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Plate Boundary Deformation Project

Subsections

Introduction

Figure 9.1: Location of existing (red), in preparation (yellow), and pending (blue) Mini-PBO sites in the San Francisco Bay area. Shown also (red) are currently operating strainmeter (circles) and BARD (triangles) stations. Blue triangles are other pending BARD stations. Black triangles are L1-system profile sites near the Hayward fault and the UC Berkeley campus.
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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 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 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. The first augments existing instrumentation along the Hayward and San Andreas faults in the San Francisco Bay area (Figure 9.1). During July 2001 to August 2002, five boreholes were drilled and equipped with tensor strainmeters and 3-component L22 (velocity) seismometers (Table 9.1). The strainmeters were recently developed by by CIW and use 3 sensing volumes placed in an annulus with 120 degree angular separation, which allows the 3-component horizontal strain tensor to be determined. One borehole station has also been equipped with a GPS receiver, Quanterra recording system, and downhole pore pressure sensor, and will eventully also include a tilt sensor. The other stations are in various stages of completion, primarily waiting for power and telemetry to be established. The GPS antennas at these stations are mounted at the top of the borehole casings in an experimental approach to achieve stable compact monuments. The GPS stations complement existing Bay Area stations of the BARD continuous network.

The 30-second GPS, and 100-Hz strainmeter and seismometer data is acquired on Quanterra data loggers and continuously telemetered by frame relay to the BSL. Low frequency (600 second) data (including strainmeters, for redundancy) is telemetered using the GOES system to the USGS. All data is available to the community through the Northern California Earthquake Data Center (NCEDC) in SEED format, using procedures developed by the BSL and USGS to archive similar 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.

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. During Summer 2001, nine new continuous GPS sites were installed (see Table 8.2) in the Parkfield area spanning about 25 km on either side of the San Andreas fault. One of the receivers was contributed by the USGS and the other eight were contributed by SCIGN, while the braced monuments for all the sites were constructed using Mini-PBO funding. 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. The NCEDC is currently assuming the responsibility for retrieving the data from these sites over their existing frame relay circuit at Parkfield. A subset of these sites will eventually be upgraded to real-time streaming and analyzed in instantaneous positioning mode.

The third component is InSAR, which supports 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 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 the time consuming nature of processing phase information. Our objective is to improve access to these data for plate boundary research within the strict guidelines set by ESA.

New Site Installations


Table 9.1: Currently operating and planned stations of the Mini-PBO network. Strainmeter installation date is given. Depth to tensor strainmeter and 3-component seismometers in feet.
Code Latitude Longitude Installed Strainmeter Seismometer Location
        depth (ft) depth (ft)  
OHLN 38.00742 -122.27371 2001/07/16 670.5 645.5 Ohlone Park, Hercules
SBRN 37.68562 -122.41127 2001/08/06 551.5 530.0 San Bruno Mtn. State Park, Brisbane
OXMT 37.49795 -122.42488 2002/02/06 662.7 637.3 Ox Mtn., Half Moon Bay
MHDL 37.84227 -122.49374 2002/08/06 520.6 489.2 Golden Gate Nat. Rec. Area, Sausalito
SVIN 38.03325 -122.52638 2002/08/29 527.0 500.0 St. Vincent CYO School, San Rafael
SMCB 37.83881 -122.11159       St. Mary's College, Moraga
WDCB 38.24088 -122.49628       Wildcat Mt., Sears Pt.


During this last year, the BSL and USGS installed the first Mini-PBO stations. Boreholes were drilled by the USGS Water Resources Division crew at five sites. The drillers used a newly purchased rig (Figure 9.2) that experienced numerous problems (hydraulics, stuck bits, etc.), which delayed the drilling considerably at several of the sites and significantly increased the costs of the project.

Figure 9.3 shows the configuration of the borehole instrument installation at the first site at Ohlone Park in Hercules (OHLN). A 6.625" steel casing was cemented into a 10.75" hole to 625'4" depth to prevent the upper, most unconsolidated materials from collapsing into the hole. Below this depth a 6" uncased hole was drilled to 676'. Coring was attempted with moderate success below 540' through poorly consolidated mudstone to about 570', and increasingly competent sandstone below. Moderately good core was obtained from 655' to 669', so this region was selected for the strainmeter installation. The section of the hole below about 645' was filled with a non-shrink grout into which the strainmeter was lowered, allowing the grout to completely fill the inner cavity of the strainmeter within the annulus formed by the sensing volumes to ensure good coupling to the surrounding rock.

The 3-component seismometer package was then lowered to 645.5', just above the strainmeter, on a 2" PVC pipe, and neat cement was used to fill the hole and PVC pipe to 565'. The pipe above this depth was left open for later installation of the pore pressure sensor in the 520-540' region. To allow water to circulate into the pipe from the surrounding rock for the pore pressure measurements, the the steel casing was perforated, a sand/gravel pack was emplaced, and a PVC screen was used at this depth. The casing was then cemented inside to 192', and outside to 16' depth. A 12" PVC conductor casing was cemented on the outside from the surface to 16' to stabilize the hole for drilling and to provide an environmental health seal for shallow groundwater flow. The annulus between the 12" conductor casing and the 6.625" steel casing was cemented from 16' to 10' depth and above was left decoupled from the upper surface to help minimize monument instability for the GPS antenna mounted on top of the steel casing.

The drilling procedures and hole instrument configuration were similar at the other four sites. At San Bruno Mt (SBRN) near Brisbane, the hole was drilled to 550' with good core through competent graywacke below 520'. Reaming of the bottom hole from the 4" core diameter to 6" was delayed considerably for retrieval of the reaming bit that broke and got lodged in the bottom of the hole. At Scarper's Ridge (OXMT) near Half Moon Bay, the hole was drilled to 712.7' depth with coring attempted below 653' through granite. Because core was poorly recovered at the lowest depths due to inadequacies with the coring system that broke up the rock, a slightly shallower depth of 660' was chosen for the strainmeter installation.

At the Marin Headlands (MHDL) site, the drilling in October 2001 encountered hard greenstone with some fractures and clay layers between 410-608' and red and green chert below to 659'. Coring at around 545' was slow and poorly recovered. A video log of the hole showed several promising strainmeter installation regions at 500-550' depths. However, containment of high volumes of artesianing fluids from the well became increasing problematic. The hole was cased to 278', sand filled on the bottom, and cemented and plugged at the top in mid-October. In August 2002, the cement and sand were rapidly drilled out, without any artesianing problems, allowing the strainmeter and seismometer packages to be successfully installed.

Figure 9.2: USGS Water Resources Division rig used to drill the Mini-PBO boreholes at the St. Vincents site.
drill rig

Figure 9.3: The Mini-PBO borehole configuration at Ohlone, showing the emplacement of the strainmeter and seismometer instruments downhole. The GPS receiver is mounted on the top. Figure courtesy B. Mueller (USGS).
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At the St Vincents (SVIN) site near San Rafael, drilling in August 2002 also entailed no shortage of problems. The first hole had to be abandoned after some tungsten grinding buttons from a defective bit dislodged and could not be retrieved from the bottom of the hole. Hammer drilling through the very hard graywacke encountered throughout the hole also proved difficult due to the lack of proper stabilization on the drill string. Rotary drilling, although relatively slow, enabled penetration to 528' in the limited time available. A video log showed a promising region devoid of open fractures near the bottom of the hole where the strainmeter and seismometer packages were installed without any further difficulties.

Due to the unexpectedly high costs of drilling, only 5 boreholes could be completed under the NSF/IF grant, although additional instrumentation was purchased in anticipation of acquiring more sites. Caltrans intends to drill boreholes at several locations for the HFN project in the coming year that might be suitable for Mini-PBO installations, depending on the quality of the rock encountered at about 600' depth. Two of the already permitted potential sites, St. Mary's College (SMCB) and Wildcat Mt. (WDCB) (Figure 9.1 and Table 9.1), would nicely complement existing instrumentation, providing additional monitoring of the northern Hayward fault and initiating monitoring of the southern Rodgers Creek fault north of San Pablo bay.

The BSL is supervising GPS, power, frame relay telemetry, and Quanterra 4120 datalogger installation at all the Mini-PBO stations. Power, telemetry, and dataloggers are currently installed at OHLN and SBRN. The frame relay circuit at MHDL is also installed, but the power hookup has been delayed due to permitting complications that should be resolved in Fall 2002. We are currently establishing power and telemetry at the most recently installed MHDL and SVIN stations. The USGS has installed solar panels at OXMT, and soon at MHDL and SVIN, to collect the low-frequency strainmeter data prior to establishing DC power at the sites.

The BSL is developing an experimental GPS mount for the top of the borehole casings to create a stable, compact monument (Figure 9.4). The antennas, using standard SCIGN adapters and domes for protection, are 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. We have installed a GPS antenna at OHLN (Figure 9.5). The antenna is attached to a metal pipe symmetrically centered with respect to the casing that is welded to a cross beam and bolted inside the top of the casing, which allows access through the top of the casing to the 2" pipe for heat flow measurements. A similar mount was constructed at OXMT, but it was found to have too much play in the area where the bolts are attached to ensure long-term stability of the monument. We are currently redesigning the mount to minimize such non-tectonic motions. Preliminary analysis of 100 days of the GPS observations at OHLN shows that the short-term daily repeatabilities in the horizontal components are about 0.5-1 mm. These values are similar to those obtained with more typical monuments, such as concrete piers or braced monuments, but it is too early to assess the long-term stability of the borehole casing monument, which might also be affected by annual thermal expansion effects on the casing.

Figure 9.4: Design of the Mini-PBO GPS antenna mount on top of casing.
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Strainmeter Data

We are in the initial stages of assessing the data quality of the Mini-PBO instrumentation. The newly designed tensor strainmeters appear to faithfully record strain signals over a broad frequency range. During the 9 months that the strainmeter at OHLN has been providing high-frequency data, the strain has been exponentially decaying (top, Figure 9.6). This large signal is most likely due to cement hardening effects and re-equilibration of stresses in the surrounding rock in response to the sudden appearance of the borehole. These effects can last for many years and are the principal reason that borehole strainmeters can not reliably measure strain at periods greater than a few months.

Figure 9.5: GPS antenna mounted on top of casing at OHLN. The final installation includes a SCIGN antenna dome and a steel protective shroud that envelopes the casing.
GPS antenna photo

At periods around 1 day, tidally induced strains are the dominant strain signal, about 3 orders of magnitude smaller than the long-term decay signal (bottom, Figure 9.6). Since the response of the strainmeter volumes is difficult to estimate independently, theoretically predicted Earth tides are typically used to calibrate the strainmeters. Figure 9.7 shows the calibrated signals in microstrain of the OHLN strainmeter over a several day interval.

At higher frequencies, strains due to seismic events are also evident. Figure 9.8 shows a comparison of the OHLN vertical velocity seismometer and one component of the strainmeter for an M=2 event that occurred within 15 km of the station. Strains from this event are about an order of magnitude smaller than the tidal strains. We are beginning to examine the strain data for other types of transient behavior, such as episodic creep or slow earthquake displacements.

Figure 9.6: 100-second strainmeter data measured by Component 1 at OHLN, in instrumental counts. Top, 9-month timeseries with instrumental offsets due to reservoir resetting removed. Bottom, 1-month timeseries bandpass filtered at 0.5-2 day to show tidal strain signals. Note the different vertical scales.
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Figure 9.7: Three-component strainmeter data measured at OHLN, in microstrain. Tidal strain is used to calibrate the sensors, allowing instrument counts to be converted to microstrain. Figure courtesy M. Johnston (USGS).
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Figure 9.8: 100-Hz strainmeter and seismometer data measured at OHLN, in instrumental counts, showing response to a M=2.3 May 2002 earthquake within 15 km of the station. Seismometer is vertical component. Strainmeter is Component 1.
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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 Mark Murray's supervision, André Basset, Bill Karavas, John Friday, Dave Rapkin, Doug Neuhauser, Tom McEvilly, Wade Johnson, and Rich Clymer have contributed to the development of the BSL component of the Mini-PBO project. Several USGS colleagues, especially Malcolm Johnston, Bob Mueller, and Doug Myren, played critical roles in the drilling and instrument installation phases. Mark Murray and Barbara Romanowicz contributed to the preparation of this chapter.



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