Geodetic detection of active faults in S. California

A new analysis of velocities of geodetic markers straddling the San Andreas Fault System in southern California reveals that interseismic deformation is localized along a dozen sub-parallel narrow belts of high shear strain r ate that correlate well with active geologic fault segments and locally wit h concentrated zones of microseismicity. High shear strain rates (0.3-0.95 mu strain/year) are observed northward and southward of the San Andreas fau lt's big bend, whereas the big bend itself is characterized by a diffuse lo w magnitude shear strain rate. Dilatational deformation is diffuse and of r elatively low magnitude (< 0.2 mu strain/year), with the highest contractio n rates occurring in the Ventura and Los Angeles basins. Because no prior a ssumptions were made regarding the geology, tectonics, or seismicity of the region, our analysis demonstrates that geodetic observations alone can be used to detect active fault segments.