Latitude: N 36°50'48"
Longitude: W 121°32'00"
Length: 776.1m
Number of Bench marks: 20
Number of Surveys to 2000: 22
Date of Initial Survey: 20 September 1975
Date of Latest Survey: 29 June 1994
Trespass Permission: Permission to trespass is required.
Directions:
The level line follows Franklin Street from the fault scarp below the Faultline
Restaurant in San Juan Bautista, southwest to Sixth Street, thence southwest
along Washington Street to its intersection with Highway 156. Bench mark
7321 is a bronze tablet in the northwest bridge abutment, about a meter above
ground level. The northeasternmost bench mark, 7328, is a bronze tablet in
the concrete abutment of a small bridge across an irrigation channel in a
field about 150 m northeast of the fault scarp.
Comments:
This line was established across the San Andreas fault to determine if the
prominent scarp along the fault at the northeast edge of San Juan Bautista
is increasing in height at the same time as horizontal creep occurs along
the fault. Several bench marks originally placed in concrete curbs, now replaced
with driveways, have been been destroyed. Bench marks Rod 6 and 7328 have
been destroyed due to farming activities.
Results:
Six levelings from October 1975 to August 1990 are compared in an accompanying
figure. Relative to a bench mark in the town, the northeast part of the line
has subsided nearly 100 mm. The line would have to be lengthened tenfold
and cross the full width of San Juan Bautista Valley to resolve the question
of valley subsidence by withdrawal of groundwater vs. tectonic uplift of
the fault scarp. A sharp boundary does n ot exist between relative uplift
and subsidence at the fault in contrast to the remarkable narrow zone of
horizontal movement documented by U.S.G.S. creepmeters northwest and southeast
of the town. En echelon cracks formed at the time of the October 1989 Loma
Prieta earthquake along the fault at the base of the scarp where it is crossed
by the leveling line. No evidence of vertical separation has been found along
the San Juan Bautista strand of the fault since that earthquake.
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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