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Bay Area Regional Deformation Network

Subsections

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., 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 $\sim $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.

Figure 8.1: Operational BARD stations (solid triangles) 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 use continuous telemetry. The 18 station Long Valley Caldera (LVC) network and 15 station Parkfield (PKFD) networks are also part of BARD. Other nearby networks (open triangles) include: Basin and Range (BARGEN), and Southern California Integrated GPS Network (SCIGN).
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\epsfig{file=bard02_1.ps, width=15cm}\end{center}\end{figure*}

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.


Table 8.1: 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 4.2 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 Stat., 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-Z12 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
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
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
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



Table 8.2: 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 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 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.

BARD Stations

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.

2001-2002 Activities

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.

Station Maintenance

In March 2002, ``copper-miners" took advantage of of the poor security at the decommissioned Point Molates naval facility to fell the power poles and remove high tension copper power lines that were used by the MOLA station. The property has been put aside for environmental cleanup before the ownership is transferred from the Navy to the City of Richmond. Due to the status of the property, the high costs to reestablish power, and the unsecured nature of the area, the station was removed from continuous GPS service. The monument and enclosure were left intact and the site is being be periodically reoccupied, approximately 2-3 days per month, in a semi-permanent mode.

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.

New Installations

Mini-PBO sites

In the summer and fall of 2001, we helped to do site reconnaissance, permitting, and installations for the NSF-funded mini-PBO project establishing collocated GPS/seismometer/borehole strainmeter observatories in the Bay Area. Boreholes were drilled to around 200 m at 4 sites, and strainmeter and seismometer instruments were installed in 3 of the boreholes (the fourth proved problematic and could not be completed until August 2002). Due to delays by utility companies in establishing power and telemetry, only the station OHLN at Ohlone Park in Hercules, about 5 km west of the northern Hayward fault, became fully operational. The GPS antenna is mounted on the top of the 6" borehole casing, in an experimental approach to obtain a stable, compact monument. GPS data from OHLN is now publicly available from the NCEDC, and included in the automated BARD processing for hourly and daily site positions. For more details about the OHLN station installation, and the other mini-PBO sites, see Chapter 9).

L1-system Profile

Figure 8.2: 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.
\begin{figure}\begin{center}
\epsfig{file=bard02_2.ps, width=9cm}\end{center}\end{figure}

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.

Figure 8.3: L1 system installation at GRIZ. This autonomous system uses solar panels (mounted in back) and Freewave radios (with Yagi antenna mounded on pole below GPS antenna). GPS receiver, radio, solar power regulators, and backup batteries are in the electronics box mounted in front.
\begin{figure}\begin{center}
\epsfig{file=bard02_2.ps, width=9cm}\end{center}\end{figure}

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.

Figure 8.4: Daily estimates of the north and east components of the BRIB to BDAM 3-km baseline. Daily repeatabilities are about 1 mm in north, and 2 mm in east.
\begin{figure}\begin{center}
\epsfig{file=bard02_4.ps, width=9cm}\end{center}\end{figure}

Data Analysis and Results

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.

Figure 8.5: Velocities relative to stable North America for the BARD stations and other stations operated in nearby networks. Data from November 1993 to July 2000 was processed by the BSL using GAMIT software. Ellipses show 95% confidence regions, assuming white noise and $1 mm/\sqrt {yr}$ random-walk noise, with the predicted Pacific-North America relative plate motion in central California shown for scale.
\begin{figure*}\begin{center}
\epsfig{file=bard02_5.ps, width=10cm}\end{center}\end{figure*}

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 $1 mm/\sqrt {yr}$ 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 19.3$\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 also developing real-time analysis techniques that will enable rapid determinations ($\sim $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.

Acknowledgements

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.

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.

Kogan, M. G., G. M. Steblov, R. W. King, T. A. Herring, D. I. Frolov, S. G. Egorov, V. Y. Levin, A. Lerner-Lam, A. Jones, Geodetic constraints on the rigidity and relative motion of Eurasia and North America, Geophys. Res. Lett., 27, 2041-2044, 2000.

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, 1998a.

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