The Plate Boundary Observatory Borehole Strainmeter Network
Meghan Miller and Mike Jackson UNAVCO
Drilling PBO borehole B005 near Port Angeles, WA
One of the objectives of NSF funded geodetic component of Earthscope, the Plate Boundary Observatory, is to capture the continuous three-dimensional deformation field across the Western United States plate boundary. To accomplish this, the observatory consists of a suite of instruments which when combined have the ability to measure strain transients with deformation rates of milliseconds out to decades. The observatory, installed by UNAVCO, was completed in October 2008 and consists of over 1100 permanent GPS stations and 74 borehole strainmeter installations. Borehole strainmeters were included because they are designed to record nanostrain-level signals over periods of hours to weeks. They therefore provide an improved ability to record the evolution of strain transients such as those associated with Episodic Tremor and Slip (ETS) events and slow fault slip. DOSECC drilled the first six PBO boreholes, B001, B003, B004, B005, B006, and B007, on the Pacific Northwest Olympic Peninsula. Strainmeters installed in those holes have successfully recorded ETS related strain transients in 2005, 2007 and 2008.
To get the best performance strainmeters must be isolated from thermal and human induced tectonic strains and to record tectonic strain, it is essential they be well coupled to the surrounding rock. Hence, PBO strainmeters were installed in steel-cased boreholes at depths of up to 800 feet. The instruments are installed below the cased section ideally in an unfractured 8-12 foot section of rock. Samples and cutting were collected every 20 feet during drilling to identify the change in lithology and hydrology with depth. When the target depth was reached the boreholes were logged with a full wave sonic tool, an acoustic televiewer, a natural gamma tool and calipers. The DOSECC holes were then cored to determine whether or not the installation zone consisted of competent enough rock to host a strainmeter. Once a strainmeter was installed additional instruments such as a seismometer and in some cases pore pressure sensors were installed it. The holes were then backfilled with cement. In the Pacific Northwest it took about 3 days to complete the coring of a hole, extracting about 30 feet of core each day at a rate of about 1.5 inches per minute. Information on how to access the core from each site is available from UNAVCO. Links to logging information, photos of drilling and the installations plus strain data recorded in each of the DOSECC drilled holes can be found at http://pboweb.unavco.org/strain_data.
Schematic drawing of PBO borehole B004 at Hoko Falls, WA. The strainmeter is installed at the bottom of the borehole in a bath of grout. When the grout cures the strainmeter is bonded to the surrounding rock. Most PBO boreholes include a seismometer installed above the strainmeter and several include pore pressure sensors. After the equipment is installed the hole is backfilled with cement.
List of 2007/2008 publications using data from DOSECC drilled holes:
Seismological Society of America 2007 Annual Meeting, Hawaii, April 11-13 2007:
Hodgkinson, K.M., Anderson, G., Jackson, M., Mencin, D., and Johnson, W. Plate boundary Observatory Borehole Strainmeters: Results from the Pacific Northwest, Parkfield, and ANZA.
Roeloffs, E. Monitoring Aseismic Deformation in Northern Cascadia with Plate Boundary Observatory Borehole Strainmeters.
Schmidt, D.A., Observations of the Recent Slow Slip Events in Cascadia Captured by the PBO Strainmeters.
Hasting, M.A., Johnson, W.C., Venator, S.C., Dittman, S.T., Stair, J.M., Tiedeman, A.P., Gottlieb, M.H., Stroeve, A.R., Mencin, D.J., and Jackson, M. The Deployment of a PBO Strainmeter Site: Four Steps to a Better Understanding of the Earth.
Fall AGU 2007, San Francisco, 10–14 December 2007:
Roeloffs, E., McCausland, W. Areal and Shear Strain Coupling of PBO Borehole Strainmeters From Teleseismic Surface Waves.
Hodgkinson, K., Anderson, G., Dittmann, T., Henderson, B., Jackson, M., Johnson, W., Matykiewicz, J., Mencin, D., Wright, J. PBO Borehole Strainmeters: Bridging the Gap Between Seismology and GPS.
McCausland, W., Roeloffs, E.,Detection of Slow Slip Events Along the Cascadia Subduction Zone Using Plate Boundary Observatory Borehole Strainmeters.
Dragert, H., Wang, K. Observing Episodic Slow Slip with PBO Borehole Strainmeters along the Northern Cascadia Margin.
UNAVCO Science Meeting, March 11-13, 2008, Boulder Colorado:
Roeloffs, E, McCausland, W. Calibration of PBO Borehole Strainmeters: Time and Frequency Dependence.
Seismological Society of America 2008 Annual Meeting, 16 – 18 April 2008, Santa Fe, New Mexico:
McCausland, W. Scanning for strain transients along the Cascadia subduction zone using Plate Boundary Observatory borehole strainmeters.