Bay Area Regional Deformation Network

Introduction

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 70 permanent stations (Figure 7.1). 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.

Figure 7.1: Operational (red) and planned (blue) BARD stations in northern California (top) and in the San Francisco Bay area (bottom). In the oblique Mercator projection expected Pacific-North America relative plate motion is parallel to the horizontal. Circled stations use continuous real-time telemetry. The 18 station Long Valley Caldera (LVC) network and 15 station Parkfield (PKFD) networks are also part of BARD. The small black triangles near BRIB are the experimental L1 stations. Mini-PBO stations are OHLN and SBRN (existing), and MHDL, OXMT, and SVIN (planned), all located along the northern Hayward and San Andreas fault. We plan to install 3 other stations at CVS, GARS, and COYS. The 2 Central Valley sites (USBR and CCID) are being installed in cooperation with the CSRC. Other nearby networks (open triangles) include: Basin and Range (BARGEN), and Southern California Integrated GPS Network (SCIGN).
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BARD currently includes 38 continuously operating stations in the Bay Area and northern California (Table 7.1), 14 near Parkfield along the central San Andreas fault, and 18 near the Long Valley caldera near Mammoth (Table 7.2). The BSL maintains 22 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, 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.


Table 7.1: Currently operating stations of the BARD GPS network maintained by the BSL or by other agencies except in the Parkfield and Long Valley caldera regions. Other agencies include: EBMUD = East Bay Mun. Util. Dist., UCD = UC Davis, SU = Stanford Univ., UCSC = UC Santa Cruz, City = City of Modesto (see also Table 1.1). Receivers: A = Ashtech, T = Trimble. See Table 3.2 for telemetry codes and for BSL sites collocated with seismic stations. Data from other agencies retrieved or pushed by FTP or from the Web.
Code Latitude Longitude Start Receiver Maint. Telem. Location
BRIB 37.91940 -122.15255 1993.58 A-Z12 BSL FR Briones Reservation, Orinda
CMBB 38.03418 -120.38604 1993.92 A-Z12 BSL FR Columbia College, Columbia
DIAB 37.87858 -121.91563 1998.33 A-Z12 BSL FR Mt. Diablo
FARB 37.69721 -123.00076 1994.00 A-Z12 BSL R-FR/R Farallon Island
HOPB 38.99518 -123.07472 1995.58 A-Z12 BSL FR Hopland Field Station, Hopland
LUTZ 37.28685 -121.86522 1996.33 A-Z12 BSL FR SCC Comm., Santa Clara
MHCB 37.34153 -121.64258 1996.33 A-Z12 BSL FR Lick Obs., Mt. Hamilton
MODB 41.90233 -120.30283 1999.83 A-Z12 BSL NSN Modoc Plateau
MOLA 37.94657 -122.41992 1993.75 T-SSE BSL   Pt. Molate, Richmond
MONB 37.49892 -121.87131 1998.50 A-Z12 BSL FR Monument Peak, Milpitas
MUSB 37.16994 -119.30935 1997.83 A-Z12 BSL R-Mi-FR Musick Mt.
OHLN 38.00742 -122.27371 2001.83 A-uZ BSL FR Ohlone Park, Hercules
ORVB 39.55463 -121.50029 1996.83 A-Z12 BSL FR Oroville
PKDB 35.94524 -120.54155 1996.67 A-Z12 BSL FR Bear Valley Ranch, Parkfield
POTB 38.20258 -121.95560 1998.92 A-Z12 BSL FR Potrero Hill, Fairfield
PTRB 37.99640 -123.01490 1998.58 A-Z12 BSL R-FR Point Reyes Lighthouse
SAOB 36.76530 -121.44718 1997.58 A-Z12 BSL FR San Andreas Obs., Hollister
SBRN 37.68622 -122.41044 2003.18 A-uZ BSL FR San Bruno Mt., Brisbane
SODB 37.16640 -121.92552 1996.33 A-Z12 BSL R-FR Soda Springs, Los Gatos
SUTB 39.20584 -121.82060 1997.33 A-Z12 BSL R-FR Sutter Buttes
TIBB 37.89087 -122.44760 1994.42 A-Z12 BSL R Tiburon
YBHB 41.73166 -122.71073 1996.75 A-Z12 BSL FR Yreka Blue Horn Mine, Yreka
CHAB 37.72412 -122.11931 1992.00 A-Z12 USGS   Chabot, San Leandro
WINT 37.65264 -122.14056 1992.00 A-Z12 USGS   Winton, Hayward
EBMD 37.81501 -122.28380 1999.18 T-SSi EBMUD   EBMUD, Oakland
QUIN 39.97455 -120.94443 1992.68 Rogue JPL   Quincy
S300 37.66642 -121.55815 1998.48 T-SSi LLNL   Site 300, Livermore
HCRO 40.81563 -121.46915 2003.50 T-SSi HCRO   Hat Creek Radio Obs.
CHO1 39.43264 -121.66496 1999.50 A-Z12 NGS   Chico
CME1 40.44177 -124.39633 1995.74 A-Z12 NGS   Cape Mendocino
CMOD 37.64130 -121.99997 2000.76 T-SSi City   Modesto
CNDR 37.89641 -121.27849 1999.27 A-Z12 NGS   Condor, Stockton
PBL1 37.85306 -122.41944 1995.50 A-Z12 NGS   Point Blunt, Angel Island
PPT1 37.18167 -122.39333 1996.00 A-Z12 NGS   Pigeon Point
SUAA 37.42691 -122.17328 1994.30 A-Z12 SU   Stanford University
THAL 37.35149 -121.93549 2003.00 A-uZ Thales   Thales, Inc., Santa Clara
UCD1 38.53624 -121.75123 1996.38 T-SSi UCD   UC Davis
UCSC 36.99279 -122.05219 2000.31 T-SSi UCSC   UC Santa Cruz
ZOA1 37.54305 -122.01594 2002.50 Novatel FAA   Fremont



Table 7.2: Currently operating stations of the BARD GPS network maintained by other agencies in the Parkfield and Long Valley caldera regions. Other agencies include: CVO = USGS Cascade Volcano Observatory (see also Table 1.1). Receivers: A = Ashtech. Data from other agencies retrieved or pushed by FTP or from the Web.
Code Latitude Longitude Start Receiver Maint. Location
CAND 35.93935 -120.43370 1999.33 A-Z12 USGS Cann, Parkfield
CARH 35.88838 -120.43082 2001.58 A-Z12 USGS Carr Hill 2, Parkfield
CARR 35.88835 -120.43084 1989.00-2003.31 A-Z12 JPL Carr Hill, Parkfield
CRBT 35.79161 -120.75075 2001.67 A-Z12 USGS Camp Roberts, Parkfield
HOGS 35.86671 -120.47949 2001.50 A-Z12 USGS Hogs, Parkfield
HUNT 35.88081 -120.40238 2001.58 A-Z12 USGS Hunt, Parkfield
LAND 35.89979 -120.47328 1999.33 A-Z12 USGS Lang, Parkfield
LOWS 35.82871 -120.59428 2001.58 A-Z12 USGS Lowes, Parkfield
MASW 35.83260 -120.44306 2001.58 A-Z12 USGS Mason West, Parkfield
MIDA 35.92191 -120.45883 1999.75 A-Z12 USGS Mida, Parkfield
MNMC 35.96947 -120.43405 2001.58 A-Z12 USGS Mine Mt., Parkfield
POMM 35.91991 -120.47843 1999.75 A-Z12 USGS Pomm, Parkfield
RNCH 35.89999 -120.52482 2001.58 A-Z12 USGS Ranchita, Parkfield
TBLP 35.91741 -120.36034 2001.67 A-Z12 USGS Table, Parkfield
BALD 37.78330 -118.90130 1999.67 A-ZFX CVO Bald Mt., LVC
CA99 37.64460 -118.89670 1999.67 A-ZFX CVO Casa 1999, LVC
CASA 37.64464 -118.89666 1993.00 Rogue JPL Casa Diablo, LVC
DDMN 37.74430 -118.98120 1999.67 A-ZFX CVO Deadman Creek, LVC
DECH 38.05150 -119.09060 2001.58 A-ZFX CVO Dechambeau Ranch, LVC
HOTK 37.65860 -118.82130 2001.67 A-Z12 CVO Hot Creek, LVC
JNPR 37.77170 -119.08470 1997.81 A-Z12 USGS Juniper, LVC
KNOL 37.65912 -118.97917 1998.58 A-ZFX CVO Knolls, LVC
KRAC 37.71330 -118.88050 2001.67 A-Z12 CVO Krakatoa-USGS, LVC
KRAK 37.71313 -118.88114 1994.73 Rogue JPL Krakatoa, LVC
LINC 37.63719 -119.01729 1998.67 A-Z12 CVO Lincoln, LVC
MINS 37.65376 -119.06090 1995.92 A-Z12 USGS Minaret Summit, LVC
MWTP 37.64052 -118.94473 1998.58 A-ZFX CVO Mammoth Water Treat Plant, LVC
PMTN 37.83130 -119.05690 1999.67 A-Z12 CVO Panorama Mt., LVC
RDOM 37.67707 -118.89794 1998.58 A-ZFX CVO Resurgent Dome, LVC
SAWC 37.68990 -118.95310 2000.65 A-ZFX CVO Saw, LVC
TILC 37.61890 -118.86280 2000.65 A-Z12 CVO Tilla, LVC
WATC 37.66440 -118.65390 2001.67 A-Z12 CVO Waterson, LVC


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 7.1). Five more of the BSL receivers will be installed next year, 2 along the southern Hayward fault, and 3 as part of the NSF-funded Mini-PBO project establishing collocated GPS, and borehole strainmeter and seismometer observatories in the Bay Area (see Chapter 8).

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. The BARD network will form the initial core of the northern California array, and the BSL recently received NSF funding to maintain 40 stations for an 18-month period. During this period, BARD and the other regional networks, such as SCIGN, BARGEN, and PANGA, will be developing plans to fold operation and maintenance of portions the existing networks into the PBO array at the end of 5 years. We are working closely with UNAVCO, Inc., who has primary responsibility for implementation of PBO, to facilitate this transition and are acting in an advisory role on siting issues for the 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, which 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.

2002-2003 Activities

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 2002-June 2003, we performed maintenance on existing BARD stations, installed a new station, assisted collaborators with the installation of two new stations, and prepared for new stations near the Hayward fault, on the San Francisco peninsula, and north Bay area regions.

BARD Stations

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 programmed to record data once every 30 seconds, observing 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 phenomenon, and to minimize differential radio propagation delays.

Data from most BSL-maintained stations are collected at 30-second intervals and transmitted continuously over serial connections (Table 7.1). Station TIBB uses a direct radio link to Berkeley, and MODB uses VSAT satellite telemetry. Nineteen stations use frame relay technology, either alone or in combination with radio telemetry. Thirteen GPS stations are collocated with broadband seismometers and Quanterra data collectors (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 and MONB in the Bay Area, 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 can be limited by telemetry bandwidth issues. 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 the two stations that have sufficient bandwidth and are not collocated with seismic instrumentation. We are currently assessing bandwidth issues at other stations and are planning to convert to 1-second sampling where possible, such as the Mini-PBO stations, in the next year.

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. We installed a uZ at SBRN and replaced the Z-12 at OHLN with a uZ in May 2003 after the clock chip on Z-12 at the site began to malfunction. We are currently considering switching to the more compact BINEX format where possible, as this will reduce some of the bandwidth limitations and allow us to convert more stations to 1-second sampling.

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.

Station Maintenance

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 may have been damaged by repeated water freezing in the PVC conduit that houses it. We intend to replace the antenna cable and improve the drainage of the conduit in September 2003.

As part of the Plate Boundary Deformation project (Chapter 8), nine new continuous GPS sites were installed in the Parkfield area (see Table 7.2) in Summer 2001 by the USGS and SIO. These sites span about 25 km on either side of the San Andreas fault and are designed to link the BARD network in central and northern California to the SCIGN network in southern California. As part of this upgrade, the new station CARH was installed at Carr Hill near the original CARR station, which had been running since 1989. After allowing both stations to run side-by-side for nearly 2 years, CARR was turned off in April 2003. In February 2003, the NCEDC assumed responsibility from the USGS Pasadena for the telemetry download of these stations over their existing frame relay circuit at Parkfield. We installed an onsite LINUX computer that controlled the sequential download of data. In June 2003, the stations were upgraded to real-time streaming using Wi-Lan radios by SIO and the USGS.

New Installations

Throughout the year, we have continued installations for the NSF-funded mini-PBO project establishing collocated GPS, and borehole strainmeter and seismometer observatories in the Bay Area. Completion of these sites have been hampered by problems with the original permitting at Ox Mt and Marin Headlands, and by the need to redesign the GPS antenna mount. The GPS system at the Mini-PBO site SBRN was installed in early March 2003. Two major changes were made since the installation of the first Mini-PBO site at OHLN. We increased the diameter of the upper part of the steel shroud, which protects the borehole casing, from 10 to 14 inches to ensure that the casing remains decoupled from the surface. We also used a new borehole adapter for the GPS mount that was machined from two stainless steel flanges. Should borehole access be needed, this adapter allows a very high level of horizontal accuracy when reinstalling the antenna. These GPS mounts will be installed at the remaining 3 sites in Fall 2003 after rainfall lessens the fire hazards posed by the required welding of the lower flange onto the casing. For more details about the Mini-PBO station installations, see Chapter 8).

We assisted Hat Creek Radio Observatory (HCRO), located in northeastern California near Mt Lassen (Figure 7.1), in designing and installing a continuous GPS station. The HCRO is installing the new Allen Telescope Array (ATA), which will consist of approximately 350 6.1-meter radio telescope dishes arrayed at the site, for both astrophysical and Search for Extraterrestrial Intelligence (SETI) studies. We previously assisted UC Berkeley astrophysicists in conducting an RTK survey of the HCRO site to determine the optimal locations for the 350 dishes using Trimble RTK equipment purchased for the project. After completion of the RTK survey, the base receiver was converted into the continuous station. The site is set amidst and underlain by extensive lava fields. After extensive reconnaissance of the site, we chose a monument location that is close to the main laboratory buildings, unlikely to be affected by future ATA dish placement, and on a reasonably stable lava flow. In June 2003, we assisted with the construction of a 12"-diameter concrete pier that is anchored to the lava flow outcrop. The Trimble Zephyr antenna was attached using a SCIGN adapter. We are currently establishing data acquisition procedures with the HCRO to archive the data at the NCEDC.

We also assisted Thales, Inc. (formerly Ashtech, Inc.) to establish a continuous GPS station on the roof of their Santa Clara office building. The chokering antenna is attached to a metal pin that was drilled and cemented into a corner of the roof's concrete parapet. Data is currently acquired daily by FTP from a server located at Thales, and we are investigating methods to acquire the data more rapidly using some of the TCP/IP capabilities of the recently developed Ashtech iCGRS receiver. Other agencies have also installed new continuous stations in the Bay Area, including an FAA site in Fremont (ZOA1) that will be used for Wide Area Augmentation System (WAAS) navigation control.

L1-System Profile

Figure 7.2: Location of L1-system (open triangles) and BARD (closed circles) stations. BSL, just southwest of the Hayward fault, is the location of the Berkeley Seismological Laboratory, where data from the 4 L1-system receivers northeast of the Hayward are telemetered.
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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.

Figure 7.3: Daily estimates of the north, east, and vertical components of the BRIB to BDAM 3-km baseline. Daily repeatabilities are about 1, 2, and 5 mm, respectively.
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In April 2002, we installed 4 sites in a 10-km profile extending normal to the Hayward fault between the UC Berkeley campus and BARD station BRIB (Figure 7.2). Due to the topography of the East Bay hills, each site acts as a repeater for other sites. Data from WLDC passes 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. 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).

Between April 2002 and January 2003, the L1 system operated reasonably well, although problems with faulty batteries solar power regulators caused some loss of data. The Freewave radio at the repeater site SPSC was replaced with an Intuicom system. The original radio was sent in for routine maintenance and was found to have a frequency crystal that was beyond its normal operating range. In mid-January 2003, the solar panel at GRIZ was stolen, which resulted in damage to the cables located outside of the protective metal enclosure. The replacement solar panel was installed in a steel channel frame welded to the vertical steel post that forms the monument base. A 0.5"-thick Plexiglas layer was inserted to protect the surface of the solar panel. Acquisition of all data failed not long after this repair. Initial tests suggested a problem at the repeater site SPSC, but subsequent efforts failed to resolve the problem. In August 2003 we isolated the problem to bad cable connections at the GRIZ sites and re-established operations of the network.

We are processing the data using the GAMIT/GLOBK analysis package, which required modifications to handle L1-only observations. 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 relative positions and 5 mm in the vertical on the shortest (several km) baselines can be achieved (Figure 7.3), 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.

Figure 7.4: Daily position of FARB on the Farallon Islands west of San Francisco relative to CMBB at Columbia College in the Sierran foothills. The greater scatter in the 1994-1998 interval is due primarily to reference frame effects resulting from the weaker fiducial network available at that time.
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Data Analysis and Results

Figure 7.5: Velocities relative to stable North America for the BARD stations derived from 1993-2003 data. Ellipses show 95% confidence regions, assuming white noise only. The 35 mm/yr motion between Columbia and the Farallones is primarily due to shear across the San Andreas fault system.
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We use the GAMIT/GLOBK software developed at MIT and SIO to process data from the BARD and other nearby continuous GPS networks. We have recently modified our processing strategies to take better advantage of recent enhancements to the GAMIT software and automated scripts. These improvements 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 and are reprocessing older data from the present to 1991 using improved orbits, which we expect to be completed by Fall 2003. 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.

The estimated relative baseline determinations typically have 2-4 mm long-term scatter in the horizontal components (Figure 7.4) and the 10-20 mm scatter in the vertical. Average velocities for the longest running BARD stations during 1993-2003 are shown in Figure 7.5, 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. Together with students in the department who are now using the GAMIT software to process survey-mode data in the San Francisco Bay area, we are working to combine the survey-mode and continuous GPS solutions into a self-consistent velocity field for northern California.

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 (Murray and Segall, 2001) predicts deep slip rates for the San Andreas, Hayward, and Calaveras/Concord faults are 19.3$\pm$1.8, 11.3$\pm$1.9, and 7.4$\pm$1.6 mm/yr, respectively, in good agreement with estimated geologic rates (17$\pm$4, 9$\pm$2, and 5$\pm$3 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.

Real-Time Processing

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. 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 (of 4 mm) at a BARD GPS receiver.

Acknowledgements

Mark Murray oversees the BARD program. André Basset, Bill Karavas, John Friday, Dave Rapkin, and Doug Neuhauser contribute to the operation of the BARD and L1 networks. Mark Murray and André Basset contributed to the preparation of this chapter.

References

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.

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.

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