Large earthquakes near doubtful sound, New Zealand, 1989–93

Abstract

Aftershock data recorded on portable seismographs deployed immediately after the M_W 6.4 Doubtful Sound earthquake of 1989 May 31 and the M_W 6.8 Secretary Island earthquake of 1993 August 10 have been used to constrain the rupture zones of these events. Both earthquakes involved slip at the interface between the subducted Australian plate and overlying Pacific plate. Their rupture zones abut rather than overlap, and the region where they meet lies vertically beneath the surface trace of the East Branch of the Alpine Fault. Slip during the deeper 1989 event was approximately in the plate convergence direction, whereas slip during the shallower 1993 event was approximately down the dip of the subducted plate. This requires slip partitioning in the shallow part of the subduction zone, and suggests that the East Branch of the Alpine Fault is active in this part of Fiordland. The 1989 earthquake produced very few aftershocks, whereas the 1993 earthquake had a rich aftershock sequence. This difference, and the mismatch in slip direction between the two events, can be attributed to changes in the frictional regime at the plate interface with depth. Static stress changes produced by slip in the 1989 earthquake promoted down‐dip thrusting on the rupture zone of the 1993 event. Thus, the 1989 event appears to have triggered the 1993 event, in the sense that it moved it closer to failure.

Keywords:

• Doubtful Sound
• Secretary Island
• earthquake
• Fiordland
• seismogenic zone
• Alpine Fault
• aftershocks
• focal mechanisms