Analysis of deformation data at Parkfield, California: Detection of a long-term strain transient

Analysis of more than a decade of high-quality data, particularly those fro m the two-color electronic distance meter (EDM),:in the Parkfield, Californ ia, area reveals a significant transient in slip rate along:the:San Andreas Fault. This transient consists of an increase in fault slip rate of 3.3 +/ - 0.9 mm/yr during 1993.0 to 1998.0. The most reliable fault creep instrume nts show a comparable increase in slip rate, suggesting that the deformatio n is localized to the fault which breaks the surface. There was also an inc rease in precipitation around 1993. It is unlikely, however, that this anom aly is due directly to hydrology, as its spatial distribution is what would be expected for increased slip on the San. Andreas Fault. The increase in slip rate corresponds temporally to a dramatic increase in seismicity, incl uding the four largest earthquakes in the period 1984-1999 that occurred al ong a 6-km segment of the fault just to the north of the EDM network. There was also a previously reported anomaly in borehole shear. strain [Gwyther et al., 1996] that closely corresponds temporally to the transient-in EDM d ata. Solely on the basis of EDM data the transient can be modeled as :a Sli p event on a 10-km-long segment of the fault. The calculated shear strains from:this model, however, are not consistent with the observed ones. A comp atible model:dan be found if there is increased aseismic slip to the northw est in conjunction. with the four earthquakes. Support for this northwester n slip is provided by a recent study of slip rate based on microearthquake activity. We speculate that this northwestern event served to load the:faul t:to the southeast, with the stress being partially released by the observe d slip.