DID THE BIG PINE SEGMENT OF THE OWENS VALLEY FAULT RUPTURE DURING THE 1872 OWENS VALLEY, CALIFORNIA EARTHQUAKE?: EVIDENCE FOR DOUBT

The normal, right-handed oblique-slip Owens Valley fault, situated along the axis of Owens Valley, southeastern California, ruptured on March 28, 1872, resulting in one of California's largest historic earthquakes. Despite the lack of careful geological study of the fault scarp until many years after the earthquake, geologists apparently without exception have accepted the notion that the 1872 fault rupture extended 100 km from south of Lone Pine to just north of Big Pine. However, geologic evidence from the Big Pine segment, which extends northward from the Poverty Hills and forms the northern 20 km of the fault, suggests that this segment did not rupture in 1872. Instead, the Owens Valley fault rupture appears to have terminated at a point several kilometers south of the Poverty Hills. Two main lines of evidence argure against an 1872 rupture of the Big Pine segment. First, along the length of the Big Pine segment where the fault cuts fanglomerate, the scarp is entirely mantled by debris and has a maximum gradient of approximately 32 degrees, the angle of repose. Nowhere is a "free face" exposed along the scarp. Wallace (1977) estimates that free faces developed along fault scarps in the Great Basin take at least several hundred years to disappear. In contrast to the Big Pine segment, the scarp along the Lone Pine segment, which clearly ruptured in 1872 and cuts fanglomeratic sediments similar to those at Big Pine, has a well-developed, 1 to 1.5 m high free face with a 55 to 60 degree gradient. The second line of evidence comes from the Crater Mountain area, where the Big Pine segment cuts Pleistocene basalt for a distance of 8 km. Desert varnish is strongly developed on all basalt exposed on the scarp. Given that visually discernible desert varnish requires more than a thousand years to develop (Liu and Broecker, 2000), it does not seem reasonable that fault rupture occurred as recently as 1872. If the 20 km long Big Pine segment did not rupture during the 1872 earthquake as proposed here, then the 7.4 to 7.6 moment magnitude estimates for the earthquake may be too large. Instead, the magnitude may have been about 7.2.