Publications

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2015
Barbour, AJ, Agnew DC, Wyatt FK.  2015.  Coseismic strains on plate boundary observatory borehole strainmeters in Southern California. Bulletin of the Seismological Society of America. 105:431-444.   10.1785/0120140199   AbstractWebsite

Strainmeters can record offsets coincident with earthquakes, but how much these represent strain changes from elastic rebound, and how much they are contaminated by local effects, remains an open question. To study this, we use a probabilistic detection method to estimate coseismic offsets on nine borehole strainmeters (BSMs) operated by the Plate Boundary Observatory (PBO) in southern California, from 34 earthquakes with a wide range of magnitudes and distances. In general, the offsets estimated for the BSM data differ substantially from the static strain predicted by elastic dislocation theory, which is well supported by other techniques, though 10% of the observed offsets agree well with theory. For one earthquake, the BSM offsets significantly disagree with collocated long-base laser strainmeter data. Comparisons with collocated seismic data provide strong evidence that the absolute errors between the observed and predicted strains scale with the level of seismic energy density but also that relative errors (normalized by the model size) do not. We conclude that apparent strain offsets are induced by seismic waves, occurring presumably because of irreversible deformation, whether in the rock or cementing materials close to the BSMs, or in the instruments themselves. Coseismic offsets seen in PBO BSM data should therefore be viewed with caution before being used as a measure of large-scale coseismic deformation.

1996
Hart, RHG, Gladwin MT, Gwyther RL, Agnew DC, Wyatt FK.  1996.  Tidal calibration of borehole strain meters: Removing the effects of small-scale inhomogeneity. Journal of Geophysical Research-Solid Earth. 101:25553-25571.   10.1029/96jb02273   AbstractWebsite

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Pinon Flat. Small-scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short-baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small-scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross-coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.