WRIGHT EDM Array
Number of Bench marks: 6
Number of Surveys to 1993: 2
Date of Initial Survey: 10 September 1990
Date of Latest Survey: 18 June 1991
Trespass Permission: Wrightwood Municipal Water District
Directions: From Wrightwood, proceed west on Angeles Crest Highway to a point one mile west of the Los Angeles - San Bernardino County line. The array spans the San Andreas fault and the highway at the confluence of Swarthout Valley and Burford Canyon.
Comments: This array was established by the National Geodetic Survey across the San Andreas fault in 1964. Bench marks are standard NGS bronze tablets in concrete posts. Previous NGS su rveys were in 1964, 1965, 1966, 1967, 1970, and 1971. It was surveyed by the California Division of Mines & Geology in May 1978. The 1971 NGS and 1978 CDMG measurements were made with the same model EDM instrumentation (Tellurometer MA-100). The 1978 CD MG measurements revealed no significant horizontal fault movement during the seven year period 1971-1978, and first-order releveling by the Los Angeles County Engineer in November 1977 similarly indicated no differential vertical movement within this surve y network during the period 1971-1978 (Bennett, 1979). The most recent UCSB survey was performed with a Wild TC2000 total station distance meter. Relative to the 1971 NGS survey, all the line lengths increased from 70 to 130 mm. The increase is probably due to different reflector constants between the two different instruments. Not all lengths originally surveyed by other agencies could be resurveyed by UCSB, because several large pine trees had grown along the lines and obliterated direct sights.
Results: We measured no significant changes from 1990 to 1991.
Bennett, J. H., 1979. Fault creep measurement. California Geology 32, 98-105.
U.S. Department of Commerce, Coast & Geodetic Survey, July 1971
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