Fault activity and stress field of shallow intraplate earthquakes in northern South Island, New Zealand

The Alpine Fault is a major transpressive right-lateral fault at the intracontinental plate boundary between the Australian and Pacific Plates. Inversion tectonics are inferred to affect a sizable proportion of the active faults in the crust of the Australian Plate, west of the Alpine Fault. However, the relationship between seismic activity and active faulting in this region remains poorly understood. This study presents new data to estimate the stress tensor in the northern South Island and to assess whether historical and recent earthquakes occurred on faults that were favorably oriented to slip. The estimated stress field in the northwestern region of the South Island favors reverse faulting, with dominant N-S faults dipping approximately 30°–60° and being the most favorably oriented for the slip. Both compressionally reactivated normal faults and newly developed reverse faults were found to be favorably oriented to slip under the current stress regime. In contrast, the estimated stress field in the northeastern part of the South Island favors strike-slip faulting, with NE-SW faults dipping approximately 75°–90°, which is the most favorable orientation for slip. Therefore, faults in the Australian and Pacific crusts have the most favorable geometry for being seismically reactivated in the current stress field. However, suboptimal fault clusters were identified west of the Alpine Fault. A previous study observed elevated Vp/Vs ratios extending continuously from the top of the subducting Pacific Plate into the crust in the northwestern South Island, suggesting that crustal overpressured fluid migration may have locally triggered fault activity. • Transpressive stress fields dominate the South Island west of the Alpine Fault. • A strike-slip stress field prevails in the Pacific crust to the northeast. • Active reverse faults are well oriented for reactivation under today's stress field. • Active unfavorably oriented faults suggest overpressured fluid in reactivation.