Detailed mapping of active strands of the Agua Blanca fault in Valle Agua Blanca, northern Baja California, Mexico, indicates that: 1) the overall sense of displacement is nearly pure strike-slip with local vertical components dependant upon individual fault strand geometries; 2) the main fault zone is locally over 0.5 km wide with multiple subparallel strands that have been active at different times throughout the Quaternary, some concurrently; 3) lateral offset and active sedimentation from distinct source areas has resulted in both, the stacking of dissimilar alluvial deposits, and the juxtaposition of alluvial deposits with dissimilar source areas; 4) the rate of late Quaternary slip, based on a 14C calibrated soil chronosequence, is best estimated to be 4 to 6 mm/yr; and 5) Holocene displacements are decipherable in trench exposures. The Agua Blanca fault is a major, active tectonic element in northern Baja California that is capable of producing moderately large earthquakes, despite the lack of significant historical seismicity. In addition, since the Agua Blanca fault appears to transfer slip directly into the southern California inner borderland fault zone, this zone may be constrained to a composite slip rate of about 6mm/yr.
