Spatial and Temporal Patterns of San Andreas Fault-Driven Distributed Deformation near Point Arena, Mendocino County, California

We have mapped and correlated a flight of late-Pleistocene marine terraces between Fort Ross and Mendocino using aerial photography and digital elevation models (DEMs) to refine estimates of a slip rate on the San Andreas Fault (SAF) and to examine spatial and temporal variations in SAF-driven distributed deformation recorded in the landscape. Such patterns of deformation may reflect SAF geometry and may be useful for inferring rupture behavior and slip history. Our terrace correlation provides an important datum that can be used to constrain patterns of deformation associated with the SAF. In this study, inner edge elevations for the three lowest terraces were extracted from 1.8 meter LiDAR-derived DEM's to measure variability in terrace elevation along the 125 km of coastline. These observations suggest surface uplift rates as much as three times higher immediately north of the SAF at Alder Creek than elsewhere in the region between Fort Ross and Mendocino. If uplift rates north of the SAF are in fact higher, this signal should also be expressed in older portions of the landscape. An analysis of local relief and channel steepness in the high topography adjacent to the marine terraces using a combination of LiDAR and 10 m DEMs confirms this hypothesis. These geomorphic metrics delineate zones of high rock-uplift rate and provide an independent test of the apparent differential uplift observed in the marine terraces. Using the observed deformation field as input, we use simple boundary element modeling to evaluate the SAF geometry and slip history necessary to generate the observed patterns. In addition to illuminating off fault deformation driven by the SAF, these observations also enhance our confidence in the terrace correlation and associated slip rate at Alder Creek.