For the last few years, a group of international scientists has been reassessing the cause of the March 2011 tsunami that inundated the Tohoku coastline.

The common view among seismologists is that the tsunami was caused solely by the magnitude-9.0 earthquake that struck in the Pacific Ocean about 150 km off the coast of Miyagi Prefecture.

But a group of scientists led by Dave Tappin, a professor of marine geology at the British Geological Survey, believes the explanation is more complex.

They now think that the earthquake also triggered a large slump in the sediment of the seabed and this contributed to the force of the tsunami.

Tappin believes this explains why the run-ups (height of the tsunami wave at the coast) reached up to 40 meters along the Sanriku coastline in Iwate Prefecture.

Tappin noted that one interesting aspect of the tsunami was the differing height of waves along the coastline.

On the Sendai plain, which was closest to the epicenter, the waves reached between 10 and 15 meters high. However, further north on the Sanriku coastline, the waves were considerably higher, he said.

Tappin’s team initially carried out modeling of the tsunami using the epicenter as the sole source.

However, they were unable to replicate the high waves seen on the Sanriku coast, even though many people argued that the coastline’s narrow inlets contributed to the height of the waves generated by the tsunami.

The team suspected a contributing factor for the high waves could be a sediment slump on the seabed.

So they looked at data from offshore pressure sensors along the Sanriku coast to examine the tsunami waves.

Using a rather complicated procedure, they judged that the structure of the waves (their frequency content) suggested they were coming from two sources instead of just one.

The team located a massive sediment slump in the seabed about 100 km north of the epicenter that, they claim, is responsible. It is also due east of the high run-ups seen on the Sanriku coast.

The slump looks like an amphitheater or crater. Tappin describes it as if “someone has taken a huge bite out of the seabed.”

It measures about 30 km by 20 km and is about 2 km thick. Inside that area the sediment has slumped “several hundreds” of meters, said Tappin. Records indicate the slump appeared after the earthquake.

Tappin said: “Our conclusion is that the majority of the tsunami in the south (along the Sendai plain) was definitely generated by the earthquake, but because the earthquake is to the south of where the high run-ups are, we suggest there’s an additional source. And our slump is on the northern margin of the earthquake rupture, and so we are suggesting that part of the tsunami, particularly that part on the Sanriku coast, is generated by a submarine slump or landslide (as they are also known).

“We are confident (of our claims) in the context that all present models of the earthquake do not explain those high run-ups in the north. A reasonable alternative suggestion is that it’s a submarine slump, but all these things we never know for certain.”

Tappin, who advises the British government on marine hazards, first presented his claims last year at a scientific gathering in the United States. He concedes that not everyone in the academic community shares his analysis.

He is preparing to submit his case for publication soon in a scientific journal.

Tappin hopes his findings will act as a wake-up call to governments across the world about the importance of submarine slumps. He hopes his research will ensure that they are factored into any future tsunami-mitigation measures.

Over the last 15 years, scientists have become more aware of the effects of submarine slumps or landslides. A submarine landslide in 1998 in Papua New Guinea caused a tsunami that killed 2,200 people, said Tappin.

“It was thought unlikely that a submarine landslide would create a tsunami killing over 2,000 people; the similarity with Tohoku is that this could well be a seminal event where we have to revise our understanding of the generation mechanisms of a tsunami,” he said.

The British Geological Survey is a government-funded body that provides expert services on geoscience. Tappin received funding from the National Environment Research Council to visit Japan. His team is made up of engineers, oceanographers and a seismologist from a number of countries, including the United States and Japan.