Geomorphic and structural mapping in pursuit of a slip rate for the Santa Susana Fault, southern California

The Santa Susana Fault (SSF) is a 38-km-long thrust fault that connects active faults along the margins of the Ventura and San Fernando basins within the Western Transverse Ranges of southern California. Despite proximity of the SSF to metropolitan areas, the slip rate on the SSF remains very uncertain; the highest rate (10 mm/yr) comes from structural reconstructions and the lowest rate (0.5 mm/yr) is an estimate based on the lack of fresh geomorphic offsets along its trace. Working towards a more robust late Quaternary slip rate, we have characterized the geology and topography of the SSF utilizing lidar datasets, photogrammetrically-derived DEMs from 1920-30s pre-development aerial photographs, and field surveying and mapping. We focused on an area along the west-central SSF north of Simi Valley where a set of alluvial fans is offset by the fault. Vertical separations are 24 m for the fan surfaces, and a minimum of 22 m for the underlying alluvium/bedrock contact. Age results from ³⁶Cl cosmogenic nuclide depth profile samples of the offset fan surfaces will assist in correlating alluvial fan surfaces and yield the first late Quaternary slip rate for the SSF derived from modern chronologic techniques. The geomorphology of the footwall valley indicates a period of widespread incision followed by aggradation, producing flat-bottomed valleys inset into the alluvial fan margins. This history suggests a complex interaction between local tectonics and/or climatic changes, which we examine through quantitative analysis of stream channel geometry. Preliminary analysis indicates a modest decrease in normalized channel steepness along the western half of the fault, which may reflect interactions with other faults to the south. Better understanding the tectonic regime, as well as constraining the slip rate for the SSF, will assist earthquake hazard forecasting for southern California and reveal the spatio-temporal patterns of sedimentation and erosion associated with tectonism for the Western Transverse Ranges.