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., 1998). 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 80 permanent stations (Figure 6.1). 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 (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 surveying and other interested communities.
BARD currently includes 80 continuously operating stations in the Bay Area and northern California (Tables 6.1 and 6.2), including 14 near Parkfield along the central San Andreas fault, and 17 near the Long Valley caldera near Mammoth. The BSL maintains 23 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, Hat Creek Radio Observatory, U. Wisconsin, Haselbach Surveying Instruments, East Bay Municipal Utilities District, the City of Modesto, the National Geodetic Survey, Thales, Inc., 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 2001, the BSL acquired 29 Ashtech Z-12 and Micro-Z receivers from a variety of funding sources, including from federal (NSF and USGS), state (CLC), and private (EPRI) agencies. The network enhances continuous strain measurements in the Bay Area and 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 6.1). Five sites have been equipped as part of the NSF-funded Mini-PBO project to establish collocated GPS, and borehole strainmeter and seismometer observatories in the Bay Area (see Chapter 7).
The number of continuous GPS stations in northern California will dramatically increase over the next 5 years, with over 250 new site installations planned as part of the Plate Boundary Observatory (PBO) component of the NSF-funded Earthscope project. UNAVCO and researchers from BARD and the other regional networks, such as SCIGN, BARGEN, and PANGA, have submitted a proposal to NSF to fold operation and maintenance of portions of the existing networks into the PBO array at the end of the 5 years. Due to incompatible management plans for real-time telemetry and site maintenance procedures between the BSL and UNAVCO, only two BSL-maintained stations (SUTB and MUSB), out of a total of 25 BARD stations, are proposed to become PBO stations. The other BSL stations are either collocated with seismic instrumentation or are located near the San Andreas fault where real-time processing of the GPS data for earthquake notification is a high priority. We are working closely with UNAVCO to facilitate the transition of the 25 stations and are acting in an advisory role on siting issues for the planned new installations.
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/).
Many of the BARD sites are classified as CORS stations by the NGS, and are used as reference stations by the surveying community. We coordinate efforts with surveying community at meetings of the Northern California GPS Users Group and the California Spatial Reference Center, and are currently developing plans to use the existing infrastructure at the NCEDC to provide a hub for a high-frequency real-time surveying network in the Bay Area. 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.
The typical configuration of a BSL continuous GPS station installation has been described in detail in previous annual reports. We here provide a brief description and highlight some of the recent changes. During July 2003-June 2004, we performed maintenance on existing BARD stations, installed 2 new stations, and prepared for a new station installation in the Marin Headlands.
Each BSL BARD station uses a low-multipath choke-ring antenna, most of which are mounted to a reinforced concrete pillar approximately 0.5-1.0 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. A low-loss antenna cable is used 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. Most use Ashtech Z-12 receivers that are programmed to record data once every 30 seconds and observe up to 12 satellites simultaneously at elevations down to the horizon. The antennas are equipped with SCIGN antenna adapters and hemispherical domes, designed to provide security and protection from weather and other natural phenomena, and to minimize differential radio propagation delays. The BSL acquired 7 Ashtech MicroZ-CGRS (uZ) receivers with NSF funding for the Mini-PBO project. These receivers, designed for continuous station applications, use less power (5.6 W) than the Z-12 receivers due to the lack of an interactive screen, provide better remote receiver control, and can support serial telemetry in both native raw format and the receiver independent BINEX format.
Data from most BSL-maintained stations are collected at 30-second intervals and transmitted continuously over serial connections (Table 6.1). Station TIBB uses a direct radio link to Berkeley, and MODB uses VSAT satellite telemetry. Most stations use frame relay technology, either alone or in combination with radio telemetry. Fourteen GPS stations are collocated with broadband seismometers and Quanterra data loggers (Table 3.2). With the support of IRIS we developed software that converts continuous GPS data to MiniSEED opaque blockettes that are stored and retrieved from the Quanterra data loggers (Perin et al., 1998), providing more robust data recovery from onsite disks following telemetry outages.
Data from DIAB, MONB, POTB, and TIBB in the Bay Area, the 4 Mini-PBO stations, and 13 stations in the Parkfield regional (all but PKDB), are now being collected at 1-second intervals. Collecting at such high-frequency (for GPS) allows dynamic displacements due to large earthquakes to be better measured, such as was demonstrated by several studies following the 2002 Denali fault earthquake. However, this 30-fold increase in data pose telemetry bandwidth limitations. Data from the Parkfield stations are collected on an on-site computer, written to removable disk once per month, and sent to SOPAC for long-term archiving (decimated 30-sec data is acquired daily via the BSL frame relay circuit). In the Bay Area, we have converted stations that have sufficient bandwidth and are currently assessing bandwidth issues at other stations. We are planning to convert to 1-second sampling where possible during the next year.
The BSL also acquired several Wi-Lan VIP 110-24 VINES Ethernet bridge radios. These 2.4 GHz spread spectrum radios use a tree structure to create a distributed Ethernet backbone with speeds up to 11 Mbps. Each system uses a directional antenna to talk to its ``parent" in the tree, and an omni-directional antenna to talk to its children, if multiple, or a directional antenna if it has only 1 child. These radios offer several advantages over the Freewave radios used at other sites, including TCP/IP Ethernet control, higher bandwidth, and greater flexibility for setting up networks. We installed a set of Wi-Lan radios at the SVIN Mini-PBO station to transmit data from the site to the frame relay circuit, and are assisting EBMUD in converting their continuous station to real-time telemetry using Wi-Lan radios.
In September 2003, data telemetry from Sutter Buttes (SUTB) was interrupted. SUTB uses a Freewave radio to telemeter data to Oroville (ORVB) where it is aggregated with the ORVB GPS and seismic data on the Quanterra dataloggers and then telemetered to BSL in real time. Two problems were found. The Freewave radio at ORVB was replaced due to a bad serial port. Also, the circuit breaker on the GPS receiver at SUTB had been tripped. After resetting the breaker, data acquisition and telemetry were restarted. It is possible that a lightning storm was responsible for both problems, although workers at the communication facility on Sutter Buttes said they had been having problems with turkey vultures flying into the power lines and shorting out the power system.
In February 2003, telemetry flow of GPS data stopped at MUSB. Access to the site was initially limited by the winter snowpack, and then by the need to coordinate the visit with Southern California Edison engineers. During a visit to the site in August 2003 continuity tests revealed that the hardline antenna cable had apparently failed. This 70 m cable apparently was damaged by repeated water freezing in the PVC conduit that houses it. We replaced the hardline antenna cable, improved the drainage of the conduit, and restarted data acquisition in October 2003.
In February 2004, the site installation at Diablo (DIAB) was refurbished, including replacement of dead batteries and cooling fans, and removal of animal waste and nesting material that was degrading the performance of the cooling system.
Throughout the year, we continued installations for the NSF-funded Mini-PBO project to establish collocated GPS, and borehole strainmeter and seismometer observatories in the Bay Area. In November 2003, we installed the GPS instrumentation at the Mini-PBO station at St. Vincents School for Boys (SVIN). In February 2004, we installed the GPS instrumentation at the Mini-PBO station at Ox Mountain (OXMT). Both of these sites employed the borehole antenna mount we designed to provide a stable, compact GPS monument. In the coming year we plan to complete the Mini-PBO GPS installations after PG&E has installed AC power at the Marin Headlands (MHDL) site. See Chapter 7 for more details about the Mini-PBO station installations.
Additional continuous GPS stations were installed in northern California by collaborating agencies during the last year, including 4 stations (GR8V, MEE1, MEE2, QCYN) by the University of Wisconsin to study the creeping section of the San Andreas fault between San Juan Bautista and Parkfield, 3 stations (PLSB, SHJB, TMSB) by Haselbach Instruments in the Central Valley for surveying applications, one station (LNC1) in the Central Valley by the US Coast Guard for navigation applications (replacing PBL1, which was decommissioned), and SLAC by the Stanford Linear Accelerator surveying group, which effectively replaces the nearby SUAA site that had been used for navigation research. Due to lack of funding, JPL discontinued the operation of KRAK on the resurgent dome in the Long Valley caldera. However, continued monitoring of this geophysically interesting location is provided by the nearby station KRAC operated by CVO.
We use the GAMIT/GLOBK software developed at MIT and SIO to process data from the BARD and other nearby continuous GPS networks. Recent improvements to GAMIT/GLOBK include better accounting of ocean-tide effects, estimating gradients in atmospheric variations, and applying elevation-dependent weighting to the data observables. We process data from more than 70 stations within hours of the completion of the day using rapid or predicted orbits. We have also reprocessed older data from the present to 1991 using improved orbits. 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. For long-term velocity estimates, we combine these solutions with global and regional solutions provided by SOPAC to better define a stable North America reference frame. We are currently porting our processing system to a Linux-based cluster that will significantly decrease the time required to analyze the data and allow us to process the much larger number of stations anticipated by the installation of the PBO network.
The estimated relative baseline determinations typically have 2-4 mm long-term scatter in the horizontal components and the 10-20 mm scatter in the vertical. Average velocities for the longest running BARD stations during 1993-2004 are shown in Figure 6.2, with 95% confidence regions assuming only white noise. The velocities are relative to stable North America, as defined by the IGS and CORS fiducial stations. In a study using only the continuous GPS stations in northern California and Nevada (Murray and Segall, 2001), we found that 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 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 in good agreement with estimated geologic rates.
We are completing a more comprehensive study that combines survey-mode and continuous GPS solutions into a self-consistent velocity field in the San Francisco Bay area (D'Alessio et al., 2004). This Bay Area Velocity Unification (BAVU) study, which employs more sophisticated three-dimensional block modeling methods that allow for complex fault geometries and enable along-strike variations to be estimated, is described in more detail in the research section (III.19).
We are assessing the BARD time series for transient deformation signals (short-term deviations from the long-term average velocities). Stations within stable continental regions typically move at constant velocities, but near plate boundaries, earthquake cycle deformation associated with strain accumulation and release on faults causes transient behavior both from coseismic static displacements and from postseismic deformation as the crust re-equilibrates to the stress changes induced by the earthquake.
Transient signals have been observed from numerous earthquakes worldwide, but in northern California, the August 1998 M=5.1 San Juan Bautista earthquake (Uhrhammer et al., 1999) was the only event to have produced a detectable earthquake displacement signal (of 4 mm at the SAOB station) prior to 2003. This changed on 22 December 2003 when the 6.5 San Simeon earthquake significantly displaced all the BARD stations along the creeping and Parkfield sections of the San Andreas fault. Most of these stations are located 50 km from the epicenter and were displaced southwest by about 15 mm. The closest continuous station, CRBT, about 25-30 km from the epicentral region, was displaced southwest by 68 mm and shows a modest postseismic transient in the first month following the earthquake (Figure 6.3). Preliminary results from our investigations of finite-fault rupture models for the San Simeon earthquake from inversions of seismic and geodetic data, including more coseismic displacements from survey-mode observations (Rolandone et al., 2004; Murray et al., 2004), are presented in the research section (III.14).
Transient deformation has also been observed using GPS from aseismic processes that can occur over hours to days, such as episodic slow earthquakes. These events, often associated with earthquake tremors, have been detected in both Japan and the northern Cascadia subduction zone, and recent studies suggest that they may also be occurring along the southern Cascadia subduction zone in northern California. In the research section (III.27), we report on our preliminary study of deformation and tremor observations in the southern Cascadia subduction zone to better constrain and characterize this possible episodic behavior.
We are developing real-time analysis techniques that will enable rapid determinations (within minutes) of deformation following major earthquakes to complement seismological information. We use GAMIT/GLOBK processing techniques to estimate independent hourly solutions at the several cm-level horizontal precision and during the past year established an extension of the REDI system where estimates of postseismic positions are attempted when 10 minutes of data become available following an earthquake (Murray et al., 2002).
We currently process 1-hour data batches available within 20 minutes of measurement from more than 20 continuously telemetered BSL and other stations providing hourly data. The hourly solutions have higher scatter than the 24-hour solutions (3-10 mm in the horizontal and 10-30 mm in the vertical), but 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 M7 earthquake on the Hayward fault. Due to the poor ability of GAMIT to resolve ambiguities from short data spans, estimates of coseismic displacements within minutes of an event have high (decimeter-level) uncertainty. We are testing a relatively new component of GAMIT that uses Kalman filtering techniques and improved ambiguity resolution methods to provide higher-precision kinematic positions. This method works well for networks with small interstation distances (e.g., near the 1999 Hector Mine earthquake), which aids ambiguity resolution, but has less success on more widely spaced networks, such as the continuous GPS stations in the vicinity of the 2002 Denali earthquake.
These rapid processing techniques can also be applied to estimating higher frequency 1-Hz GPS displacements, which have been used to detect surface from large earthquakes and can potentially add valuable information about the seismic source. We are also developing methods to rapidly estimate finite-source models from coseismic GPS displacements and to use these models to predict strong-ground motions and improve ShakeMap depictions of these motions as rapidly as possible after an earthquake. The first step of this project has been the development of a new methodology to improve prediction of strong ground motion from a simple uniform-slip geodetic model of the source. This methodology is based on a simple assumption that the large slip should take longer to terminate. We use well known scaling relations between stress drop and slip velocity to develop a spatio-temporal slip model. We have tested this model on the 1992 Northridge earthquake and found that the predicted ground motions agree well with observed motions and with other models derived from combinations of seismic and geodetic data (Rhie et al., 2004).
Mark Murray oversees the BARD program. André Basset, Wade Johnson, Rich Clymer, Cedric de La Beaujardiere, Bill Karavas, John Friday, Dave Rapkin, and Doug Neuhauser contributed to the operation of the BARD network.
Bürgmann, R., D. Schmidt, R.M. Nadeau, M. d'Alessio, E. Fielding, D. Manaker, T. V. McEvilly, and M. H. Murray, Earthquake potential along the northern Hayward fault, California, Science, 289, 1178-1182, 2000.
D'Alessio, M. A., I. A. Johanson, R. Bürgmann, D. A. Schmidt, and M. H. Murray, Slicing up the San Francisco Bay Area: Block kinematics from GPS-derived surface velocities, J. Geophys. Res., in prep., 2004.
King, N. E., J. L. Svarc, E. B. Fogleman, W. K. Gross, K. W. Clark, G. D. Hamilton, C. H. Stiffler, and J. M. Sutton, Continuous GPS observations across the Hayward fault, California, 1991-1994, J. Geophys. Res., 100, 20,271-20,283, 1995.
Murray, M. H., and P. Segall, Continuous GPS measurement of Pacific-North America plate boundary deformation in northern California and Nevada, Geophys. Res. Lett., 28, 4315-4318, 2001.
Murray, M. H., R. Bürgmann, W. H. Prescott, B. Romanowicz, S. Schwartz, P. Segall, and E. Silver, The Bay Area Regional Deformation (BARD) permanent GPS network in northern California, EOS Trans. AGU, 79(45), Fall Meeting Suppl., F206, 1998.
Murray, M., Neuhauser D., Gee, L., Dreger, D., Basset, A., and Romanowicz, B., Combining real-time seismic and geodetic data to improve rapid earthquake information , EOS. Trans. AGU, 83(47), G52A-0957, 2002.
Murray, M.H., D.C. Agnew, R. Bürgmann, K. Hurst, R.W. King, F. Rolandone, J. Svarc, GPS Deformation measurements of the 2003 San Simeon earthquake, Seism. Res. Lett., 75, 295, 2004.
Perin, B. J., C. M. Meertens, D. S. Neuhauser, D. R. Baxter, M. H. Murray, and R. Butler, Institutional collaborations for joint seismic and GPS measurements, Seismol. Res. Lett., 69, 159, 1998.
Rhie, J., D. Dreger, and M. H. Murray, A prediction of strong ground motions from geodetic data for PGV ShakeMaps, Geophys. Res. Lett., in prep., 2004.
Rolandone, F., I. Johanson, and R. Bürgmann, Geodetic observations of the M6.5 San Simeon earthquake with focus on the response of the creeping segment of the San Andreas fault, Seism. Res. Lett., 75, 293, 2004.
Romanowicz, B., B. Bogaert, D. Neuhauser, and D. Oppenheimer, Accessing northern California earthquake data via Internet, EOS Trans. AGU, 75, 257-260, 1994.
Uhrhammer, R., L. S. Gee, M. Murray, D. Dreger, and B. Romanowicz, The 5.1 San Juan Bautista, California earthquake of 12 August 1998, Seismol. Res. Lett., 70, 10-18, 1999.
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