This year was the second of the funded BARD project for the period 2007-2010. In consequence, this year, we have continued to push forward the efforts initiated in the previous year. One new site was installed (UCSF). According to our specifications, this site is collecting 1Hz data and buffering 5Hz data. The scientific efforts have focused on the processing of the high-rate GPS data in order to include the GPS solutions (static offsets and dynamic waveforms) in the existing monitoring system for the seismic activity in Northern California (magnitude determination, moment tensors, and the Elarms system). With Doug Dreger, we are about to release a slip distribution model for the 2004 seismic event based on high rate GPS waveforms only.
Of the 30 sites, ten (BRIB, CMBB, FARB, HOPB, MHCB, ORVB, PKDB, SAOB, SUTB, and YBHB) are collocated with broadband seismic stations of the BDSN with which they share continuous frame-relay telemetry to UC Berkeley. These sites use the Quanterra data loggers to store and retrieve the GPS data converted to MiniSEED format (Perin et al., 1998). The MiniSEED approach provides more robust data recovery from onsite backup on the Quanterra disks following telemetry outages.
Another five stations (SVIN, MHDL, OHLN, OXMT, and SBRN) have been installed in the last 3 years in the SFBA and along the Hayward Fault as the Berkeley part of a multi-institutional effort funded by the NSF/MRI program to improve strain monitoring in the SFBA using an integrated approach, with significant participation of the USGS/MP (Murray et al., 2002a). These stations include borehole tensor strainmeters, three-component borehole seismic velocity sensors, downhole pore pressure and tilt sensors, and GPS receivers. This project served as a prototype for the strainmeter installations planned for PBO, which faces many of the same station installation, configuration, and data retrieval issues we have addressed. Consequently, these 5 stations have received the nickname mini-PBO. From July 2001 to August 2002, five boreholes were drilled to about 200-m depth and equipped with tensor strainmeters recently developed by CIW and 3-component L22 (velocity) seismometers. For this project, we developed a self-centering GPS antenna mount for the top of the borehole casings, which are mechanically isolated from the upper few meters of the ground, to provide a stable, compact monument that allows access to the top of the borehole casing for downhole maintenance. The 5 GPS receivers were progressively installed and connected to Quanterra 4120 data loggers, which provide backup and telemetry capabilities. The completion of the last station (MHDL), located in the Marin Headlands, took longer because it required AC power, which PGE installed in December 2005. The site is operational since September 1, 2006. In addition, 10-minute interval data, which are retrieved from all the sites by the USGS via a backup GOES satellite system, show that all the sites are successfully measuring strains due to tidal effects and to local and teleseismic earthquakes (Murray et al., 2002b).
Each BSL/BARD station uses a low-multipath choke-ring antenna, most of which (except the ``mini-PBO'' ones discussed above) are mounted to a reinforced concrete pillar approximately 0.5-1.0 meter above the ground level. The reinforcing steel bars of the pillar are drilled and cemented into a 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 stations are equipped with aging Z-12 receivers, which were originally programmed to record data once every 30 s 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 have been installed at the ``mini-PBO'' stations, and two have been used to replace failing Z12s at other stations (CMBB and MODB). At these sites, the data are collected using only direct serial connections and are susceptible to data loss during telemetry outages.
There is growing interest in collecting higher rate data for a variety of applications. For example, GPS measurements can accurately track the propagation of earthquake dynamic motions both on the ground (e.g., Larson et al., 2003) and in the atmosphere (e.g., Artru et al., 2001, Ducic et al., 2003), providing complementary information to seismic observations (calibration of integrated acceleration and velocity sensor data) and estimates of earth structure (direct observation of surface wave propagation over the oceans). We started collecting 1 Hz observations at 2 stations (DIAB and MONB) in 2003. In the last year, we have progressively upgraded the telemetry to continuous 1 Hz telemetry at 3 additional stations (BRIB, HOPB, and PTRB), where the bandwidth of the existing telemetry system allowed it. At stations collocated with broadband seismic sensors, the seismic data has priority for telemetry, because it is used in the Northern California real-time earthquake notification system (see http://www.cisn.org/ncmc/) making this upgrade more difficult and in general not feasible with the current Z12 receivers because of insufficient data compression. All data collected from BARD/BSL are publicly available at the Northern California Earthquake Data Center (NCEDC; http://www.ncedc.org/ncedc/access.html#gps).
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 3.16). During the last two years 10 NETRS have been purchased via the UNAVCO purchase program. Theses receivers will help to upgrade the network to full high-rate capabilities. Three receivers are operating today (BRIB, MHDL and DIAB).
The number of continuous GPS stations in Northern California is significantly increasing with over 250 new site installations planned by 2008 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, are funded by NSF to fold operation and maintenance of about 200 existing stations, which constitute the PBO Nucleus network, into the PBO array by 2008. Two BSL-maintained stations (SUTB and MUSB) are included in the PBO Nucleus network. 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. Another 23 Northern California stations, including most of the Parkfield network, will be included in the PBO Nucleus, and we are working with UNAVCO to facilitate their transition to UNAVCO control.
The majority of the BSL BARD stations use 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 a 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.
Data from most BSL-maintained stations are collected at 15 or 30-second intervals and transmitted continuously over serial connections (Table 3.11). 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 BRIB, CMBB, DIAB, HOPB, MHCB, MHDL, MONB, OHLN, OXMT, PTRB, SBRN, SRB1, SVIN, TIBB, and UCD1 in the Bay Area, and 13 stations in the Parkfield region (all but PKDB), are now being collected at 1-second intervals. All high-rate observations collected by these stations are currently available from the NCEDC. Collecting at such high-frequency (for GPS) allows dynamic displacements due to large earthquakes to be better measured; however, this 30-fold increase in data can pose telemetry bandwidth limitations. We are planning to convert additional stations to 1-second sampling where possible during the next year. The acquisition of the 5 NETRS bundles will help to complete this project (see Subsection 5.3). In the Bay Area, we have converted stations that have sufficient bandwidth and are currently assessing bandwidth issues at other stations. Prior to the September 28, 2004 M6 Parkfield earthquake, data from the Parkfield stations were 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 response to the earthquake, we modified the procedures to download 1-second data converted to compact RINEX format at hourly intervals, which does not significantly impact the telemetry bandwidth.
As part of the activities funded by the USGS through the BARD network, the NCEDC has established an archive of the 7000+ survey-mode occupations collected by the USGS since 1992. The NCEDC continues to archive non-continuous survey GPS data. The initial dataset archived is the survey GPS data collected by the USGS Menlo Park for northern California and other locations. The NCEDC is the principal archive for this dataset. Significant quality control efforts were implemented by the NCEDC (Romanowicz et al., 1994) to ensure that the raw data, scanned site log sheets, and RINEX data are archived for each survey. All of the USGS MP GPS data has been transferred to the NCEDC, and virtually all of the data from 1992 to the present has been archived and is available for distribution. We are also archiving additional high-precision GPS data sets from Northern California (mainly Parkfield measurements). Together with graduate 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 C-GPS solutions into a self-consistent velocity field for Northern California. The campaign velocity field computed from campaign measurements by UCB and USGS groups has been published by d'Alessio et al., (2005).
Data from five of our sites (HOPB, MHCB, CMBB, OHLN, and YBHB) are sent to the National Geodetic Survey (NGS) in the framework of the CORS (Continuous Operating Reference Stations) project (http://www.ngs.noaa.gov/CORS/). The data from these five sites are also distributed to the public through the CORS ftp site.
Permit requests: The permit releases of the sites PTP1 and BDM have encountered administrative delays during the year. However, an agreement has been reached, and the permits are currently at UC Berkeley to be signed by the Real Estate office.The installation of these sites should be completed during the next year.A new site has been installed on the roof top of a UCSF building. This site, as well as the other NETRS site is transmitting real-time 1Hz data and is recording on-site 5Hz data available in case of emergency.
Real-Time Kinematic (RTK) service: In the framework of the collaboration with EBPARK, BSL is distributing RTK corrections for some sites. This experimental project aims at developing collaborations with private users or local institutions in Northern California. We hope to densify the network and reduce monumentation and telemetry costs associated with the installation and operation of new sites. This year we have been encountering issues related to the Trimble user of our RTK network. After a series of tests, we requested assistance from the local reseller to complete a reliable connection between one of our NETRS and the Trimble remote device.
Meteorological Sensors and troposphere: In February 2008, we received funding to install three meteorological sensors at GPS sites, in collaboration with UC Riverside. Two of these sensors will be installed at the site SBRN.
SBRN move: The site SBRN has been damaged 3 times this year. The staff decided to move this site to a safer location near the existing one. For a period of time, two receivers will be operated simultaneously in order to check the stability of the current site. It is likely the new site will not keep the code SBRN.
Replacement of CHAB site: We are looking for a new site to replace the existing site CHAB. This 1992 installation is of foremost importance due to its proximity to the Hayward Fault; however, it might be difficult to restore if service is discontinued.
The BARD dataset has been processed in the ITRF2000 (Altamimi et al., 2002). The solutions (Houlié and Romanowicz, in prep) are in good agreement with campaign solutions (BAVU and USGS) previously released (d'Alessio et al., 2005). The new coordinates release for the BARD network includes currently operating sites and velocities for the sites transferred from BSL to PBO during the last two years.
All the BARD sites have been processed jointly with IGS sites in California. No a priori constraints have been assumed during the processing. All the velocities included in the first release of California Reference Frame (CALREF) are given in Table 3.12. The CALREF will provide velocities and coordinates of sites located in the Bay Area at specific epochs. Each solution will be associated with error estimations (formal and real). Every surveyor will be able to control the reference site coordinates for a given survey.
The CALREF processing is being reprocessed to integrate the meteorological data collected at some sites during the last decade.
In addition to the daily and long-term processing, we developed an additional channel of processing dedicated to the quick solutions. This processing focuses on locating 4 sites in order to provide quick solution offsets in less than 10 minutes. These offsets will thus be available for use in the local moment tensor inversions.
A series of products are released on the new BARD website (http://seismo.berkeley.edu/bard/). The list of products released covers various domains (from time-series to daily troposphere maps) that can potentially benefit from GPS data and will encourage collaboration with BSL researchers and others. All products are updated daily.
Preliminary work completed during the previous year led to a project funded by the Southern California Earthquake Center (SCEC). We are processing years 2006, 2007, and 2008 of the GPS data for the San Gabriel Valley and extending this study to the east. This year, we will apply similar processing to the BARD network. Two meteorological packages will be operated at SBRN during August 2008 and likely at BRIB during the year 2008/2009. These sensors (Figure 3.20) were designed by John Friday. We hope to minimize the effect of the troposphere on BARD solutions and improve their repeatability.
This year, we have focused our efforts on the high-rate data and their application to seismological products. We have produced high-rate GPS time-series. In collaboration with Doug Dreger, a slip distribution model based on GPS waveforms only has been computed. This slip model was presented at the AGU Fall Meeting 2007. We confirm that the GPS waveforms can be used to constrain fault slip model for large seismic events ().
Barbara Romanowicz oversees the BARD program. Nicolas Houlié, Rich Clymer, Bill Karavas, Rick Lellinger, John Friday, and Doug Neuhauser contributed to the operation of the BARD network. The operation of the BARD network is partially supported by funding from the USGS/NEHRP program (grant number 07HQAG0031) and funding from the NSF/UNAVCO PBO nucleus grant (number 0453975-09).
Altamimi, Z., P. Sillard, and C. Boucher , ITRF2000: A new release of the International Terrestrial Reference Frame for earth science applications, J. Geophys. Res., 107(B10), 2214, doi:10.1029/2001JB000561, 2002
Artru, J., P., Lognonné and E., Blanc, Normal modes modeling of post-seismic ionospheric oscillations, Geophys. Res. Lett., 28, 697-700, 2001
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., 110, B06403, doi:10.1029/2004JB003496, 2005.
Ducic, V., J. Artru and P. Lognonné, Ionospheric remote sensing of the Denali Earthquake Rayleigh surface waves, Geophys. Res. Lett., 30, 18, 2003
Houlié, N. and Romanowicz, B., CALREF, a stable reference frame for the Northern California, submitted.
Larson, Using 1-Hz GPS Data to Measure Deformations Caused by the Denali Fault Earthquake, Science, 300, 1421-1424, 2003
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.
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.
Romanowicz, B., B. Bogaert, D. Neuhauser, and D. Oppenheimer, Accessing northern California earthquake data via Internet, EOS Trans. AGU, 75, 257-260, 1994.
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