HOLOCENE FAULTING BETWEEN SAN JACINTO AND SAN ANDREAS FAULTS, SAN BERNARDINO AREA, SAN BERNARDINO COUNTY, CALIFORNIA

Faulting investigations using aerial-photograph, GPR, seismological, and trenching methods in the San Bernardino Valley region have revealed several previously unrecognized faults that displace the ground surface. Geomorphic, C-14 dating, and soil-stratigraphic analyses indicate the displaced materials are of Holocene age, possibly as young as about 7000 years. Individual faults appear to be short but they are associatd with clusters of lineaments indicating wider and longer zones. Relationships are obscured to the northwest by erosion and deposition within Lytle Creek and Cajon Washes, and by urban development in the city of San Bernardino area to the southeast. Aerial photographs of 1930s vintage indicate that these clusters of faults and lineaments align with faults in Sycamore Canyon and Cajon Wash (Glen Helen fault)in the mountains to the northwest and with subtle lineaments within the city of San Bernardino to the southeast. The area of heaviest lineament and fault concentration coincides with a dense cluster of earthquakes occurring during the past few decades. Stratigraphic offsets measured in the trenches indicate vertical separations on the order of 1 m on some faults and horizontal separations up to about 3 m on another. These offsets appear to have occurred during one or two events. These faults and lineaments, and their association with ongoing earthquake activity suggest the possibility of a significant active fault extending from the San Gabriel Mountains into San Bernardino Valley, along the southwest side of the Shandin Hills, subparallel to the main San Jacinto fault along Rialto Bench and to the San Andreas fault. Empirical fault-length/earthquake-magnitude and displacement-magnitude relationships suggest such a fault is capable of generating a magnitude 6.5 to 6.75 earthquake. Displacement relationships indicate that the slip rate is very slow, perhaps on the order of <1 mm/yr, and the recurrence interval is very long, on the order of several thousand years.