Seismotectonics of the Arthur's Pass region, South Island, New Zealand
The spatial distribution of the seismicity of the Arthur's Pass region in South Island, New Zealand, shows that earthquakes occur within the crust in definite subparallel zones, each with an east-northeast trend. Composite focal mechanism solutions indicate that the compressive axis is oriented approximately west-northwest, with an average slip direction of N72°E. Comparisons between the 1973 study described here and one conducted in 1972 reveal no differences in the spatial and temporal distributions of the seismicity. Tectonically, the region can be considered as one in which transference of motion between the Alpine fault and the Hikurangi Trench is occurring. This tectonic situation, coupled with the shoaling of the Benioff zone associated with a southward migration of the Hikurangi Trench, suggests that the deformation in the Arthur's Pass region has occurred within the past 2 m.y. This region has marked similarities with the Transverse Ranges area of the San Andreas fault zone in southern California. Both regions are located where two different tectonic and seismic “styles” merge – namely, where a master fault, with its associated seismic activity consisting of infrequent great earthquakes separated by long periods of quiescence, branches into a system of splay faults having a moderate to high level of continuous activity, with the occurrence of large earthquakes. The regions do differ, however, in that the long quiet zone on the Alpine fault is oblique to the slip vector between plates, whereas similar zones of the San Andreas fault are both parallel and nonparallel to the slip vector. This suggests that long quiet zones of strike-slip faults are not a consequence solely of the orientation of a fault with respect to the direction of relative motion between plates.