Calibrated Plate Boundary Observatory Borehole Strainmeter Data

The Plate Boundary Observatory (PBO), funded by NSF as part of the Earthscope program and installed and maintained by UNAVCO, includes 75 borehole strainmeters (BSMs), which makes it one of the largest strainmeter arrays in the world. Co-located with seismometers, and embedded within the PBO continuous GPS network, the strainmeters expand the bandwidth of the Observatory enabling the capture of signals with periods of days to minutes. Six years after installation of the first strainmeter, over 70% of the network is in compression and over 85% of the instruments have a strong signal to noise ratio in the M2 tidal band. UNAVCO's BSM engineers ensure the network usually collects over 95% percent of possible data. UNAVCO makes the BSM Level 0 (raw) and Level 2 (processed) data products available to the community via the Northern California Earthquake Data Center (NCEDC), the IRIS DMC and UNAVCO's own web site. Processed BSM data includes gauge, areal, differential and tensor shear strains plus data edits and time series corrections for barometric pressure, earth tides, ocean load and borehole trends. Before strain data can be incorporated into a geophysical model with confidence, however, an instrument response must be found that relates the gauge measurements to the regional strains (i.e., an in-situ calibration is needed). In this presentation we describe the method UNAVCO will use to calibrate PBO strainmeters using earth tides as a reference signal and assuming an anisotropic instrument setting. The calibrated data will be released in a simple delimited ASCII format and will be included with the processed data set that is currently updated every 24 hours. In addition to the 5-minute Level 2 data set, UNAVCO will include the calibrated areal and shear strains at 1-sps for significant events anywhere in the world as part of its Special Event series. In order to meet Earthscope goals of data transparency and processing repeatability, the expanded processed data sets will include a summary of the calibration method, tidal observations, predictions upon which the calibrations are based and the strain matrix used to generate the areal and shear data. This presentation will also describe the new file naming convention that will allow the user to 1.) select a preferred calibration method for their data and 2.) allow UNAVCO the flexibility of including new methods of calibration in the future.