Australian tsunami experts say the small tsunami triggered by an earthquake in New Zealand on Wednesday was the 41st to strike the "shaky isles" in the past 190 years, and demonstrated that our region is vulnerable to tsunamis.
The quake's epicenter was 160 kilometres west of Invercargill, 33kms below the sea floor and was followed by a second quake about 20 minutes later measuring 6.1. The US Pacific Tsunami Warning Center issued a tsunami warning lasting 2.5 hours after the first quake hit at 9:22pm New Zealand local time. The 7.8 magnitude tremor struck around the tip of New Zealand's South Island.
Australian tsunami experts from the University of New South Wales were conducting research in the area at the time of the event. The researchers, who monitored Wednesday's quakes, have assembled the world's most comprehensive tsunami database recording events dating back 5000 years.
The purpose of the database is to expand scientific understanding of tsunamis because written historical records are too recent to offer meaningful insights in to the magnitude and frequency of past tsunamis, or what could be expected in the future. Specifically, they are searching for matching records of tsunamis on both sides of the Tasman Sea in New Zealand and along the NSW coastline.
New Zealand's north and south islands lie on the margin of two colliding tectonic plates, the Pacific and Indo-Australian Plates. Earthquakes are common, particularly in the southwest of the South Island and in the central North Island.
This week's events raise important questions about the frequency, size and risks posed to exposed low-lying exposed coastal communities, and how best to protect them, according to Associate Professor Dale Dominey-Howes of the UNSW Australian Tsunami Research Laboratory.
"The historic record of tsunamis in Australia and New Zealand simply will not allow us to answer some of those important risk related questions," he says. "In geological terms, the written record of tsunamis in this region is just a blink of the eye. Therefore, to extend our knowledge and understanding beyond the 19th century we are looking to sources beyond written records."
For the past 15 years researchers have been examining the sedimentary evidence of past tsunamis in Australia and New Zealand. The UNSW scientists have supplemented this geological data with geo-morphological, archaeological and anthropological information to establish the most comprehensive national paleotsunami database anywhere in the world. Palaeotsunamis are tsunamis that occurred prior to the historical record of a region.
Prehistoric Maori oral histories are rich with accounts of what were most likely tsunamis. These oral histories have been interpreted in the context of geological and geomorphologic data that help to add depth to their meaning.
"For example, one Maori legend dating back to 1300AD tells of the Māhuhu canoe that landed at the mouth of the Kaipara where people settled at a place called Taporapora," says Dominey Howes. "The people lived here for many years but that one day ‘the place was shaved off by sea,' the land disappeared and ‘all were carried away by the sea.'"
"The nature of the event is difficult to determine from this account, but other elements of this tradition have been recorded. However, finding parallel records of events such these Maori legends will tell us something about region-wide tsunamis that simultaneously impacted both countries."
Dale Dominey-Howes, 0401 647 959
Dan Gaffney, UNSW science media, 0411 15 60 15