FAULT SYSTEM DEFORMATION AND EVOLUTION AT METER TO KILOMETER LENGTH SCALES OVER SINGLE TO TENS OF EARTHQUAKE CYCLES

Continental fault systems are divided into a seismogenic portion bounded by shallower and deeper stably sliding zones of anastamosing fault surfaces bounding elongate blocks. Earthquake ruptures propagate through the upper zone and leave their record on the landscape with tectonically produced relief sculpted by surface processes. Characterizing the geometry, senses of slip and relative motions, and the rheology of the fault surfaces and blocks they bound over timescales of one to tens of earthquake cycles is essential for studies of fault mechanics and applications to seismic hazard. Much of the structural and surface process phenomena observed in the evolution of fault systems is governed by processes operating at the meter scale or finer. Therefore, high resolution (> 1 sample per sq. meter) topography is a powerful tool for the study of fault systems. Much of the San Andreas fault (SAF) system has been scanned using airborne laser swath mapping, providing a treasure trove for studies of active faults with variable slip and earthquake recurrence rates situated within a range of rock type, terrain, and climate regimes. Differencing before and after LiDAR scans of the 2010 M7.2 El Mayor Cucupah earthquake demonstrates that most of the coseismic deformation was accommodated by slip along faults discontinuous in strike and dip and by 10^-3 strains in the fault zone blocks. Slip in repeated earthquakes and creep along the Parkfield section of the SAF has been persistently localized as indicated by the development of fabrics in the unconsolidated materials and by association with composite fault scarps. In some cases, adjacent fault surfaces were activated in successive earthquakes, suggesting the process of dislocation of fault blocks within the shear zone. Finally, as the overall SAF trace rotates from N45W to N60W going southeast into the Big Bend, sub parallel active fault traces are replaced by discontinuous strike-slip faults producing local uplift and subsidence and normal and reverse faults accommodating right lateral shear across as much as 2 km with persistent activity for at least 10s of kyr. These results and related studies suggest that shallow fault zone structure is governed by velocity strengthening faults embedded within a elasto-plastic rheology with moduli diminished by repeated coseismic stress pulses.