Latitude: N 36°50'48"
Longitude: W 121°32'00"
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.
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.
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.
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.
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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