Tsunami impacts Pacific culture history
Tsunami researchers are urging archaeologists working around the Pacific to look for indications of the impact of past big waves, following the widespread devastation caused by last year's Japanese tsunami and earthquakes.
The suggestion comes from Victoria University archaeologist Dr Bruce McFadgen and Dr James Goff, Professor of Tsunami Studies at the University of New South Wales.
They, and two other Australia-based colleagues, give the message in an article recently published in the journal The Holocene.
McFadgen has previously proposed a theory that tsunamis in the 15th century had a severe impact on Maori coastal populations.
The new article says the March 2011 Japanese event was the largest of several tsunamis in the Pacific in recent years.
A growing database of paleotsunamis suggests recurring events in the Pacific of a similar size, or larger, to the recent tsunami, the article says.
That evidence provides an opportunity to re-evaluate hypotheses used to explain the punctuated history of human settlement patterns across the Pacific.
In particular, the almost two millennia "long pause" in eastward migration starting about 2800 years ago, and the abandonment of long distance sea voyaging in the 15th century, may both be related to paleotsunamis.
We do have tsunami deposits in Pacific Islands that date to the correct time, Dr Goff, formerly a Niwa group manager, says.
Despite that, the potential influence of paleotsunamis on Polynesian prehistory has received little attention from archaeologists, the article says.
Considering Polynesians were a largely coastal people who relied on boats for travel, that was a significant gap in the understanding of the region's cultural past.
Some researchers considered rapid-onset natural hazards such as earthquakes and tsunamis to be too localised in space and time to be important.
In their view, such events added a layer of complexity to Pacific human history, but did not have the widespread long term impacts associated with climate change and human-caused modifications.
But the article argued that tsunamis could have caused the end of sea voyaging until people recovered skills and the inclination to make long distance voyages again.
"Given the region-wide impacts of historical tsunamis and the archaeological evidence of their effects, a revision of the view of tectonic hazards as an agent of change in the human history of Polynesia would be timely," the article says.
McFadgen said many scientists had taken seriously the idea that tsunamis could have had a major impact on Pacific communities, but many also did not.
Much more work needed to be done to test the proposal, he said.
"It may turn out to be an absolute ‘no no’, that there's nothing supporting it at all. Until we look, we aren't going to find it."
He hoped that when people were doing archaeological work in the Pacific they at least considered the possibility that coastal sites would be within the range of tsunami, and there may be tsunami evidence at the site.
While tsunami evidence could be extraordinarily difficult to find, if researchers persisted they could uncover something.
Dating tsunami to the period the authors were interested in - the 15th century - was also hard.Radio carbon dating was the only technique currently available, but it had a quirk which tended to cluster dates, McFadgen said.
It was known that between 1300 and 1500, the early part of the pre-European Maori period, there had been a tsunami at Houhora which had a height of about 32m, and which penetrated at least 800m inland.
Dating it was tricky, but he thought it was later rather than earlier in the period, probably sometime in the 15th century.
It was also known a tsunami struck the southeast Wairarapa coast, penetrating inland some 300m to the back of the coastal platform where it ran up against cliffs. That was probably in the 16th century.
The evidence for those two events is pretty clear, while in some other cases there was second tier evidence, McFadgen said.
An example of that was D'Urville Island where it appeared there had been a tsunami in the 15th or 16th century. Such an event was well recorded in Maori traditions.
Other work McFadgen has under way includes an investigation into the possibility of uplift and subsidence events along the Kapiti Coast.
If those hypotheses survived the test of future archaeological investigations it would strongly suggest significant seismic events on the coast had influenced Maori occupation, he says in an earlier article.
Uplift could explain the finding in middens at Raumati of sub-tidal tuatua, which lived from low tide down to about four metres below low tide and were rarely found in archaeological sites.
Sites along the Kapiti Coast also indicated a rise in the groundwater table, possibly around the end of the 15th century or in the early 16th century, which was possibly a result of subsidence.
There were indications of subsidence at Pekapeka about the same time as subsidence resulted in the inundation of archaeological sites at Porirua Harbour 32km to the south, the article said.
"If the Porirua inundation is related to the subsidence at Pekapeka, and to the rise in groundwater on the Kapiti Coast, then the event was very widespread, and significant."
Tsunamis are also a possibility. At least one is recorded for the Pre-European Maori period on Kapiti Island “although its effect on the mainland is currently unknown” the article says.
Considering the large numbers of people now living in the coastal zone, not only on the Kapiti Coast, but elsewhere in New Zealand, “there is… an opportunity to find out a lot more about past seismic events at the coast by recording and interpreting the evidence contained in archaeological sites and in their surroundings,” the article says.
“The more that is known (about their effects on past Maori communities), the better prepared future communities will be”.