A medium term precursor to the Loma Prieta Earthquake?

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.