate predicted by the NUVEL-1A Pacific-North America Euler pole. Two-dimensional modeling of the observed fault-parallel strain accumulation predicts deep slip rates for the San Andreas, Hayward, and Calaveras/Concord faults are 19.31.8, 11.31.9, and 7.41.6 mm/yr, respectively, in good agreement with estimated geologic rates (174, 92, and 53 mm/yr, respectively). Most of the 46 mm/yr of relative motion is accommodated within a 100-wide zone centered on the SAF system and a broader zone in the Basin and Range Province in Nevada.
We are also developing real-time analysis techniques that will enable rapid determinations (minutes) of deformation following major earthquakes to complement seismological information and aid determinations of earthquake location, magnitude, geometry, and strong motion (Murray et al., 1998c). We currently process data available within 1 hour of measurement from the 18 continuous telemetry BSL stations, and several other stations that make their data available on an hourly basis. The data are binned into 1 hour files and processed simultaneously. The scatter of these hourly solutions is much higher than the 24-hour solutions: 10 mm in the horizontal and 30-50 mm in the vertical. Our simulations suggest that displacements 3-5 times these levels should be reliably detected, and that the current network should be able to resolve the finite dimensions and slip magnitude of a M=7 earthquake on the Hayward fault. We are currently investigating other analysis techniques that should improve upon these results, such as using a Kalman filter that can combine the most recent data with previous data in near real-time. The August 1998 M=5.1 San Juan Bautista earthquake (Uhrhammer et al., 1999) is the only event to have produced a detectable earthquake displacement signal at a BARD GPS receiver.
Mark Murray oversees the BARD program. André Basset, Bill Karavas, John Friday, Dave Rapkin, Doug Neuhauser, and Rich Clymer contribute to the operation of the BARD and L1 networks. Mark Murray and André Basset contributed to the preparation of this chapter.
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