Latitude: N 33°35'00"
Longitude: W 116°39'35"
Number of Bench marks: 32
Number of Surveys to 2000: 33
Date of Initial Survey: 24 January 1981
Date of Latest Survey: 34 June 1992
Most of the array is on private property with limited access to authorized persons.
From Riverside take Interstate 15E 40 miles south to State 79 South, then proceed east 18 miles to State 371, and then east (left turn) on Hwy 371 for 15 miles to Anza. At the intersection of 371 and Hill St. in Anza, turn left (north) on Hill Street Driv e and proceed 1.1 miles to Mitchell Street. Turn right and drive 0.5 mile to Bohlen Street (after the Hamilton School). Turn left on Bohlen Street and drive 0.8 mile to the end of the road; there a gate leads to USFS road 6520. Drive through the left gate to Pine Oaks and Rancho de la Detras. The line is 0.3 miles from the gate.
This irregular-shaped line conforms, in part, to the physiography of a right-offset canyon across the San Jacinto fault, two miles north of Anza.
This line has been surveyed more than any other line in our inventory, largely in response to the occurrence of nearby earthquakes, or the concern that an earthquake was imminent, judging from observations by other observers of ground water level changes. An a ccompanying diagram portrays the results of four levelings representative of the time period 1984 to 1992, and bench mark A01 on the valley side of the fault is arbitrarily held constant. Prior to 1984, from 1981 to 1984, no significant changes were detec ted from bench mark to bench mark or along the entire line, even on 15 June 1982 when the M 4.8 earthquake occurred benath Anza, 3 km south of line, when the line was being surveyed (Sylvester, 1989). One may interpret the southwest tilt of the northeast part of the line, from 1984 to 1992, as being due to the progressive, tectonic uplift of the mountain side (northeast) side of the fault, but it is more probable that the tilt reflects the lowering of the water table in Anza Valley during to the 1986-1991 drought. This would be depicted clearly if a bench mark at the northeast end of the line were held fixed.
Sylvester, A. G., 1989. Triggered vertical strain across active strike-slip faults in southern California. Abstracts Volume, 28th International Geological Congress, Washington, D. C., v. 3, 207-208.
Sylvester, A. G., J. Helm, C. Hitchcock, and N. Howe, 1990. Minor vertical,
but no horizontal nearfield displacement across the San Jacinto fault at
Anza and San Bernardino, California, from 1988 to 1990. Seismological Research
Letters 61 (1), p. 22.
Up one level
Return to the Title Page.