Reconnaissance studies of landslides and other ground damage caused by the Mw7.2 Fiordland earthquake of 22 August 2003

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Hancox, G.T.; Cox, S.C.; Turnbull, I.M.; Crozier, M.J. 2003 Reconnaissance studies of landslides and other ground damage caused by the Mw7.2 Fiordland earthquake of 22 August 2003 . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2003/30 32 p.

Abstract: The Mw Fiordland earthquake of 22 August 2003 was located ~10 km northwest of Secretary Island at the mouth of Thompson Sound at a depth of ~20 km. It was the largest shallow earthquake in New Zealand for 35 years. Preliminary seismological and GPS data from the epicentral region after the event show that the earthquake did not occur on the Alpine Fault, but involved thrusting along the shallow part of the underlying subduction interface between the Australian and Pacific plates. The earthquake was strongly felt over much of Otago and Southland, and caused minor but sometimes spectacular damage throughout the region, with items thrown off shelves in Te Anau and Queenstown, and minor cracking to some concrete structures. By the end of September 2003, more than 2100 claims for damage had been received by EQC. Significant landsliding was reported in the mountains west of Te Anau, with minor damage along SH 94 between Te Anau and Milford Sound, and the Wilmot Pass road from Lake Manapouri to Deep Cover was closed by a slip. The main earthquake was followed by a large aftershock of ML 6.2 and more than 20 aftershocks greater than magnitude 5 were recorded over the next 4 weeks. The reconnaissance studies have revealed some interesting relationships between landslide development and slope aspect and steepness, with preferential failure on north and west facing slopes clearly indicated. Further analysis of the landslide distribution using GIS is recommended to explore the relationships of landsliding to slope aspect and slope angle in more detail. There is also merit in carrying out further studies on the very large prehistoric landslides in Fiordland, including more systematic mapping, analysis and dating of the landslides to determine their palaeoseismic significance. (auth/DG)