Surface effects and geological observations following the 1988 Te Anau and 1989 Doubtful Sound earthquakes, Fiordland, New Zealand

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Forsyth, P.J.; Turnbull, I.M.; Beanland, S.; Thomson, R. 2006 Surface effects and geological observations following the 1988 Te Anau and 1989 Doubtful Sound earthquakes, Fiordland, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/29 29 p.

Abstract: The MW6.7 Te Anau earthquake of June 1988 was centred beneath central Fiordland at a depth of about 60 km. A report of felt intensity of MM9 was recorded at Manapouri. The earthquake was felt from the southern South Island to Westport and Wellington. Earthquake effects observed immediately or soon after the event included slumping of river and stream deltas into Lake Te Anau, liquefaction (sand volcanoes, sand boils), landslides from steep slopes, and minor damage to structures such as huts and jetties. Ground accelerations were estimated (from slumping of gravel stockpiles alongside roads) at not much greater than 0.1 g. Two surveys of earthquake effects were filed but never published. Another moderate (MW6.4) earthquake occurred in May 1989, and was centred near Doubtful Sound. It was shallower but smaller than the 1988 event, and caused less damage. A limited survey of ground effects was carried out but not published. A much larger earthquake in 2003 (August 22, Secretary Island, MW 7.2) caused slumping, landslides and property damage. However, liquefaction and delta slumping were not noted to the same extent as in 1988. Reports of investigations into this event have been published elsewhere. Delta collapse, liquefaction and landsliding may be expected from future moderate and large earthquakes. The severity of damage will depend on the type of earthquake, distance from the epicentre, and site characteristics such as slope, soil/rock type, and saturation of soil. It is recommended that GNS Science and GeoNet staff review the systematic earthquake response programme carried out after moderate or large earthquakes. This should include surveys for surface effects, and routinely re-inspecting sites which have experienced damage in previous earthquakes, to improve knowledge of failure parameters. The observations in this report should be made available to relevant organisations including Department of Conservation and district and regional councils. (auth)