No abstract

Two continuous borehole plane strain monitoring sites have been operational in California since late 1983, using borehole tensor strain monitors implanted at a depth of 150 m. Shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals. At Pinon Flat Observatory, data indicate a constant shear strain accumulation of 0.6 microstrain per annum with the axis of maximum compression oriented 50° ± 5° west of north. This result differs significantly from regional geodetic estimates, the amplitude being dominated by continued viscoelastic response of the hole. Variations of measured strain rate exceeding 100 nstrain/yr would be significant at this instrument site. At San Juan Bautista, measurements of the angle of maximum compression (10° ± 3° west of north) agree closely with previous geodetic and hydrofracture estimates. After 2 years of operation the residual shear strain rate is 1 μstrain/yr. Preliminary analysis of strain steps observed at San Juan Bautista during the Morgan Hill earthquake of April 24, 1984, show good agreement with calculations from seismically determined source parameters for this event.

The Loma Prieta earthquake (Ms=7.1, 17 October 1989) provided unique near field borehole tensor strain observations. The medium term data at the San Juan Bautista site showed a clear anomalous change in the fault parallel shear strain rate beginning about a year before the event. The anomaly ultimately reached 30% of the coseismic offset. The signal shows similarities with changes in strain rates reported from the geodetic record, suggesting a broad regional anomaly. The limited spacial sampling available prevents determination of a causal link useful for prediction between these data and the earthquake.