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PBO Strainmeters: Distribution, Design and Data Products

Kathleen Hodgkinson, Greg Anderson, Mike Hasting, Bob Mueller

PBO will install and operate up to 143 three-component borehole strainmeters and five long-baseline laser strainmeters along the Pacific-North American plate boundary over the next four years. Drilling for the first borehole strainmeter, located in the Cascadia region of Washington, will start in October/November 2004 with instrumentation being installed in early 2005. Installing the first borehole station in Cascadia at that time should allow us to capture the next anticipated silent earthquake on the Cascadia subduction zone. In the second year of PBO, four new borehole strainmeters will be installed at Parkfield, 11 more in the Cascadia region, and up to four on Vancouver Island. The remaining strainmeters will be distributed so as to improve our understanding of tectonic and volcanic processes along the entire plate boundary.

Three-component borehole strainmeters (BSM) measure change in diameter of the borehole in 3 directions oriented 120 degrees apart. The 3 measurements are then combined to determine the change in areal and shear strains. The BSMs will be installed at depths of between 150 m and 240 m. Long-baseline laser strainmeters (LSM) measure change in the relative position of end monuments hundreds of meters apart using an unequal-arm Michelson interferometer. The LSMs will be installed on the surface.

Each BSM station will have a 2 Hz, 3 component, passive borehole seismometer installed above the strainmeter making the PBO BSM network the second largest borehole seismic network in the world. Each BSM station will monitor pore and atmospheric pressures, and some will monitor rainfall, temperature, wind speed, and relative humidity. Some BSM stations will also be colocated with PBO GPS stations.

The BSM data, collected at 20 samples per second (sps), will be buffered on-site and downloaded in near real time to PBO via direct Internet connections, along with environmental data. BSM data will be transferred to a central quality-checking system and then passed to the Strainmeter Analysis and Archive Centers. LSM data, collected at 1 sps, will be buffered on-site and downloaded at least daily to PBO. It will then be sent to Scripps Institution of Oceanography for analysis and to the archives for storage. Strainmeter data analysis will include: instrument calibration, removal of spikes and offsets, identification of strain induced by changes in atmospheric and pore pressure, removal of borehole relaxation trends in BSM data, and production of an earth tide model. Borehole seismic data will likely be sent to the Array Network Facility (ANF) located at Scripps for analysis, quality-checking, distribution and arching. The ANF also process and analyses data for the USArray arm of EarthScope.

PBO strain data products will be divided into 3 levels. The raw data will be considered the Level 0 product. Level 1 data will consist of clean, scaled, gauge data. The Level 2 data set will contain derived products: areal and shear strains, tidal and pressure corrections. Level 2 data will be further divided into 3 sub-levels ranging from a rapid solution produced every 24 hours to a final verified data set updated every 3 months. The data will be available in XML and SEED format from the EarthScope Data Access System. Metadata such as site information and scale factors will be retrievable from the PBO Operational Database.

Last modified: Monday, 20-Jul-2015 14:20:45 UTC


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