The Fiordland earthquake of 10 August, 1993
A reconnaissance report covering tectonic setting, peak ground acceleration, and landslide damage
On 10 August, 1993, a ML 6.7 (Ms 7.1, Mw 7.0) earthquake occurred c. 10 km offshore of western Fiordland, New Zealand (45.2° S, 166.7° E). Its hypocentre is approximately 20 km deep which places it on, or close to, the interface between the subducted Australian plate and the overriding Pacific plate. The focal mechanism for the mainshock indicates reverse faulting on either a steep west-dipping, or shallow southeast-dipping plane. Analysis of a subset of the over 7,000 recorded aftershocks defines a shallow (c. 15°) southeast-dipping plane, roughly 25 km long and 15 km wide, that probably represents the rupture surface of the mainshock.
The earthquake was strongly felt by fishermen offshore in the epicentral region. Onshore, there were no reports of damage to man-made structures. The maximum peak ground acceleration recorded was 0.08 g at Te Anau, about 73 km from the epicentre. The attenuation of peak horizontal ground acceleration for this event is similar to the attenuation of other shallow crustal earthquakes in New Zealand.
The number of landslides triggered by this event is at least an order-of-magnitude less than the number of pre-existing landslide scars. The highest concentration of new slides appears to be in the Vancouver Arm/Hall Arm region, c. 45 km south-southeast from the epicentre. Many of the new slides were narrow, shallow seated failures, or small reactivated portions of older slides. The two largest earthquake-triggered landslides observed are located near Hall Arm, and in the Freeman Burn north of Lake Manapouri. Except perhaps for these two slides, all other observed earthquake-triggered slides will be indistinguishable from storm-generated slides once re-vegetated.
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Copyright (c) 1994 R. Van Dissen, J. Cousins, R. Robinson, M. Reyners
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