Introduction

Figure 8.1: Map of San Francisco Bay and San Juan Bautista area with existing borehole strainmeter and continuous GPS stations. The 5 new Mini-PBO stations (blue) complement the existing configuration of BARD GPS (red triangles) and borehole strainmeter sites (red circles) and BDSN and borehole HFN seismic stations (yellow).
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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 was 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 initiated 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 characterize broadband surface deformation. Also, ultrasensitive borehole seismometers monitor microearthquake activity related to subsurface deformation.

The project has three components: 1) the installation of broadband deformation stations in the San Francisco Bay area; 2) the installation of GPS stations in the Parkfield region; and 3) support for skeletal operations of a 5-m X-band SAR downlink facility in San Diego to collect and archive radar data, and develop an online SAR database for WInSAR users. The BSL has participated in the first two of these components. Additional details about the Parkfield GPS stations, installed in 2001 to link the BARD network in central and northern California to the SCIGN network in southern California and currently operating in real-time streaming mode with instantaneous position analysis, are provided in the BARD chapter of this report. The remainder of this chapter describes San Francisco Bay area broadband deformation station component of this project.

The broadband deformation stations augment existing instrumentation along the Hayward and San Andreas faults in the San Francisco Bay area (Figure 8.1). During July 2001 to August 2002, five boreholes were drilled and equipped with tensor strainmeters and 3-component L22 (velocity) seismometers (Table 8.1). These were the first deployments of a new type of strainmeter developed by CIW that use 3 sensing volumes placed in an annulus with 120-degree angular separation (Figure 8.2, which allows the 3-component horizontal strain tensor to be determined. All of the stations include pore pressure sensors and 2-component tiltmeters. Four of the stations now are equipped with GPS receivers recording at 1 sample per second and Quanterra recording systems that provide 100-Hz seismic and strainmeter data. The GPS antennas at these stations are mounted at the top of the borehole casings to achieve stable compact monuments. These stations complement existing Bay Area stations of the BARD continuous network, the BDSN and HFN seismic networks, and borehole dilatometers along the southern Hayward fault and in the San Juan Bautista region.

Although the EarthScope project decided not to install the CIW hydraulic strainmeter systems due to logistical considerations, many of the techniques developed during this project have been adopted for use in the Plate Boundary Observatory strainmeter network, including the compact GPS antenna mount.

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