The 2001 ML 6.2 Jackson Bay earthquake sequence, South Island, New Zealand

Abstract

Following the 2001 December 7 Jackson Bay earthquake (ML 6.2, MW 5.8), a temporary network of five portable seismographs was deployed in the region to record aftershock activity. Data recorded by the temporary network and nearby New Zealand National Seismological Network stations have been used to define a velocity model for the region and station corrections for each recording station. The locations of the best recorded aftershocks and the revised location of the mainshock indicate that the Jackson Bay earthquake sequence occurred 3–10 km to the east of the Alpine Fault, which is vertical in this region. A fault plane solution obtained from body‐wave modelling suggests the mainshock was primarily a reverse event (rake = 103°) centred at c. 4 km depth on a fault striking northeast‐southwest (48°) and probably dipping to the southeast (45°), which is roughly consistent with the Harvard CMT solution for this earthquake. However, on examination of the aftershock locations, such a fault plane is not clear, nor is any other. The aftershocks are located mainly in two clusters near each other at depths between 3 and 8 km and aligned approximately north‐south. Their positions are in accord with induced stress considerations and the mainshock fault plane lying between the clusters. Individual focal mechanisms for 33 aftershocks have a wide range of solutions. As a group, however, their P and T axes are reasonably well aligned and consistent with the background stress regime in the region as determined by direct inversion of P‐wave polarities. The Jackson Bay earthquake was the third thrust earthquake of magnitude >6 to occur just east of the Alpine Fault in a 7 yr period. Consideration of the mechanics of this earthquake, and the previous two, suggests that the regional stress is at a high level, in accord with the long elapsed time since a large Alpine Fault event. Although the area is small, the Jackson Bay mainshock induced a mainly positive change in Coulomb Failure Stress (CFS) on the closest section of the Alpine Fault, up to c. 0.7 MPa (7 bars).