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Precursors of the Great Tohoku Earthquake found

Scientists think they have discovered traces of several earlier quakes of similar size to the magnitude 9 Tohoku earthquake that hit Japan in March 2011.

A Japanese-German team studying sediment cores taken off Japan’s east coast reported this and other preliminary findings of their four-week research cruise aboard the German research vessel the Sonne at the European Geoscience Union’s annual assembly this week in Vienna.

The team recovered 16 cores, of 95 metres in total length, along the bottom of the 7,000 metre-deep Japan Trench. The cores seem to contain striking fingerprints – including microfossils normally not found at such depths, and sections of chaotically disturbed sediment layers– of at least three quake-triggered huge submarine landslides which may have moved large amounts of sediment from shallower waters into the trench.

“It was a most exciting moment when we opened the cores on board and I saw under the microscope that there are coccoliths in the sediment,” says Michael Strasser, a geologist at the Swiss Federal Institute of Technology in Zurich who was on board the Sonne. These calcifying algae would normally dissolve long before they reach the deep sea.

The evidence of the past events appears consistently across the 16 samples, Strasser says. But he warns that the cores have not yet been dated and he would not speculate on when any of the previous quakes and landslides might have occurred.

The results, together with ongoing research on paleo-tsunami deposits on Japan’s Sendai plain, will shed light on the recurrence interval of large Tohoku-type quakes in the Japan Trench, says Kiyoshi Suyehiro, president of the Integrated Ocean Drilling Program.

Meanwhile, a research team on board the Japanese drilling ship CHIKYU has begun to drill into the fault zone in the Japanese Trench and measure frictional heat there.

“Only drilling can provide information on the nature of the extraordinary fault slip that has occurred along the rupture zone,” says Suyehiro.

To reach the fault zone, the team plans to drill more than 1,000 metres into the seafloor off the Oshika Peninsula, at a location 7,000 metres underwater. Recovering a piece of the fault that slipped 30 to 50 metres during the Tohoku quake, triggering the deadly tsunami, should help researchers understand the quake’s elusive rupture mechanism.

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