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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 Piñon 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.

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An instrument capable of deep borehole measurement of vector plane strain to 0.3 nstrain and tilt to 1.0 nrad has been developed for deployment in crustal deformation and earthquake prediction studies. The instrument has been deployed in California where shear strains dominate the deformation. The 125-mm-diam package is grouted in 175-mm boreholes at depths of approximately 200 m. The wall thickness and the grout thickness are chosen to match instrument strength to expected rock parameters. The instrument is capable of flat response from dc to 10 Hz on any single channel. The electronics package is stable to three parts in 108 over the temperature range 10 to 45° C. Reliable shear strain data is available immediately on installation when simple volume strain meters show only bond curing effects or thermal recovery signals.

Over the past 6 years, repeated strain steps recorded on a tensor strainmeter installed close to the San Andreas fault trace at San Juan Bautista in California correlated closely with episodic creep events registered on a nearby creepmeter. The strain events were remarkably similar in character, were of about 1 hour duration, and were followed within hours to days by creep events of 2–6 days duration. The episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data. We postulate that the additional stress transferred to this region by the Loma Prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region, thus accounting for the increased frequency of the creep/strain events following the Loma Prieta earthquake.