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 rate that correlate well with active geologic fault segments and locally with concentrated zones of microseismicity. High shear strain rates (0.3--0.95 &mgr;strain/year) are observed northward and southward of the San Andreas fault's big bend, whereas the big bend itself is characterized by a diffuse low magnitude shear strain rate. Dilatational deformation is diffuse and of relatively low magnitude (<0.2 &mgr;strain/year), with the highest contraction rates occurring in the Ventura and Los Angeles basins. Because no prior assumptions 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. ¿ 2001 American Geophysical Union