WITBAARD, ROBERT, Harding Lawson Associates, Santa Ana, CA, HOWARD SHIFFLETT, Long Beach City College, Long beach, CA, and ROSWITHA GRANNELL, California State University, Long Beach, CA

ABSTRACT: Neotectonic Movement on En Echelon Faults of the Southern San Andreas System, Mecca Hills, California

Movement on the southern San Andreas fault has been the focus of much attention in recent years. Laser surveys across the San Andreas fault in the Indio Hills and at Mecca Beach have indicated that the fault segments in those areas creep at a rate of 2-4mm/yr. This creep includes sympathetic movement following earthquakes on other faults. Creep meters at North Shore indicated no movement during the period before the Landers earthquake. Laser surveys across the San Andreas fault segment southeast of Box Canyon Wash in the Mecca Hills have indicated an asperity with no movement during the last 6 yr. The Painted Canyon and Eagle Canyon faults are en echelon to the San Andreas in the Mecca Hills and appear to converge into one

major fault in Hidden Spring Wash, approximately 3 mi east of the asperity. The Painted Canyon and Eagle Canyon faults crossing Hidden Spring Wash have been surveyed for movement during the last 2 yr. The results of surveys performed on a quarterly basis indicate that creep has occurred on this segment of these faults. This creep appears to be consistent with the movement north and south of the asperity on the San Andreas fault. Several other studies have been conducted where the Painted Canyon and Eagle Canyon faults cross through the alluvium in Hidden Spring Wash, including gravity, magnetics, resistivity, active seismics, and ground penetrating radar (GPR) to further identify other aspects of neotectonic activity. A 2-ft-high fault-related scarp in the alluvium has been studied wi h GPR and trenching revealing a continuous fault to the ground surface. A change in the density of vegetation in the wash from the northeast to the southwest side of the fault zone suggests a possible groundwater barrier and has been studied with active seismics and resistivity. The nature of the bedrock beneath the alluvium in the wash has been studied with gravity and magnetics. No obvious variation in the behavior of horizontal angle change across the faults in the wash was observed from surveys conducted prior to and following the activity northeast at Landers and Big Bear. A change in horizontal distances was observed across the faults in Hidden Spring Wash and at the asperity prior to and following the seismic activity at Landers and Big Bear.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.