Shore shapes made tsunami deadlier: study


The shape of the coastlines along Kamaishi Bay, Ishinomaki Bay and four other areas in Tohoku may have amplified the power of the tsunami that struck there last March, a study by tsunami researchers said Saturday.

A simulation by a group of researchers led by Fumihiko Imamura, professor of tsunami studies at Tohoku University, found that a “resonance phenomenon” may have increased tsunami heights at six places in Iwate, Miyagi, Aomori and Fukushima prefectures. Tsunami in those areas may have been as much as three times higher than those that ravaged other areas on the Pacific coast.

Tsunami can be amplified when their frequencies match certain features of the coast, such as shape and depth — especially where coastlines are deeply indented. This phenomenon occurred at Ofunato Bay in Iwate Prefecture, when tsunami triggered by a major quake in Chile hit the area in 1960.

Imamura and other researchers collected tidal data from wave recorders installed off the Sanriku coast and analyzed the frequencies and shapes of tsunami that struck the region last March.

They found the tsunami had two patterns — a short frequency of about 10 minutes with large movements, and a long frequency of about one hour with moderate movements — and replicated the walls of water in computer simulations.

The six locations also include coastal areas in the towns of Oirase and Hashikami in Aomori Prefecture, Yamada Bay in Iwate Prefecture and Onahama Port in Fukushima Prefecture.

In the city of Higashimatsushima in Miyagi Prefecture, west of Ishinomaki Bay, tsunami reached 10 meters, much higher than the previously predicted maximum of 4 meters for the area, destroying more than 70 percent of the houses in the area.

The researchers said, however, it is difficult to confirm whether the resonance phenomenon actually happened, as all the tidal observatories in the area were ravaged by tsunami last March.

Imamura warned that massive damage could be caused again by tsunami amplified by the phenomenon.

“Residents must flee right away and never return home, even if tsunami appear to have subsided,” he said.