GOLD HILL Level Line

Latitude: N 35°48'04"
Longitude: W 120°21'48"

Length: 6845m
Number of Bench marks: 28

Number of Surveys to 2000: 4
Date of Initial Survey: 16 August 1987
Date of Latest Survey: 6 July 1993

Trespass Permission: The ends of the line are on private ranch property. Obtain trespass permission from the ranch foremen.

From Hwy 46, proceed 6 miles northwest on the Parkfield Road to a left deflection in the road and the entrance to Jack Ranch. The central part of the line follows the ranch fenceline along the southwest-trending segment of the fence and along the drive on to the ranch.

This line was established across the San Andreas fault and the Cholame pull-apart by the U.S. Geological Survey in 1987. The entire line has been surveyed only once.

Results of four partial to complete surveys from bench mark GOLD in the center of Cholame Valley northeast across the San Andreas fault to the end of the line beyond Gold Hill are compared in the graph below. The principal result is the subsidence of Cholame Valley on the southwest side of the San Andreas fault relative to the Gold Hill block northeast of the fault. The horizontal rate of creep is about 11 mm/yr at this locality, so it would be reasonable to infer that the subsidence is tectonic, aseismic creep. It is more probable that the subsidence is due to lowering of the water table in the valley due to pumping at the ranch and the lack of recharge during the 1985-1991 drought.  The continued subsidence of the line into 1993 belies that simple conclusion.

Sylvester, A. G., 1995. Nearfield vertical displacement in the creeping segment of the San Andreas fault, central California, 1975 to 1994. Tectonophysics 247, 25-47.

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