New Zealand Journal of Geology and Geophysics abstracts
The 2000 Thompson Sound earthquake, New Zealand
Russell Robinson
Terry Webb
Peter McGinty
Jim Cousins
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
D. Eberhart-Phillips
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, 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 x
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
G02035; Received 4 July 2002; accepted 6 May 2003; online publication date
10 September 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 331-341
0028-8306/03/4603-0331 $7.00/0 © The Royal Society of New Zealand
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