The 2000 Thompson Sound earthquake, New Zealand

Abstract

The Mw 6.1 Thompson Sound earthquake occurred on 1 November 2000, with an epicentre near the Fiordland, New Zealand, coastline (–45.112^°, 166.952°). Aftershocks, recorded on temporary seismographs as well as the National Seismograph Network, define a 12.5 × 12.5 km planar zone, taken as the mainshock fault, with a strike of 175° and dip of 65°W, ranging in depth from c. 12 to 24 km. This is in accord with the mainshock focal mechanism determined by a body‐wave inversion that indicates a rake of c. 59°, that is, mainly thrust motion with a component of left‐lateral strike‐slip. This event follows a series of moderate to large earthquakes in the Doubtful Sound region: Te Anau, 1988, Mw 6.7, depth 60 km; Doubtful Sound, 1989, Mw 6.4, depth 24 km; Secretary Island, 1993, Mw 6.8, depth 22 km. The Secretary Island event was a thrust event near, or on, the subduction interface, with a dip of 30–40°SE, and our interpretation is that both the Doubtful and Thompson Sound events were oblique thrusts (with a left‐lateral component) above the interface and shoreward of the Secretary Island earthquake. The Coulomb failure stress induced by all three large events prior to the Thompson Sound event would have loaded closer to failure faults such as that at Thompson Sound. Together, the four events induced a pattern of Coulomb failure stress on the nearby Alpine Fault that varies in sign with depth. Overall, little can be said about potential triggering or bringing forward/retarding of a large Alpine Fault event. Strong motion recordings for the Thompson Sound event are few, but peak accelerations are in accord with existing attenuation relationships.

Keywords:

• Thompson Sound
• Fiordland
• earthquakes
• aftershocks
• subduction
• stress interactions
• triggering