HR: 11:20h
AN: G22A-05 INVITED    [Abstracts]
TI: Areal and Shear Strain Coupling of PBO Borehole Strainmeters From Teleseismic Surface Waves
AU: * Roeloffs, E
EM: evelynr@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683, United States
AU: McCausland, W
EM: wmccausland@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683, United States
AB: In order to compare borehole strainmeter data with tectonic models, we must know the coupling parameters relating elastic deformation of the strainmeter to strain in the surrounding rock. At least two coupling parameters are required: the ratios of instrument areal and shear strain to formation areal and shear strain, respectively. These coupling parameters depend on the relative elastic moduli of the formation, grout, and strainmeter, and typical elastic moduli yield nominal coupling parameters of 1.5 and 3. More accurate coupling parameters must be determined by analyzing each strainmeter's response to a known deformation source after the instrument has been grouted into the borehole. 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. Teleseismic Love and Rayleigh waves that produce fractional gauge elongations > 10^-7 , such as those from the M8.3 Kuril Islands earthquake of November 15, 2006, can be used to constrain the coupling parameters. A planar Love or Rayleigh wave is expected to have a simple strain field that produces the same waveform on all four gauges of a strainmeter. The two-parameter coupling model is consistent with the variation of surface wave amplitudes as functions of azimuth for the borehole strainmeter data analyzed to date, although most of the PBO strainmeters require that differences in the relative gains of the four gauges be estimated as well. Fits to the data can be improved for some strainmeters by allowing for two distinct shear strain coupling parameters, and/or for a small (<10 degrees) error in the orientation of the strainmeter as measured during installation. However, data from more earthquakes will need to be analyzed before these refinements can be called significant. The Rayleigh wave data provide tight constraints on the ratio of shear to areal coupling. For borehole strainmeters in northern Washington state, these ratios are in the range 5 to 10, in contrast to the ratio of 2 for the nominal coupling parameters. This behavior could be explained by grout with a relatively low elastic modulus. 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.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1294 Instruments and techniques
DE: 7255 Surface waves and free oscillations
SC: Geodesy [G]
MN: 2007 Fall Meeting