Understanding local source tsunami: 1820s South Island tsunami

Southland lies adjacent to an offshore section of the plate boundary between the Australian and Pacific tectonic plates. The plate boundary zone consists of numerous active faults that could potentially rupture the seafloor in large earthquakes and trigger tsunami with devastating consequences for the Fiordland and Southland coasts. However little is known of the frequency and size of earthquakes likely to be generated in this southern part of New Zealand’s plate boundary, and even less is known of the potential for tsunami to be triggered in this area. The historical record provides some indication that such events are a real threat for Southland and Fiordland. Historical reports of a tsunami killing a group of Māori on the beach near Orepuki in the 1820s and large earthquakes and a possible tsunami occurring in Fiordland in 1826 are investigated in this report.

Further assessment of historical records and collection of additional reports suggests the 1820s Southland tsunami and the 1826 Fiordland earthquake are separate events so the exact date of the 1820s Southland tsunami remains unknown. The source of this tsunami also remains unknown but modelling indicates it is more likely to have been a Puysegur subduction zone earthquake than a Fiordland subduction zone or Alpine Fault earthquake. The effects of the 1826 earthquake are considered consistent with a large Fiordland subduction zone event that caused uplift of the coast near Doubtful Sound, extensive landsliding and possibly a local tsunami.

Physical evidence of these events was searched for at three sites on the Fiordland coast. At Cascada Bay near the mouth of Doubtful Sound, a series of raised bedrock platforms and recent revegetation of an 8 m high surface could be the result of tectonic uplift occurring in previous large earthquakes and vegetation stripping in a past tsunami. However further investigation of these features and confirmation of their age would be required before attributing them to the known historical events. Evidence of older earthquakes and / or tsunami appears to be preserved in sedimentary sequences at Martins Bay in northern Fiordland and at Goose Cove in southern Fiordland. Therefore, although the steep topography, high rainfall and strong wind and wave regime of Fiordland make it a challenging place for any evidence of past earthquakes and tsunami to be preserved, there is potential onshore to derive further information about behaviour of the offshore faults.

Three sections of the plate boundary considered most likely to be tsunamigenic were modelled: the offshore section of the Alpine Fault, the Fiordland subduction zone and the Puysegur subduction zone. Models were developed for the predicted deformation of the sea floor in a large earthquake on each of these structures (elastic dislocation models). Tsunami propagation models were used to predict the behaviour of the resultant tsunami from generation to impact. The offshore section of the Alpine Fault is considered capable of rupturing in a magnitude 7.8 earthquake, which would trigger a tsunami with water elevations at the shore of at least 4 m and maximum impact between Milford Sound and West Cape. The Fiordland coast also endures the greatest impact (water elevations1 at the shore of 4 m) from a tsunami triggered by a Fiordland subuction zone earthquake of magnitude 7.7-7.9. The Southland coast is most vulnerable to earthquakes in the Puysegur subduction zone where an earthquake of magnitude 8.5-8.6 is considered possible and would trigger a tsunami with water elevations at the shore of 4 m at numerous points along the Southland coast.