Spatiotemporal evolution of a transient slip event on the San Andreas fault near Parkfield, California

In 1993 several baselines of the two‐color electronic distance meter (EDM) network at Parkfield, California, deviated from their long‐term rates, coincident with anomalous observations from nearby strain meters and a creep meter, as well as an increase in microseismicity. Between October 1992 and December 1994, three M ∼ 4.5 earthquakes occurred beneath Middle Mountain, near the hypocenter of the 1934 and 1966 Parkfield M 6 events. We analyzed the two‐color EDM data using a Kalman‐filtering based technique to image the spatiotemporal evolution of slip on the fault at Parkfield between the mid‐1980s and 2003. This method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines. We find that a slip rate increase occurred between January 1993 and July 1996 on the upper 8 km of the fault near Middle Mountain. The peak estimated slip rate during this time was 49 mm/yr, which exceeds the long‐term geologic rate of ∼35 mm/yr. The slip rate evolution appears episodic, with an initial modest increase after the M 4.3 earthquake and a much larger jump following the shallower M 4.7 event in December 1994. This temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip. The EDM data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 M 6 Parkfield earthquakes. However, transient slip and its associated stress release in the hypocentral area of previous Parkfield events is consistent with the nucleation of the 2004 M 6 Parkfield earthquake elsewhere on the fault.