Focal mechanisms in the southeastern South Island of Aotearoa New Zealand indicate scale-dependent partitioning of transpressional strain

The classic Andersonian model of faulting is difficult to apply to plate boundaries with oblique motion, as displacement is accommodated across oblique-slip faults, or it is partitioned into distinct strike-slip and dip-slip faults. Here, we investigate how faults accommodate oblique plate motion by using the focal mechanism solutions of 126 MLV 1.3-4.3 earthquakes in the transpressional southeastern South Island of Aotearoa New Zealand. Focal mechanisms were assigned an A-D quality, and of the 91 C or better quality solutions, 57 are strike-slip. In addition, when incorporated into a stress inversion, these focal mechanisms indicate a strike-slip stress state with an WNW-trending maximum principal compressive stress. By contrast, constraints on active crustal-scale faulting from the New Zealand Community Fault Model indicate reverse faulting in this region. A high stress shape ratio can partly account for the coexistence of reverse and strike-slip faults. However, we also propose that the focal mechanisms are typically sampling slip on optimally-oriented small-scale faults in intact crust, while the larger magnitude reverse faulting reflects local stress rotations within pre-existing faults and shear zones in the southeastern South Island. Our study therefore demonstrates how inherited structures influence the scale and orientation of faults onto which transpressional strain is partitioned.