A research team has found evidence of a series of tsunami caused by earthquakes concentrated in the Tokai region inside layers of earth below a lake in Owase, Mie Prefecture, the team said Saturday.
Yoshinobu Tsuji, an assistant professor at the Earthquake Research Institute of the University of Tokyo, said his team discovered evidence that seawater and pieces of wood were carried by tidal waves into Lake Oike on at least nine occasions as far back as 2,500 years ago.
Tsuji and other researchers will make a presentation on the discovery at the 2001 Japan Earth and Planetary Science Joint Meeting, which begins Monday in Tokyo.
The traces, including some from a prehistoric era, will play a key role in revealing the history and scale of cyclical earthquakes, which are generally believed to occur every 100 to 150 years.
The lake’s circumference is about 2 km and its depth is 2 to 4 meters. It is isolated from streams, tidal waves and human activities because of its mountainous surroundings, researchers said.
The research began last year after the discovery of two lower points in the lake that are thought to provide an entry point for seawater in the event of a huge tidal wave.
Researchers discovered nine different layers where sand, wood, shells and seawater were carried. The layers were found in samples taken from a humus-decay accumulative layer about 2.5 meters below the lake bottom, they said.
Of the nine layers, the ages of wood in six layers were ascertained by radiation. They were from around 1297, 1023, between 695 and 772, 160, 1 and 530 B.C., they said.
The researchers then identified a possibility that wood found in the 1297 layer was carried into the lake by a tidal wave caused by earthquakes that occurred in either 1361 or 1498, and that wood from the 1023 layer was carried by a tidal wave caused by a 1096 quake.
There were, however, no tsunami traces left by a 1944 Tokai region cyclical quake. The researchers said that the quake may not have been big enough to cause a tidal wave.
But traces of larger Tokai quakes that occurred in 1707 and 1854 were not found. The researchers said that the tidal waves caused by those quakes were probably too strong and scraped the layers away.
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