JPL Level Line

Latitude: N 34°12'05"
Longitude: W 118°09'52"

Length: 1899.7m
Number of Bench marks: 41

Number of Surveys to 2000: 12
Date of Initial Survey: 14 June 1984
Date of Latest Survey: 13 July 1995

Trespass Permission: Written permits to trespass are required.

Take the 210 Freeway from/to Pasadena to the Windsor Avenue offramp. Drive North about 1.5 miles on Windsor Avenue to where the road has three forks: Ventura Street to the right; Gabrielina Trail Road with a locked yellow bar gate straight ahead; and a do wnhill road to JPL parking lot on the left. The first BM is in the traffic island at the three-fork intersection. It is an NGS Class A (?) rod mark labeled Z1306 in a capped (threaded) pipe. A wrench is needed to get the cap off.

This line was established across the Sierra Madre fault in Arroyo Seco adjacent to the Jet Propulsion Laboratory, two miles east of La Canada. A key and written permits are required to drive a vehicle beyond the locked gate at the entrance to the access r oad.

The southernmost bench mark, Z1036, was damaged between January and August, 1986, when the traffic island was repaved. Many of the other bench marks have been damaged by vandals.

Re lative to the mountain block (north), bench marks in the block south of the fault have subsided as much as 10 mm, fluctuations that are probably due to general lowering of the water table south of the fault. The Pasadena Department of Water & Power maint ains four major water wells south of the fault in the Monk Hill sub-basin. They were pumped heavily before we established our leveling line in 1984, and peak cumulative production was achieved in 1983-1984. Pumping decreased steadily from that time, but subsidence continued until recovery began in 1985. The apparent anomalous behavior of JP39 relative to other bench marks is due to vandalism.

We did not detect apparent vertical displacement across the fault in response to the 1 October 1987 Whittier Narrows earthquake (M 5.9), 16 km southeast of the array (Sylvester, 1989), or to the 28 June 1991 Sierra Madre earthquake (M 5.4), 16 km east of t he array.

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.

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