Slip-partitioned fault breaks have been mapped for a 70-km stretch of the 450-km surface rupture of the 14 November 2001 Kokoxili earthquake. Simultaneous dip-slip and strike-slip motion on parallel faults has been proposed before, but the new observations demonstrate unequivocally that it can occur in a single earthquake and allows the mechanical processes to be scrutinized. Observed normal fault offsets were between 0.5 and 1 m and strike-slip offsets were 3–5 m. The partitioned stretch of faulting has a strike that differs by 2–3° from the pure strike-slip faulting to the east and west. This allows a horizontal opening vector of 0.25 m to be determined for the partitioned region. The distance between the two faults is greatest (∼2 km) at the center of the partitioned portion and diminishes toward the ends.
The faulting is modeled to result from strains due to a buried oblique slip-fault dipping at 80° to the south. The depth to the top of the buried fault is shown to vary commensurately with the separation of the surface faults. Clear surface rupture is observed where the predicted model mechanisms are colinear and where substantial faults can develop into a kinematically stable partitioned system. In a few interesting examples fragmentary, oblique surface ruptures occur where the predicted mechanisms are not colinear, but they are not associated with long-term surface faulting.
The proposed mechanism for slip partitioning requires that rupture propagates upward from depth. For the Kokoxili surface breaks this is a consequence of coseismic, dynamic rupture traveling faster at depth than near the surface, leaving the surface deformation to catch up. While the mechanism we propose requires slip weakening and localization to create faults or shear zones, it does not require that faults with different mechanisms have different frictional behavior.