Increase in Seismic Hazard at Isabella Dam: The Active Kern Canyon Fault

Evidence has recently been found showing that the 140-km-long, north-striking Kern Canyon fault (KCF) that passes under the right abutment of the Auxiliary dam at Lake Isabella has been active within the Holocene epoch (last 10,000 years). As part of a larger Lake Isabella safety evaluation and remedial design project, an extensive evaluation of the KCF has been undertaken to better understand the risk that it poses to the two embankment dams and appurtenant structures. Lake Isabella is located in the southern Sierra Nevada, California; failure of either of the dams could lead to severe flooding in Bakersfield, located 35 miles downstream, putting a population of over 300,000 inhabitants at risk. The Auxiliary dam sits primarily on an alluvial foundation containing zones that are potentially liquefiable. While other seismic sources have been recognized as contributing to the seismic hazard at Lake Isabella, the KCF has recently been confirmed as a major contributor to the seismic hazard at the site through strong ground shaking and surface rupture. Our on-going study of the fault has involved interpretation of LiDAR data and aerial photography along nearly its entire length, geologic mapping at numerous sites, geophysical surveying at the Auxiliary dam and across the upstream delta, exploratory drilling at both dams and two nearby sites along the fault, fault trenching at four sites, and regional seismicity analysis. Radiocarbon and optically stimulated luminescence analyses of samples recovered from the fault trenches and cosmogenically derived isotope analysis of rock samples from the surfaces of fault-displaced glacial moraines provide numerical age control for fault rupture events. Samples collected from exploratory trenches about 4 miles south of the reservoir indicate that the KCF displaces Holocene sediments, contrary to the belief at the time of the project construction in the late 1940s and early 1950s, that the fault had been dormant for at least the last 3.5 million years. Recent sense of slip on the fault is predominantly normal with the west side up, although there has been in the past and may continue to be some right lateral component of slip. 24 pp., 12 figures.