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.
Directions:
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.
Comments:
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:
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.
Reference:
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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