Modeling stress changes induced by earthquakes in the southern Marlborough region, South Island, New Zealand

Between 1888 and 1994 a series of seven earthquakes of M-W 5.9-7.2 occurred within a 150 km by 150 km area (southern Marlborough region) directly east of the Alpine fault. Major strike-slip faults in this region, including the Hope, Kakapo, and Clarence faults, serve to transfer plate motion from the Alpine fault northeastward to the Hikurangi subduction zone. We have modeled induced changes in Coulomb failure stress (bCFS) to determine whether stress triggering could have occurred during this sequence and to determine how the earthquakes have affected stress along the North Westland segment of the Alpine fault, which appears to have last ruptured in this region in A.D. 1717. We included the effects of secular stress accumulation on the four most rapidly slipping faults in the region (Alpine, Awatere, Clarence, and Hope), but neglected the effects of viscoelastic relaxation. Our results suggest that at least six of seven earthquakes, and possibly all seven events (when uncertainties in earthquake location and focal mechanism are considered), occurred in regions where deltaCFS exceeded 10 kPa, indicating stress triggering may have occurred. Although (5CFS exceeds 10 kPa along most of the North Westland segment of the Alpine fault at depths of 10 km or less, deltaCFS is less than 10 kPa at depths of 10 km or more. If rupture for large Alpine fault events initiates at depths of 10 km or more, then this result implies that the induced stress since 1848 has not brought this segment significantly closer to failure.