Newly discovered tsunami deposits suggest the Japanese coastline was hammered by a series of massive waves thousands of years ago. The finding adds to growing evidence that the region is regularly pounded by killer waves, and could help in planning for future inundations.
The northern Japanese island of Hokkaido is nestled up against the Kuril-Kamchatka trench, a place where the Pacific tectonic plate dives beneath the Eurasian plate, and home to terrible earthquakes in excess of magnitude 8.0.
Now Wesley Nutter and a team of researchers say nine waves, each at least 33 feet high, battered the coastline before the dawn of civilization on the island.
"In recorded history, tsunamis have hit the Hokkaido coast over and over again," Wesley Nutter of Earlham College in Indiana said. "But something of that size has never been recorded here."
Nutter and a team of researchers dug down into the sediments of a saltwater marsh on the island looking for signs of past tsunamis. Team member Kazuomi Hirakawa of Hokkaido University had first noticed a series of sand deposits several years ago there that had no business in a marsh mostly made of peat.
Tracing the sand deposits away from the coast, the team found they extend up to more than a mile inland and get thinner further from the sea.
In theory, huge storm surges could have deposited the sand, but a tempest with a 13-foot surge raked the region several years ago and left no sign of its passing in the marsh, which is protected by 33-foot-high cliffs.
Nutter believes the deposits have tsunami origins. And they must have been big: In 2003 a magnitude 8.3 earthquake in the Kuril trench generated a wave 13 feet high, not nearly enough to reach the marsh.
The deposits also seem to repeat every 500 years or so, suggesting the Kuril is capable of regularly ripping off huge earthquakes that could have devastating results.
"The new research should help define the inundation hazard that the tsunamis pose," Brian Atwater of the United State Geological Survey said in an e-mail to Discovery News. "The research may also lay groundwork for improved estimates of the size and recurrence intervals of the associated earthquakes."
In a paper published last year, Atwater pointed out the Japanese government has already recalculated the tsunami hazard based on the team's initial results. In the case of an extraordinary earthquake, the resulting tsunami could destroy 5,600 homes and kill 850 people, even though the country has an advanced tsunami warning system in place.