ANOMALOUS SHEAR STRAIN AT PARKFIELD DURING 1993-94

Three borehole tensor strainmeters (BTSM) installed near Parkfield and separated by more than 5km have established a good baseline of data s panning the past eight years. A significant change in the accumulation rate of shear strain (0.5 and 1.0 microstrain per year at two of the sites) commencing approximately in mid-1993 and continuing to 1995 is evident in these data. Some supporting evidence for a change in the st rain rate at that time is seen in dilatometer data, geodetic measureme nts, fault zone properties and microearthquake locations. There was al so an increase in medium level seismic activity (M=4.7 on 10/20/92; M= 3.9 on 10/26/92; m=3.5 on 3/13/93; M=4.4 on 4/3/93; M=4.8 on 11/14/93) in the Middle Mountain area. While a rainfall induced strain may have caused the observed strain anomaly, we propose a more likely explanat ion is that aseismic slip at depth on a section of the fault south of Middle Mountain produced the observed strain anomaly. This anomaly is unique in the current data set.