Modern tectonic cracking near the Garlock fault, California

Abstract

Tectonic strain accumulation along the locked, eastern segment of the Garlock fault is probably sufficient to cause ground cracking and related fissuring at the Naval Weapons Center, China Lake, California. A 7 mm/yr slip rate measured along the western segment of the fault apparently is not being relieved along the 155-km-long eastern segment by creep, and no major earthquakes have occurred along this segment during the past 1,000 ± 500 yr. Therefore, the unrelieved slip available for elastic strain may be between 3.5 and 10.5 m. During 1970, a zone of erosion-modified fissures was observed near the Randsburg Wash Access Road, ∼5 km north of the Garlock fault. The fissures did not significantly enlarge between 1970 and 1983. Following a severe thunderstorm the night of August 18, 1983, two new zones of ground cracks appeared. The first zone included the earlier fissures but also extended to the north and south for a total length of 732 m. The second zone intersected the first near its midpoint, at a 30° angle, and had a total length of 686 m. Analysis of the cracks suggests that the first zone resulted from tensional stresses and that the second resulted principally from shear. The tectonic stress field assumed responsible for the formation of these zones is consistent with that deduced for the western segment of the Garlock fault. The morphology of the cracks indicates that they occurred shortly after the rainfall and flash flooding ceased, and there was probably no causative relationship between them. Trenching studies indicate that the cracks and fissures are a part of a long-term process and that the new cracks are only the latest of a series of events.