Friday, August 12, 2016
Tsunami and Tupaia
Tupaia, the great Tahitian high priest and navigator, testified to Captain Cook that Polynesians had sailed great distances from Tahiti, and history tells us that one of those great distances involved the 3,000-kilometer voyage from the cradle of Polynesia to New Zealand. In the meantime, however, the lore had been lost -- when Tupaia arrived in New Zealand in 1769, no one from Tahiti had been there for the past three or four hundred years.
Something had happened. But what?
According to geological theory, the east coast of New Zealand had been struck by a huge tsunami sometime in the fifteen century. An immense wall of water, probably 12 meters high, had struck the coast and roared inland, leaving devastation in its wake. Gardens, plantations, house, and fishing beds were destroyed -- along with the massive voyaging canoes.
Great men had died -- the craftsmen, the shipwrights, the priests, and the star navigators. Voyages back to Hawaiki -- Raiatea -- became impossible, because not only the craft, but the lore had been lost.
Or so the theory went. Now, it has been demonstrated that the theory is right. The tsunami really happened. An article in the Dominion Post today describes it.
A new study shows the East Coast was struck by three or four large tsunamis as high as twelve metres over the past 1200 years.
The GNS Science study which involved the digging of a 90metre long, two-metre-deep, trench at Puatai beach, 30km north of Gisborne, revealed shells and wood that could be radiocarbon dated to establish the dates of numerous earthquakes and tsunamis.
The tsunamis produced waves between nine and twelve metres high and would have travelled up to hundreds of metres inland. The last tsunami occurred about 300 years ago.
"The study provides the first really concrete evidence of what we have long thought likely, and it shows that hazard preparation in the east coast area is on the right track," said lead author Nicola Litchfield.
"The tsunamis would have affected the entire East Coast of the North Island. It is hard to estimate from just this one study how big a particular tsunami would have been at various places along the coast. However, we know from past fieldwork and from modelling that 5metre-plus tsunamis can strike anywhere along the East Coast," Litchfield said.
It has been known for a long time that the area is seismically active and that there is a high risk of tsunamis, but hard evidence about tsunamis has been rare. The study provides solid evidence that the tsunamis occurred.
The investigation showed there were three large earthquakes in the region around 1800, 1200 and 400 years ago. Each quake was around magnitude 7.2 and uplifted coastal land by about 3.5metres.