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Permanent GPS Network: Bay Area Regional Deformation Array

Mark Murray, Ray Baxter, Bill Karavas, Roland Bürgmann

Subsections


Introduction

The Bay Area Regional Deformation (BARD) network of permanent, continuously operating Global Positioning System (GPS) receivers monitors crustal deformation in the San Francisco Bay area and northern California. It is a cooperative effort of the Berkeley Seismological Laboratory at UC Berkeley (BSL), the US Geological Survey (USGS), and several other academic, commercial, and governmental institutions. Started in 1991 with 2 stations spanning the Hayward fault, BARD now includes 32 permanent stations and will expand to 40 stations in 1998 (Figure 4.1). 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 system in the San Francisco 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.


  
Figure 4.1: Operational (solid triangles) and planned (open triangles) BARD stations in northern California (top) and in the San Francisco Bay area (bottom). The oblique Mercator projection is about the NUVEL-1 Pacific-North America Euler pole so that expected relative plate motion is parallel to the horizontal. Circled stations (will) use continuous telemetry. Courtesy of M. Murray
BARD Site Map

BARD presently includes 32 permanent, continuously operating stations (Table 4.1): 15 are maintained by the BSL (including two with equipment provided by Lawrence Livermore National Laboratory, and the Satloc Corporation), 4 by the USGS, Menlo Park, California, 2 by Trimble Navigation, and 1 by Stanford University. Other stations are maintained by institutions outside of northern California, such as the National Geodetic Survey, the Jet Propulsion Laboratory, and the Scripps Institution of Oceanography, as part of larger networks devoted to real-time navigation, orbit determination, and crustal deformation. In addition, 3 stations are operated by UC Davis in a semi-permanent mode: receivers at these sites operate continuously except for brief periods when they are used for campaign-mode surveying.


  
Table 4.1: Operational and Planned BARD Stations
BARD Site Table

In 1996, the BSL acquired 13 Ashtech Z-12 receivers with Dorne-Margolin design choke ring antennas from a combination of federal (NSF), state (CLC), and private (EPRI) funding. Nine of these receivers have been installed during the past two years to densify the continuous strain measurements in the San Francisco Bay area and to consolidate the regional geodetic network. One particular focus of the station locations is several profiles between the Farallon Islands and the Sierra Nevada in order to better characterize the larger scale deformation field in northern California (Figure 4.1). Two more of the BSL receivers and 2 receivers owned by UC Santa Cruz will be installed during the next year. In addition, the BSL will assume operation and maintenance of the 2 semi-permanent receivers located in the south Bay region currently operated by UC Davis, and with recent federal (USGS NEHRP) funding, has purchased 2 Ashtech receivers to be permanently installed at these sites during 1998-99.

The 2 remaining BSL receivers will be part of Self-Continuous Autonomous Mobile Positioning Stations (SCAMPS) that can be deployed for short intervals within dense local subnets around the Hayward fault and in the north Bay area. Three years funding for this project, which will combine GPS and INSAR observations to infer deformation along the Hayward fault, was award by NASA in early 1998, and preliminary reconnaissance and surveys to establish suitable GPS sites are currently underway. Testing and evaluation of possible hardware systems, including less expensive L1-only receivers, will be conducted in collaboration with UNAVCO during the next year.

In 1996, researchers from the BSL, the USGS, Stanford University, LLNL, UC Davis and UC Santa Cruz formed a consortium of institutions involved in studies of the 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. They agreed in principle to the continued development of the network of permanently deployed GPS receivers, to the development and maintenance of a pool of GPS receivers for campaign-mode operations that, when not used in campaigns, will be deployed in semi-permanent mode in the San Francisco Bay area, to archiving of all data at the NCEDC, and to the development of a coordinated data analysis facility that will process permanent, semi- permanent, and campaign-mode data.

Site Installations

During fiscal year 1997-98 three new sites (SAOB, MUSB, and DIAB) of the BARD network were permitted, designed, constructed and equipped with Ashtech Z-12 receivers by the BSL staff. Several other sites were also prepared in July-August 1998: MONB and PTRB became operational and a monument pier was installed at S300 (Figure 4.1).

In general, each new site uses a low-multipath choke-ring antenna mounted to a reinforced concrete pillar approximately one meter above local ground level. If possible, the reinforcing steel bars of the pillar are drilled and cemented into rock outcrop to improve long-term monument stability. Low-loss antenna cables are used to minimize signal degradation on the longer cable setups that normally would require signal amplification. Continuous telemetry is provided by a combination of radio modem or frame-relay technologies. Use of low-voltage cutoff devices to improve receiver performance following power outages was also introduced last year.

New sites

The first site (SAOB) was installed in July 1997 near the San Andreas Geophysical Observatory near Hollister, monitoring the south San Francisco Bay area near the intersection of the San Andreas and Calaveras faults. At SAOB, due to a lack of exposed rock outcrop, a 20 cm diameter hole was drilled five meters deep. Vertical steel reinforcing bars run from the bottom of the hole into the exposed pillar which serves as the antenna mount. The vertical hole was filled with concrete and extended monolithically to include the exposed antenna mount portion.

The second site (MUSB) was installed in November 1997 on Musick Mountain, in the Sierra Nevada range southwest of Mammoth. This site, combined with CMBB, ORVB, and QUIN, provides valuable constraints on the tectonic stability of the Sierra Nevada range, and its relative motion with respect to the Great Valley (these two provinces are often assumed to form a rigid block), and to the central Coast Ranges. MUSB uses a one-meter-high, reinforced-concrete monument attached to exposed granite outcropping near the summit of Musick Mountain. Power and telemetry to this site are supplied by Southern California Edison from their colocated microwave facility. The data is first telemetered via radio modems to the microwave facility, and then via microwave to the next microwave facility where it joins the data stream from the KCC broadband station, which is located 20 km north of MUSB. UC Santa Cruz will provide an Ashtech Z-12 receiver to replace the existing BSL receiver during 1998-99.

The third site (DIAB) was installed in May 1998 in the Mount Diablo State Park. Mount Diablo is one of the most prominent peaks in the east Bay area. Located near the northern Calaveras, Concord, and Livermore faults, this site fills in an important gap in the Farallon-Sierra Nevada profiles, currently making it the most stable BARD station closest to the west side of the Great Valley. The DIAB site is close to a public road near the summit, so a short, half-meter, reinforced-concrete monument was attached to an exposed rock outcropping at this site to limit public visibility. Municipal utilities pass near the site and supply power and frame-relay telemetry.

Laboratory personnel also installed a reinforced concrete monument at the Site 300 explosion testing facility (S300) of the Lawrence Livermore National Laboratory in July 1998. During 1997-98 BSL staff worked closely with LLNL and Trimble representatives to design a multi-purpose system that will primarily be operated by LLNL with a continuous telemetry feed to the BSL. This system will provide real-time kinematic mode surveying capability for precise (cm-level) mapping of archealogical sites and other features at Site 300, publicly available differential corrections for real-time meter-level navigation and position capability in the general vicinity of the site, and real-time telemetry of the raw data stream for the fault monitoring activies at the BSL. The site was chosen for its good site sky visibility, for its availability of rock outcrop enhancing monument stability, and for its proximity to the Great Valley. Because it is located at one of the easternmost sites in the Diablo Range, it will provide valuable constraints on the total deformation accommodated between the Sierra Nevada range and the San Andreas fault system. S300 will become operational in October 1998.

Two more sites were installed in July and August 1998 in the Point Reyes National Park (PTRB) and on Monument Peak (MONB) in the Mission Hills of the east San Francisco Bay area. PTRB, located near the historic Point Reyes lighthouse at the westernmost point of land in the north Bay midway between the San Andreas fault and the Farallon Islands, will provide valuable constraints on deformation west of the San Andreas fault. MONB is located in a tectonically complicated region near the intersection of the Calaveras and southern Hayward faults, where the seismicity appears to step over along the Mission fault.

Installations are also currently being permitted and prepared at several other sites, including Barnabe Peak (BARB), just east of the San Andreas fault in the north Bay area and Potrero Hills (POTB) in the north Bay near the Great Valley. In addition, the BSL will assume operational responsibilities for the LUTZ and SODA sites in the south Bay after replacing the the semi-permanent Trimble receivers currently operated by UC Davis with recently purchased BSL Ashtech receivers, which will be permanently installed, and upgrading to continuous telemetry.

During 1997-98, the USGS installed a receiver near Mammoth (JNPR), so that 4 receivers are now monitoring the volcanic unrest at the Long Valley caldera. In 1998-99, the USGS plans to install four receivers in the Parkfield area to supplement the PKD1 and CARR receivers that are currently monitoring this segment of the San Andreas fault that is expected to rupture in a M6 earthquake in the near future. Also during 1998-99, UC Davis plans to improve the telemetry of the UCD1 receiver located on their campus, and UC Santa Cruz is is preparing to install a semi-permanent receiver on the roof of one of their campus buildings.

Low-loss antenna cables

At SAOB, MUSB, and DIAB, low-loss antenna (<1.5 db per 30 meters at 1.5 GHz) cable was used. Normally, the signal from the receiver antenna is amplified to a high level, and subsequently attenuated over the length of the cable connecting the antenna to the receiver. This attenuation limits the physical distance between the receiver and the antenna as the signal-to-noise degrades. Traditionally, line amplifiers have been added as a remedy. The amplifiers may drift or become electrically unstable as they age. This would result in an impedance mismatch and an unknown but apparent change in the phase center of the antenna over the life of the experiment. In addition to eliminating the loss of signal which necessitated the line amplifier in the first place, the low loss cable is passive, and will not change in impedance with time.

In addition to the three new installations, the CMBB and HOPB stations were upgraded from high-loss antenna cable and line amplifiers to low-loss antenna cables.

Low-multipath antenna and radomes

Antennas at two older BSL stations, CMBB and HOPB were replaced with Dorne-Margolin design choke-ring antenna that minimize the effects of multipath signals and are the accepted standard for the IGS global permanent GPS network. The older microstrip Ashtech antennas still used at two BSL stations (BRIB and TIBB) will be replaced in 1998-99 with choke-ring antennas when the hemispherical radome designed by SCIGN becomes widely available. Antenna radomes cover the antennas to provide security and protection from the weather and other natural phenomenon. Many BSL stations do not currently use radomes, particularly if they are in isolated areas with temperate climates. FARB uses a prototype hemispherical radome designed by UNAVCO to protect the antenna from birds during nesting season. Other stations, such as CMBB and MUSB, use the Ashtech conical design radome to minimize snow buildup on the antenna during the winter season. However, because radomes cause additional delays to the GPS signals, they can alter the antenna phase pattern and its average phase center. The CMBB radome was damaged in December 1997 when vandals apparently kicked a large (10 cm) hole into one side of the conical dome, which introduced a 2-cm apparent offset in the east coordinate of the CMBB position (Figure 4.5). The new SCIGN radome designed for the Dorne-Margolin antennas 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 far more resistent to damage and will completely cover the dome and cable connections for added protection. The BSL will purchase these radomes for all of the choke-ring antennas after their first production run in late 1998, and will install them at all the sites to ensure the most uniform and internally consistent antenna configuration for the BARD network.

Low-voltage cutoffs

With the charging circuit off, the receiver and telemetry equipments continue to draw constant power from the batteries. (Electrically, power = voltage x current). As the battery voltage gets lower, the current increase to the point of failure. In the case of the Ashtech receivers BSL engineers have found the on/off switch is a circuit breaker. The breaker trips at low voltage (high current) levels. After an extended power outage, the receiver appears to have been turned off. The low voltage cut-off device protects equipment, and eliminates the need to visit a station after an extended power outage. Low-voltage cut-off devices were installed at all new stations and stations at SAOB, YBHB, FARB, SUTB and CMBB were upgraded with the devices.

Continuous Telemetry

The UCB Seismological Laboratory currently maintains and retrieves data from 15 Ashtech Z-12 receivers. Data from these sites is collected at 30-second intervals, transmitted continuously over serial connections, collected into 24-hour raw serial files and processed daily. The serial connections to eleven sites use frame relay technology, one site (TIBB) has a direct radio link to Berkeley and three sites (SUTB, MUSB, and FARB) use a combination of radio and frame-relay technologies. We have developed software to interpret and collect the raw serial output into hourly files, which is then converted to the standard interchange RINEX format using software provided by NGS and UNAVCO. In 1997-98 we converted the last two of our sites with dial-up telemetry (HOPB and CMBB) to continuous serial telemetry.

Nine current GPS sites are co-located with broadband seismometers and Quanterra data collectors. With the support of IRIS we have developed software for the Quanterra that will allow the storage and retrieval of the continuous serial output of Ashtech and Trimble GPS receivers during and after a telemetry outage. This software has been developed and tested and hardware connections to allow its use have been made at four BSL sites. Upon Quanterra's completion of new system software, this datalogging software will be tested and submitted to UNAVCO for general public distribution.

Data Archival and Distribution

The raw and RINEX data files are imported in a timely fashion into the UCB/USGS Northern California Earthquake Data Center (NCEDC) data archive maintained at UCB (Romanowicz et al., 1994), where they are immediately accessible 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 from the 15 BSL sites, and the 20 other sites run by BARD collaborators are archived at the NCEDC. Before being submitted to the archive each daily data set is checked to verify data integrity, quality, completeness, and conformance to the RINEX standard. Any data that fail to meet these criteria are held back from the public until the problem can be corrected or it can be established that no more data is forthcoming.

As part of the attempt to verify data quality and integrity, we have begun periodic checks of the signal-to-noise ratio associated with the incoming GPS signals. Currently we create skyplots that show the signal-to-noise associated with each region of the sky for each of our stations (Figure 4.2). We will use these plots to give us early warning of antenna or receiver failure, or other problems such as changes in the multipath environment at the sites due to alterations in buildings, vehicles and other local structures.

Data and ancillary information about BARD sites 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 USGS imports data daily from their stations into the NCEDC. In addition, the BSL automatically retrieves data from other continuously operating stations in northern California from other GPS archives, such as at SIO, JPL, and NGS, using modified UNIX scripts provided by SIO. The NCEDC currently archives nearly all high-precision continuous GPS measurements collected in northern California.

Many of the BARD sites have been added by the NGS to their database of CORS sites, which are used as references stations by the surveying community, after having established their locations in the WGS84 coordinate system. All permanent sites operational in July 1998 will be included in a new statewide adjustment of WGS84 coordinates, which makes the data more useful to the surveying community. 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.

The BARD Project and the NCEDC are cooperating with UNAVCO and others in the GPS community to establish a seamless data archive. This project will enable a user to find and access GPS data no matter where they are archived and to be assured that this data is correct and up to date. In 1997-98 members of the BARD project participated in a working group organized by UNAVCO to define the GPS Seamless Archive Centers (GSAC) proposal and necessary meta-data file structures, and work is underway to create the databases that will describe the holdings of the NCEDC to the GSAC community.


  
Figure:4.2 Skyplots and L2 signal-to-noise ratios for four BARD stations. Skyplots show satellite azimuth and elevation at each 30-sec epoch for 24 hours on 23 May 1998. The zenith direction is at the center of the circle, and circle grids are $30^{\circ }$ elevation angles down to the horizon. The absence of low elevation data in the north direction is due to the geometry of the satellite orbits. All receivers can track down to the horizon, as is mostly possible at SUTB. Satellite tracks that cut off above the horizon indicate local obstructions. The cutoff in the NE and NW directions at DIAB are due to the summit of Mount Diablo itself, which is to the north of the receiver. The greyscale of each symbol shows the signal-to-noise ratio of the L2 phase observable (SN2). High SN2 (darkest greys) are normally near the zenith and the low SN2 (lightest greys) are normally near the horizon. Unusual deviations from this pattern might indicate signal multipathing due to reflectors in the local site environment. Courtesy of M. Murray
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c) MUSB & d) SAOB \\
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Data Analysis

The data from the BARD sites generally are of high quality and measure relative horizontal positions at the 3-5 mm level. The 24-hour RINEX data files are processed daily with an automated system using high-precision IGS orbits. Final IGS orbits, available with 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. Due to the large number of BARD and fiducial stations, the BARD network is split into a "San Francisco Bay" subnet and a "Northern California" subnet, with the 5 fiducial stations and CMBB and FARB in common to both to help tie the subnets together. The weakly constrained solutions are combined using Kalman filter techniques using the GLOBK software developed at MIT that can apply tight a posteriori constraints. This helps to ensure a self-consistent reference frame for the final combined solution. The subnet solution 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 3-5 mm RMS scatter about a linear fit to changes in north and east components and the 15-25 mm RMS scatter in the vertical component. Figure 4.3 shows several time series from the automated 24-hour solutions for 1997-1998. These examples show some of the more unusual weather and tectonic activity recorded by the BARD network. The El Niño winter of 1997-1998 produced heavy rainfall along the coast and snowfall in the Sierra Nevada. The hillslope at the NUNE site became unstable during one rainstorm, causing the box housing the receiver to slide downhill and damage the antenna, which was more stably anchored at greater depth. This damage to the antenna caused an apparent 2-cm shift in position to the north (Figure 4.3a). MUSB, located at high altitude on Musick Mountain in the southern Sierra Nevada, becomes snowbound and inaccessible at times during the winter. Two episodes of large vertical motions occurred during this period in 1998, each beginning gradually but ending abruptly (Figure 4.3b). We speculate that these episodes were caused by the antenna becoming covered with snow that then abruptly fell off during melting. The Long Valley caldera experienced an episode of volcanic unrest, with increased levels of seismicity and high deformation rates, that began around May 1997 and abated in early 1998. The baseline length between CASA and KRAK, which crosses the caldera, shows the high levels of deformation (nearly 10 cm increase) measured during this period (Figure 4.3c). A similar deformation episode was measured by two-color laser EDM in 1989.


  
Figure:4.3 Time series of BARD station positions and baseline components from January, 1997 to August, 1998. Data are 24-hour estimates about their mean, with one standard deviation uncertainties, relative to the ITRF96 reference frame as defined by five North America and Hawaii IGS fiducials stations. a) The north component of NUNE. Typical relative scatter of horizontal components is 3-5 mm. The anomalous motion in early 1998 is non-tectonic, and is due to the antenna being damaged when the box housing the receiver slid downslope due to near-surface instability follow a severe winter storm. b) The vertical component of MUSB. Typical relative scatter of horizontal components is 15-25 mm. The anomalous motions in early 1998 are non-tectonic and are mostly likely due to heavy snow either covering the antenna or inducing severe multipath at the site. c) The length of the baseline from CASA to KRAK, which crosses the Long Valley caldera. The Long Valley caldera experienced increased seismicity and deformation, consistent with inflation of one or more magmatic sources at depth within the center and along the south edge of the caldera, during the latter half of 1997. The caldera became quiescent again in 1998. A similar episode of rapid deformation was measured by two-color laser EDM in 1989. Courtesy of M. Murray
Example Time Series

Average velocities for the longest running BARD stations during 1997-1998 are shown in Figure 4.4, with 95% confidence regions. Anomalous site behavior, such as the NUNE and MUSB episodes described above, have been removed from this analysis. The velocities are relative to stable North America, as defined by the five IGS fiducial stations. 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. Most of the Sierra Nevada sites (CMBB, QUIN, and ORVB), as well as SUTB in the Great Valley, show little relative motion, indicating that the northern Sierra Nevada-Great Valley is tectonically stable. The motion of these sites relative to North America is consistent with spreading in the Basin and Range Province. The sites near Long Valley caldera (CASA, KRAK, MINS, and JNPR) show anomalously high radial motions away from the center of the caldera, consistent with inflation of magmatic sources at depth. Deformation along the coast is dominated by the active San Andreas fault system, which accommodates about 35 mm/yr of right-lateral shear. These BARD results are being combined with older VLBI and spatially dense Geodolite EDM and campaign GPS measurements currently being collected by the USGS, Stanford University, and UC Berkeley. The combined velocity map will provide significantly improved constraints on three-dimensional locking depth and deep-slip models of strain accumulation, which will be used for seismic hazard assessment along the San Andreas fault system.


  
Figure:4.4 Average velocities of BARD stations in northern California (top) and in the San Francisco Bay area (bottom). The projection is the same as in Figure 4.1. Velocities are relative to stable North America. The NUVEL-1A predicted Pacific-North America 46 mm/yr relative motion is shown for scale. Courtesy of M. Murray
BARD Velocities

One of the goals of the BSL effort is to develop real-time analysis techniques that will enable rapid determinations ($\sim$minutes) of motion following major earthquakes to complement seismological information and aid determinations of earthquake location, magnitude, geometry, and strong motion. We currently process data available within 1 hour of measurement from the 15 continuous telemetry BSL stations, and 3 U.S. Coast Guard stations. The data are binned into 1 hour files and processed simultaneously. Figure 4.5 shows a 5-day interval of hourly position estimates for 12 of the continuous telemetry stations and the 3 U.S. Coast Guard (USCG) stations (CME1, PBL1, PPT1). 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; however, displacements 3 times these levels should be reliably detected. Several stations exhibit higher scatter than others, particularly TIBB and the USCG stations. The cause of this higher scatter is not well understood yet, but may be due to the higher multipath environment at these sites, which would have a greater effect on short interval data sets. The 2-cm apparent displacement caused by the damage to the CMBB antenna radome is discernable in the east component.


  
Figure:4.5 Results of automatically processed hourly position solutions for 5 days in December 1997 at 12 of the continuously telemetered BARD stations and 3 USCG stations, which provide data hourly. Solutions are available within 15 minutes after the end of the hour. Shown are the scatter about the average for the north, east, and vertical (note scale change) components. Horizontal offsets at the several-cm level are detectable at most sites. Note the 2-cm offset at CMBB in the east component on Day 337 that was caused by damage to the antenna radome. Courtesy of M. Murray
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