Tearing of the Subducted Australian Plate During the Te Anau, New Zealand, Earthquake of 1988 June 3

The ML= 6.1 earthquake of 1988 June 3 was widely felt in the South Island of New Zealand, and caused landslides in Fiordland. It occurred at a depth of 57 km near 45.10°S 167.17°E, in a region where the Australian plate is subducting beneath the Pacific plate. Immediately after the event, portable seismographs were installed in the Te Anau area. Data from a well-recorded subset of aftershocks have been used to invert for the seismic velocities of the lower crust and upper mantle of the region, and for station terms. The large range in station terms obtained (3.3 s for P-waves) emphasizes the structural complexity of the Fiordland region. The aftershocks, as relocated with the new velocity model, occur in a cigar-shaped zone which extends from about 40 to 70 km in depth, and dips southeast at approximately 65°. The orientation of the aftershock zone in relation to the focal mechanism of the mainshoek suggests that the earthquake involved down-dip tearing of the crust of the subducted Australian plate, with the NE part of the plate having moved up and to the east-southeast relative to the SW part. Down-dip tearing of the crust of the Australian plate is also inferred for an earthquake of ML= 5.9 which occurred on 1988 July 19. This was located directly down the dip of the subducted plate from the June 3 event, at a depth of 122 km. Taken together with other seismological data, these two events suggest the existence of a major tear in the subducted plate, and that the shallow part of the plate to the northeast of this tear is resisting subduction. The resistance may result from subduction of a region of relatively thick crust. This would provide an explanation for the uneven distribution of both shallow and intermediate-depth seismicity in Fiordland, the differing dips of the subducted plate along the margin, fault-plane solutions, and spatial variations in the b value of intermediate-depth events. Subduction of a region of relatively thick crust also provides a mechanism for tilting up the western part of the Fiordland block.