Coastal deformation and tsunami deposit observations following the July 15, 2009, Mw 7.8 Dusky Sound earthquake

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Wilson, K.J.; Litchfield, N.J.; Turnbull, I.M. 2009 Coastal deformation and tsunami deposit observations following the July 15, 2009, Mw 7.8 Dusky Sound earthquake. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/46 61 p.

Abstract: A MW 7.8 earthquake occurred at Dusky Sound, southwest Fiordland, New Zealand, at 9.22 pm on the 15th July, 2009. The earthquake was the largest earthquake in New Zealand for the past 80 years and the most significant subduction earthquake along either of New Zealand’s subduction margins to have occurred in modern history (post-1840 AD) Eyewitnesses reported a significant tsunami in Dusky Sound shortly after the earthquake and preliminary models suggested there may have been uplift and subsidence of the upper plate. This report details a reconnaissance trip undertaken 19-21st August, 2009, to assess the geological evidence of tsunami inundation and coastal deformation in Dusky Sound. We also recorded some new landslides and checked whether active faults in the area had any surface rupture. Seven locations around Dusky Sound were checked for evidence of coastal deformation and tsunami inundation. No evidence of coastal uplift was seen. In general we saw possible evidence for 10 – 20 cm of coastal subsidence. Evidence for this came from the inundation of coastal flora by 10 – 20 cm of water at spring high tide levels. We cannot be certain that this was due to subsidence because some of the plants are salt-tolerant and we do not have suitable calibration data to be sure that they cannot normally live in conditions with 10 – 20 cm of sea water inundation. Further monitoring of the area in future months to check if there is vegetation die-off may resolve this. Evidence for tsunami run-up was seen at Passage Point, and probable evidence was seen at Goose Cove. At Passage Point fish, shells, gravel and subtidal-dwelling-starfish were deposited 1.16 m above and 8 m inland of the spring high tide line, suggesting that the tsunami at this location was at least 2.3 ± 0.2 m in height. At Goose Cove, flattened pingao (golden sand sedge) up to 0.4 m above spring high tide on the barrier was probably bent over by currents associated with the withdrawal of the tsunami from the lagoon. The nature of the Passage Point tsunami deposit suggests that the presence of loose, unconsolidated material on the beach face is an important requirement for leaving a tsunami deposit. Along the rocky coastlines that are prevalent in Dusky Sound there is little loose material to entrain, hence no deposits remain. Where tsunami deposits were emplaced they are composed of relatively fragile material and are unlikely to last long, therefore reconnaissance soon after the event is important. Several new landslides were recorded from Doubtful Sound to Dusky Sound. During helicopter flights over three active fault scarps within 12 km of the earthquake epicentre we did not see any evidence of recent surface rupture implying that the subduction interface earthquake did not trigger secondary slip along the nearby upper plate faults. (auth)