BAT CAVES EDM Array
Number of Bench marks: 8
Number of Surveys to 1993: 3
Date of Initial Survey: 22 March 1992 (UCSB)
Date of Latest Survey: 6 August 1992
Trespass Permission: None, although the Bureau of Land Management may believe that it ought to be informed.
Directions: From Mecca, drive southeast about 20 miles on State Highway 111 along the northeast side of Salton Sea to Range Road. Turn left, drive across the South Pacific Railroad tracks, and proceed north about 5 miles to an unmarked and unsigned dirt track that pr oceeds west about two miles to the top of Durmid Hill. The central bench mark of the array, SALTON, is clearly located at the crest of the hill. Other bench marks are referenced to that bench mark.
Comments: This array was initally established by the U.S. Geological Survey in May 1978 to monitor horizontal strain across the San Andreas fault over Durmid Hill. The USGS measured slope lengths of the lines with an HP3808A distance meter and calculated horizont al lengths. The first UCSB survey used both HP3808A and Wild TC2000 distance meters. Subsequent UCSB surveys have used only the Wild TC2000 where horizontal line lengths are recorded directly.
Results: The lengths of lines have changedto an extent greater than one standard deviation of the uncertainty from the first USGS survey in 1978 to the first UCSB survey of 22 March 1992 in a pattern consistent with right shear across the fault (Sylvester et al., 1 993). The lengths of the lines did not change from March 1992 to August 1992, the time that brackets the 28 June 1992 Landers earthquake sequence.
Sylvester, A. G., R. Bilham, M. Jackson, and S. Barrientos, 1993. Aseismic uplift of Durmid Hill, 1985 to 1991, south easternmost San Andreas fault, California. Journal of Geophysical Research.
Clark, M. M., 1984. Map showing recently active breaks along the San Andreas fault and associated faults between Salton Sea and Whitewater River - Mission Creek, California. U.S. Geological Survey Map I-1483, 1:24,000.
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