March quake linked to active seabed fault

400-km rift off Pacific coast also puts Hokkaido at risk, study says


A new study indicates the Great East Japan Earthquake may have been caused by an active fault in the seabed — a finding that contradicts the prevailing view that such faults aren’t directly involved in ocean trench temblors.

A group of researchers from Japanese universities including Toyo University and Hiroshima University has concluded that a 400-km fault line off the northeast coastline may have been responsible for the magnitude 9.0 earthquake that devastated the region March 11.

“To make quake predictions more realistic, information regarding active faults in the seabed should be taken into account,” said Takashi Tanaka, a professor emeritus at Hiroshima University, who participated in the study.

The catastrophic quake — which with the tsunami it spawned left around 15,800 people dead and more than 4,000 missing — has been categorized as an ocean trench earthquake. These types of temblors are believed to be caused by the shifting of crustal plates deep below the seabed.

According to the study, the epicenters of quakes recorded in the area since the latter half of the 19th century closely match the locations of the fault line that stretches offshore from Iwate to Ibaraki prefectures, the researchers said.

The researchers also believe another active fault off Hokkaido could cause a massive temblor in the future.

The study says no major temblors have been recorded in the area along the newly discovered 120-km fault line off the east coast of Hokkaido, suggesting that a powerful quake with a magnitude of around 8 could occur there.

The findings were reported to the Association of Japanese Geographers on Saturday.