Stress Changes Induced by Earthquakes and Secular Stress Accumulation in the Buller Region, South Island, New Zealand (1929–2002)

Between 1929 and 1968 the Buller region, located west of the strike-slip Alpine fault system, experienced two Mw > 7.1 earthquakes along high-angle reverse faults. We have modeled induced changes in Coulomb failure stress (DCFS) to determine whether stress triggering could have occurred during the 1929 and 1968 sequences and to determine how these earthquakes have affected stress along the Alpine fault and neighboring strike-slip faults located to the east of the Alpine fault. We have included the effects of secular stress accumulation on the five most rapidly slipping fault systems in the region (Alpine, Wairau, Awatere, Clarence, and Hope), but have neglected the effects of viscoelastic relaxation. Our results suggest that larger aftershocks of the 1929 Buller mainshock and moderate (Mw < 6.0) events following the 1968 Inangahua mainshock may have been triggered or hastened by the mainshocks. The 1929 mainshock does not appear to have been significantly hastened by previous Mw > 7.0 events in 1848 and 1888 along strike-slip faults to the east of the Buller region or by secular stress accumulation since 1848. The 1929 Buller earthquake may have delayed the 1968 Inangahua mainshock. Present values of DCFS along segments of the major strike-slip faults located east of the Buller region indicate that every fault segment except the North Westlands North segment of the Alpine fault contains regions of negative DCFS that are related to the coseismic effects of Mw > 7.0 earthquakes occurring between 1848 and 1968. The complex variation in DCFS along most major strike-slip faults in the region highlights the difficulty in evaluating which faults may presently be the closest to failure