Calibration of PBO Borehole Strainmeters: Time and Frequency Dependence Abstract

abstract

  • Borehole strainmeters installed by the National Science Foundation-funded Earthscope Plate Boundary Observatory (PBO) consist of four gauges, sampled at 20 Hz, that measure extension along distinct azimuths. To compare these measurements with data from tectonic models, the coupling parameters relating elastic deformation of the strainmeter to strain in the surrounding rock must be determined. Rayleigh wave data show that the ratio of shear to areal coupling for many of the PBO borehole strainmeters is in the range 5 to 10, in contrast to the ratio of 2 for the nominal coupling parameters. However, surface waves are well recorded by some strainmeters that do not record earth tides, demonstrating that for some borehole strainmeter installations, coupling may decrease at long periods, and calibrations based on short-period signals may be inapplicable to longer term strain variations. The strainmeter response to atmospheric pressure can in principle be used to estimate frequency dependence of the strainmeter sensitivity. We investigated whether frequency dependence or time dependence is present in the response to atmospheric pressure. For some strainmeters, there is a phase difference between the strain and atmospheric pressure at periods of 10 -16 days. The atmospheric pressure coefficients typically also vary with time, which should be accounted for when using cross-spectral techniques to estimate frequency dependence of the response. Accurate characterization of the atmospheric pressure response allows smaller tectonic signals, such as aseismic slip events, to be resolved in the strainmeter data.

publication date

  • 2008

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