Holocene Faulting, San Bernardino Valley Area, San Bernardino County, California

Abstract

Aerial photographic, seismologic, and trenching investigations in the San Bernardino, California, region have revealed several previously unrecognized faults that displace the ground surface. Geomorphic and soil-stratigraphic analyses indicate the displaced strata are of Holocene age, possibly as young as about 5,000–7,000 years old. Stratigraphic offsets measured in the trenches indicate vertical separations on the order of 0.3 to 1 m (1 to 3 ft) on several faults and horizontal separations of up to about 3 m (10 ft) on another. Individual observed faults are short and discontinuous, but they are associated with clusters of lineaments that extend for several kilometers. Analyses of 1930s vintage aerial photographs indicate that the trenched faults and associated lineaments form alignments with faults in Sycamore Wash and Cajon Wash (Glen Helen fault) in the mountains to the northwest and with subtle lineaments to the southeast within the City of San Bernardino and within the Santa Ana River flood plain. Geophysical investigations by others indicate offsets of basement and valley-fill sediments coinciding with the southern part of the alignment. The area of greatest lineament and fault concentration coincides with a dense cluster of small earthquakes that have occurred during the past few decades. These faults and lineaments and their association with ongoing earthquake activity and basement-involved deformation indicate a significant active fault or fault zone extending from the San Gabriel Mountains into San Bernardino Valley, subparallel to and between the San Jacinto fault and the San Andreas fault. Empirical fault-length/earthquake-magnitude relationships and displacement-magnitude relationships indicate that such a fault could be capable of generating a magnitude 6.0 to 6.75 earthquake. Displacement relationships, although poorly constrained, indicate that the slip rate is very slow, perhaps on the order of 1 mm/yr (0.04 in./yr), and the recurrence interval is long, perhaps on the order of several thousand years.