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: 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). The strainmeters were recently developed 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. All of the stations include pore pressure sensors and 2-component tiltmeters. Three of the stations now are equipped with Quanterra recording systems that provide 100-Hz seismic and strainmeter data, and two of the stations now include a GPS receiver. 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.
During the period July 2002-June 2003, the BSL and USGS began the installation of the broadband deformation stations at Marin Headlands (MHDL) in the Golden Gate National Recreation Area near Pt. Reyes, and at St. Vincent's School for Boys (SVIN) near San Rafael. Additional equipment installation and maintenance was performed at the first three stations, including the installation of tiltmeters at all stations, and the GPS monument and receiver at San Bruno (SBRN).
The BSL directly supervised the drilling operations at St. Vincents during the August 2002. The boreholes were drilled by the USGS Water Resources Division using a relatively new rig 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. At St. Vincents, 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.
The USGS supervised the drilling 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 8.3 shows the typical configuration of the borehole instrument installation. A 6.625" steel casing was cemented into a 10.75" hole to 500-650' depth to prevent the upper, most unconsolidated materials from collapsing into the hole. Below this depth a 6" uncased hole was drilled to the target region for the strainmeter and seismometer packages. Coring, in order to identify the region with the most compentent rock for the strainmeter, was attempted with only moderate success at a few of the holes and was not attempted at St. Vincents. We found that video logs provided a reasonable substitute. The target region of each hole 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 just above the strainmeter, on a 2" PVC pipe, and neat cement was used to fill the hole and PVC pipe to entirely enclose the package. The pipe above this depth was left open for later installation of the pore pressure sensor. 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. At each hole, the casing was then cemented inside to about 200', and outside to about 20' depth. A 12" PVC conductor casing was cemented on the outside from the surface to 20' 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 to about 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.
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 8.1 and Table 8.1), 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 broadband deformation stations. Power, telemetry, and dataloggers are currently installed at OHLN, SBRN, and SVIN. The frame relay circuit at OXMT is also installed, but the power hookup has been delayed due to permitting complications that should be resolved in Fall 2003. Permitting complications have also delayed the establishment of power and telemetry at MHDL. Our original plans and permitting to use phone line connections became prohibitively expensive, so we are currently seeking permits to establish radio telemetry from the site either to a nearby telephone pole where a frame relay circuit can be installed or from the site directly to the BSL via a radio repeater on the ridge above the station. The USGS has installed solar panels at OXMT and MHDL to collect the low-frequency strainmeter data prior to establishing DC power at the sites. Telemetry at SVIN was established in June 2003 using Wi-LAN radios, a new type of radio that the BSL is currently beginning to adopt. These radios act as ethernet bridges, providing superior access to console control on the Quanterras. The radios can also provide a spanning tree network structure for a regional wireless network, which allows greater flexibility for future network installations.
The BSL is developing an experimental GPS mount for the top of the borehole casings to create a stable, compact monument (Figure 8.4). 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 is cemented fully to the surrounding rock. Our original mount design used at OHLN, which consists of 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, was found to have too much play in the area where the bolts are attached to ensure long-term stability of the monument.
We therefore redesigned the mount to minimize such non-tectonic motions. The current GPS mount design (Figures 8.4 and 8.5) consists of two 11- inch diameter stainless steel flanges. The lower slip- and- weld type flange is welded onto the top of the 6 5/ 8"- inch borehole casing providing a level surface for the second flange . The upper blind-type flange, to which the 1 1/ 4" stainless steel pipe used to connect to the SCIGN DC3 adaptor is attached, is bolted to the lower flange using four 3/ 4" by 3" stainless steel bolts. Two half- inch stainless steel dowels are press fit with high location precision (radius 7.500" +/- 0.001" ) into the lower flange. Two matching holes are machined into the upper flange with a high location precision (radius 7.500" +/- 0.001" ) and hole diameter precision (between +0.005" and -0. 000"). One of the dowels is offset to insure unique directional alignment. This mount was installed at SBRN in March 2003, and we are preparing to install this mount at the other broadband deformation stations in Fall 2003, after rainfall lessens the fire hazards that result from the welding.
Analysis of GPS observations at OHLN and SBRN 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.
Two-component tiltmeters were installed at all the stations by the USGS in Spring 2003. Data from these sensors are recorded at 10-minute intervals and telemetered using the GOES system. Pore pressure sensors are also installed at all the stations and data are recorded at 1 Hz on the Quanterra dataloggers, except at Marin Headlands, where 10-minute interval data are also recorded on the Zeno datalogger. After the server for the pore pressure channels was initiated in Spring 2003, the Quanterra data loggers have occasionally encountered memory overwrite problems that cause them to cease operating. We believe the problem is due to the server, which Quanterra is currently investigating. We currently are running the pore pressure sensors on a trial basis on the system at Ohlone, which seems to behave more robustly than the system at San Bruno.
We are addressing minor problems at several of the stations. Highly correlated low-amplitude noise is contaminating the seismic and strain channels at the recently installed SVIN station. We are still in the process of investigating the source of this noise, which we believe is due to deficiencies in the power grid at the maintenance yard at the school where the data loggers are housed. The vertical seismic channel at OHLN also shows poor long-period characteristics compared to the other channels, and recently displayed a non-linear response to a local earthquake. The source of this problem is probably in the Quanterra electronics, which we intend to swap out in the near future. The USGS and CIW are also investigating anomalies in the strainmeter channels, including unusual steps in the SBRN instrument and a poor long-period response of one of the channels at OXMT, both of which are probably due to electical grounding problems.
We are in the initial stages of assessing the data quality of the broadband deformation instrumentation. The borehole seismic packages provide good signal to noise characteristics compared to the NHFN stations due to their relatively deep installation. The systems have the best signal to noise near their 2-Hz characteristic frequency, but typical microseismic noise around 0.1 Hz is not evident (Figure 8.6). It is possible that the microseismic noise could be resolved if the systems included a pre-amplifier. We are planning to test this at OXMT and MHDL when the power and telemetry issues at those sites are resolved. These stations currently sample at 100-Hz, so they miss some of the seismic energy at high frequencies that are observed on the 500-Hz Parkfield borehole stations.
The newly designed tensor strainmeters appear to faithfully record strain signals over a broad frequency range. During the 2 years that the strainmeter at OHLN has been providing high-frequency data, the strain has a long-term exponential signal (Figure 8.7). 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. We are currently developing techniques to automatically clean the outliers and step offsets (due usually to valve resetting operations) seen in the raw data.
At periods around 1 day, tidally induced strains are the dominant strain signal, about 3 orders of magnitude smaller than the long-term exponential signal (Figure 8.8). Since the response of the strainmeter volumes is difficult to estimate independently, theoretically predicted Earth tides are typically used to calibrate the strainmeters. Figure 8.8 shows the approximate microstrain of the OHLN strainmeter over a several month period interval, and some of the steps required to clean the data, including removing the tides and atmospheric pressure effects. The remaining signal is highly correlated with rainfall, indicating the extent that hydrologic events can affect strain.
At higher frequencies, strains due to seismic events are also evident. Figure 8.9 shows borehole strain measurements with clear seismic phases at OHLN for the M7.9 Denali Fault, Alaska earthquake on November 3, 2002. This figure also shows measurements of pore pressure, which responds to variations in volumetric strain although not necessarily in a linear fashion. Thus pore pressure provides both an independent check on the strainmeter observations and complementary information about the surrounding rock that will aid in determining the true tectonic strains. We are beginning to examine the strain data for other types of transient behavior, such as episodic creep or slow earthquake displacements.
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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