ASSESSMENT OF LOCAL QUATERNARY STRATIGRAPHY AND POTENTIAL SEISMIC SOURCES NEAR SUCCESS DAM, CENTRAL CALIFORNIA

The southeastern California Central Valley contains alluvial sediments from the Kaweah, Tule, and Kern River watersheds that have been deposited across the N-trending western margin of the Sierra Nevada between Visalia and Bakersfield. North of Visalia, the NW-trending edge of the Sierra Nevada is associated with low-slip-rate, normal dextral-oblique faults. South of Visalia, the range-margin faults are poorly characterized as potential seismic sources; this analysis was completed to update the seismic-source characterization and probabilistic seismic hazard analysis (PSHA) for Success Dam, which is 43 m (142 ft) high and impounds the Tule River upstream of Porterville. Previous stratigraphic interpretations along the western margin of the Sierra Nevada north of Tule River show the presence of Turlock Lake, Riverbank, and Modesto deposits of middle to late Quaternary age. Along the western range front south of Tule River, analysis of high-resolution LiDAR-derived shaded relief maps, aerial imagery, and field relationships delineates deposits that are probably correlative with Turlock Lake-, Riverbank-, and Modesto-age deposits. This analysis confirms geomorphic expression of fault-related lineaments associated with the previously mapped Rag Gulch fault, and extends the fault 58 km north from Poso Creek to the Tule River. The fault contains multiple strands in a 5-km-wide zone, and consists of a 17-km-long northern section and a 41-km-long southern section, which are demarked by changes in fault strike and width near Fountain Spring Gulch. Fault-related geomorphic features are restricted to areas underlain by middle Quaternary deposits (probable Turlock Lake Fm.), and are not developed on younger alluvial deposits (probable Riverbank and Modesto Fms.). Areas north of Tule River underlain by probable late Pleistocene alluvium do not contain distinct fault-related lineaments associated with the Rag Gulch fault. Other NW-trending, potentially fault-related topographic lineaments within the range block appear to be overlain by unfaulted Riverbank- or Modesto-age deposits. The PSHA assigns probabilities of activities for the Rag Gulch fault and other local potential seismic sources, and evaluates strong ground motions and response spectra for assessing the seismic stability of Success Dam.