The enigma of the Arthur's Pass, New Zealand, earthquake 2. The aftershockdistribution and its relation to regional and induced stress fields

The aftershock distribution of the 1994 Arthur's Pass earthquake, M(w)6.7, is unusual for a reverse faulting event in that it extends 12 km NNW and 30 km SSE of the actual fault plane, which strikes NE-SW. We have used severa l methods to infer the regional stress field in the region, including geode tic results, earthquake mechanisms, and inversion of P wave polarity data f or the stress tensor orientation. The inversion method is new and does not require the focal mechanisms of the events used. It also incorporates the C oulomb failure criterion. All results point to a stress field favoring stri ke-slip faulting: not thrusting. with near-horizontal sigma 1 and sigma 3 p rincipal axes striking at 298 degrees and 28 degrees. Using dislocation the ory, we calculate the stress induced by the Arthur's Pass earthquake and it s largest aftershock (a strike-slip event) and add this to the regional fie ld. There is a fair correspondence between the hypocenters of aftershocks a way from the mainshock fault plane and regions of high induced Coulomb Fail ure Stress (CFS) on optimally oriented fault planes. However, there are reg ions of high induced CFS that are devoid of aftershocks. It appears that ea rthquake slip in this region of oblique (19 degrees) plate convergence is, as observed elsewhere, partitioned into components parallel and perpendicul ar to the plate margin. Most of the slip is parallel, as occurs on the near by dextral Alpine fault, the boundary between the Pacific and Australian pl ates. However, occasional reverse events, such as the Arthur's Pass earthqu ake, account for at least some of the perpendicular component of slip and t he uplift that produced the Southern Alps.