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Summary.Parkfield is a town located on the San Andreas Fault in central California. It is well-known for a series of M6 earthquakes which had appeared to occur at, on average, 22 year time intervals. One such earthquake took place in 1966; a subsequent earthquake was in 2004. In anticipation of another M6, Parkfield became the focus of intense geophysical monitoring starting in the mid-1980s.

One monitoring network, called the two-color EDM (electronic distance measuring) network, was used to precisely measure the distance between different points in the Parkfield area. The goal was to see how these distances changed through time in response to movement on the San Andreas and the build-up of strain in the earth’s crust.

In 1993 several baselines of the two-color EDM network at Parkfield, CA deviated from their long-term rates, coincident with anomalous observations from nearby strainmeters and a creepmeter, as well as an increase in microseismicity. Between October 1992 and December 1994 three ~M 4.5 earthquakes occurred beneath Middle Mountain, near the point on the fault (called the hypocenter) where the 1934 and 1966 M6 earthquakes initiated.

In this study we analyzed the two-color EDM data using a technique that enables us to infer both the spatial and temporal evolution of slip on the fault from the mid 1980s until 2003. We find that a fault slip rate increase occurred between January 1993 and July 1996 on the upper 8 km of the fault near Middle Mountain, site of the 1966 and 1934 M6 earthquakes. The peak estimated slip rate during this time was 49 mm/yr, which exceeds the long-term rate of ~35 mm/yr. The slip rate evolution appears episodic, with an initial modest increase after the M4.3 earthquake in 1992 and a much larger jump following the shallower M4.7 event in December, 1994. This temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the fault to slip aseismically (at speeds much slower than during an earthquake). The EDM data cannot resolve whether the area that slipped aseismically included the hypocentral location of the 1934 and 1966 M6 Parkfield earthquakes. However, the occurrence of aseismic slip and its associated stress release in the hypocentral area of previous Parkfield events is consistent with the fact that the 2004 M6 Parkfield earthquake did not initiate beneath Middle Mountain.