The enigma of the Arthur's Pass, New Zealand, earthquake 1. Reconciling a variety of data for an unusual earthquake sequence

The 1994 Arthur's Pass earthquake (M(w)6.7) is the largest in a recent sequ ence of earthquakes in the central South Island, New Zealand. No surface ru pture was observed, the aftershock distribution was complex, and routine me thods of obtaining the faulting orientation of this earthquake proved contr adictory. We use a range of data and techniques to obtain our preferred sol ution, which has a centroid depth of 5 km, M-0=1.3x10(19) N m, and a strike , dip, and rake of 221 degrees, 47 degrees, 112 degrees, respectively. Disc repancies between this solution and the Harvard centroid moment tensor, tog ether with the Global Positioning System (GPS) observations and unusual aft ershock distribution, suggest that the rupture may not have occurred on a p lanar fault. A second, strike slip, subevent on a more northerly striking p lane is suggested by these data but neither the body wave modeling nor regi onal broadband recordings show any complexity or late subevents. We relocat e the aftershocks using both one-dimensional and three-dimensional velocity inversions. The depth range of the aftershocks (1-10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the ma inshock fault plane. This structure and the Harper fault, similar to 15 km to the south, appear to have acted as boundaries to the extensive aftershoc k zone trending NNW-SSE. Most of the M(L)greater than or equal to 5 aftersh ocks, including the two largest (M(L)6.1 and M(L)5.7), clustered near the H arper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in th e south, is able to match the observed displacements. The occurrence of thi s earthquake sequence implies that the level of seismic hazard in the centr al South Island is greater than previous estimates.