Slip-rate increase at Parkfield in 1993 detected by high-precision EDM andborehole tensor strainmeters

On two of the instrument networks at Parkfield, California, the two-color E lectronic Distance Meter (EDM) network and Borehole Tensor Strainmeter (BTS M) network, we have detected a rate change starting in 1993 that has persis ted at least 5 years. These and other instruments capable of measuring crus tal deformation were installed at Parkfield in anticipation of a moderate, M6, earthquake on the San Andreas fault. Many of these instruments have bee n in operation since the mid 1980s and have established an excellent baseli ne to judge changes in rate of deformation and the coherence of such change s between instruments. The onset of the observed rate change corresponds in time to two other changes at Parkfield. From late 1992 through late 1994, the Parkfield region had an increase in number of M4 to M5 earthquakes rela tive to the preceding 6 years. The deformation-rate change also coincides w ith the end of a 7-year period of sub-normal rainfall. Both the spatial coh erence of the rate change and hydrological modeling suggest a tectonic expl anation for the rate change. From these observations, we infer that the rat e of slip increased over the period 1993-1998.