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Plate Boundary Deformation Project

Subsections

Introduction

The Integrated Instrumentation Program for Broadband Observations of Plate Boundary Deformation, commonly referred to as ``Mini-PBO'', is a joint project of the BSL, the Department of Terrestrial Magnetism at Carnegie Institution of Washington (CIW), the IGPP at UC San Diego (UCSD), and the U.S. Geological Survey (USGS) at Menlo Park, Calif. It augments existing infrastructure in central California to form an integrated pilot system of instrumentation for the study of plate boundary deformation, with special emphasis on its relation to earthquakes. This project is partially funded through the EAR NSF/IF program with matching funds from the participating institutions and the Southern California Integrated Geodetic Network (SCIGN).

Because the time scales for plate boundary deformation range over at least 8 orders of magnitude, from seconds to decades, no single technique is adequate. We have proposed an integrated approach that makes use of three complementary and mature geodetic technologies: continuous GPS, borehole tensor strainmeters, and interferometric synthetic aperture radar (InSAR), to provide a broadband characterization of surface deformation. In addition, ultrasensitive borehole seismometers will monitor microearthquake activity related to subsurface deformation.

The project has three components. One is focused on the San Francisco Bay area, and augments existing instrumentation along the Hayward and San Andreas faults. Two borehole sites were drilled during the summer of 2001, at Ohlone Park (OHLN) in Hercules and on San Bruno mountain (SBRN) near Brisbane, to complement 6 existing ones (Figure 7.1). These holes are equipped with newly fabricated borehole tensor strainmeters and seismometers, and downhole pore pressure and tilt sensors will be added in the near future. One site in the Marin Headlands (MHDL) is scheduled to be drilled before the end of September 2001, and two additional sites will be drilled by the end of 2001, at the Knox/Miller Park (KNOX) near Richmond and on Ox Mountain (OXMT) near Half Moon Bay. The first two instrumented sites are also awaiting installation of electronics, GPS receivers, and Quanterra recording systems.

The GPS antennas will be mounted at the top of the borehole casings, wherever possible, in an experimental approach to achieve stable compact monuments (Figure 7.2). The GPS stations will complement 18 existing Bay Area stations of the Bay Area Regional Deformation (BARD) continuous GPS network, which includes more than 50 stations in northern California. The data from the GPS, strainmeters, and seismometers will be acquired on Quanterra data loggers and continuously telemetered over frame relay to U.C. Berkeley, while data from other low frequency sensors will be telemetered using the GOES system to the USGS. Sampling rates are 100 Hz for strainmeters and seismometers, 1 Hz atmospheric, and 30 second GPS through the Quanterra data loggers, and 600 second for low frequency data (including strainmeters, for redundancy) over the GOES system.

All data will be made available to the community through the Northern California Earthquake Data Center (NCEDC). In preparation for this, the BSL and USGS have worked out procedures to archive data from 139 sites of the USGS ultra-low-frequency (UL) geophysical network, including data from strainmeters, tiltmeters, creep meters, magnetometers, and water well levels. These data are available in SEED format and set the stage for the archiving and distribution of data from the Mini-PBO stations.

The second component of this project is to link the BARD network in central and northern California to the SCIGN network in southern California. The distribution of these sites allows measurement of both near-field deformation from fault slip on the San Andreas and regional strain accumulation from far-field stations. As of September 2001, seven new continuous GPS sites are operational (6 constructed through Mini-PBO and one by USGS) and two more are under construction. The sites are located in an array centered in the Parkfield area and span about 25 km on either side of the San Andreas Fault. Thus, the new array augments the considerable geophysical instrumentation already deployed in the area and contributes to the deep borehole drilling on the San Andreas fault (SAFOD) component of Earthscope. The data are currently downloaded daily by SCIGN and archived by SOPAC. These sites will eventually be upgraded to real-time streaming and analyzed in instantaneous positioning mode.

The third component is InSAR, which supports skeleton operations of a 5-m X-band SAR downlink facility in San Diego to collect and archive radar. In addition, some funds are used to develop an online SAR database for WInSAR users. The ERS-1/2 SAR data, which extend from 1992 until present, offer the only means for monitoring plate boundary deformation at high spatial resolution over all of western North America. This data set is largely unexplored mainly because data distribution is restricted by ESA and working with the phase information requires a significant investment of a researchers time. Our objective is to improve access to these data for plate boundary research within the strict guidelines set by ESA.

Figure 7.1: Location of existing (yellow) and planned (blue) Mini-PBO sites in the San Francisco Bay area. Shown also (red) are currently operating strainmeter (circles) and BARD (triangles) stations in the area.
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\epsfig{file=pbd01_1.ps,bbllx=72, bblly=72,bburx=503,bbury=679, width=15cm}\end{center}\end{figure*}

Siting and Hardware Issues

This year, the BSL has focused primarily on site selection for the integrated network in the Bay Area, in cooperation with the USGS. Figure 7.1 shows the planned configuration. Two borehole sites were drilled during the summer of 2001, at Ohlone Park (OHLN) in Hercules and on San Bruno mountain (SBRN) near Brisbane. These holes are equipped with newly fabricated borehole tensor strainmeters and seismometers, and downhole pore pressure and tilt sensors will be added in the near future. One site in the Marin Headlands (MHDL) is scheduled to be drilled before the end of September 2001, and two additional sites will be drilled by the end of 2001, at the Knox/Miller Park (KNOX) near Richmond and on Ox Mountain (OXMT) near Half Moon Bay. The first two instrumented sites are also awaiting installation of power, telemetry, electronics, GPS receivers, and Quanterra 330 recording systems.

Permitting of five additional sites for 2002 installations are in the preliminary stages. These include: sites on U.S. Coast Guard property on Yerba Buena Island in the San Francisco Bay (YERB) and near the former Hamilton Field military base in Marin (HAML), a site at St. Marys College in the East Bay (STMC), and two sites in the south Bay in the Rancho San Antonio Open Space Preserve (RSAN) and Castle Rock State Park (CSTR). Site evaluation of geologic properties, sky visibility, ease of permitting, and power and telemetry, have been performed at most of the sites. We are continuing to evaluate the Yerba Buena site where a location suitable for both GPS and strainmeters has proven difficult to find.

After careful review and testing of several current generation GPS receivers, we have decided to purchase Ashtech MicroZ receivers, which use about half the power of the Ashtech Z-12 receivers currently used in the BARD network. We are also designing an experimental GPS mount for the top of the borehole casings to create a stable, compact monument (Figure 7.3). The antennas, using standard SCIGN adapters and domes for protection, will be attached to the top of the 6-inch metal casing, which will be mechanically isolated from the upper few meters of the ground. The casing below this level will be cemented fully to the surrounding rock. Although this design takes advantage of the deep anchoring of the casing, we will need to assess in the future whether other effects, such as daily or annual thermal expansion of the upper few meters of the casing, limit the long-term stability of the monument.

Acknowledgements

This project is sponsored by the National Science Foundation under the Major Research Instrumentation (MRI) program with matching funds from the participating institutions and the Southern California Earthquake Center (SCEC).

Under Barbara Romanowicz's general supervision, and with Mark Murray as head guru, André Basset, Bill Karavas, John Friday, Dave Rapkin, Doug Neuhauser, Tom McEvilly, Rich Clymer, and Roland Bürgmann contribute to the development of the BSL component of the Mini-PBO project. Mark Murray and Barbara Romanowicz contributed to the preparation of this chapter.

Figure 7.2: Design of the Mini-PBO borehole installation, showing the emplacement of the instruments downhole and the GPS receiver on the top.
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Figure 7.3: Design of the Mini-PBO GPS antenna mount on top of casing.
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\epsfig{file=pbd01_3.ps, width=8cm}\end{center}\end{figure*}


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