In 2012, a team of 27 scientists from ten countries undertook a 50-day expedition on a Japanese drilling vessel. Using specially designed deep-water drilling equipment, they drilled three holes in the Japan Trench area in order to study the rupture zone of the 2011 earthquake, which took place along a fault where the Pacific and North American plates meet.

The slip along the fault that took place in the Tohoku earthquake was the largest ever recorded, amounting to some 30–50 metres. This runaway rupture thrust up the seafloor, causing the tsunami.

The drilling results revealed a number of factors that helped to account for the unexpectedly violent slip between the two plates. First, the fault itself is very thin – less than five metres thick in the area sampled. ‘To our knowledge, it’s the thinnest plate boundary on Earth,’ said Christie Rowe of McGill University, one of the study’s authors. In contrast, California’s San Andreas fault is several kilometres thick in places.

Second, the clay deposits that fill the narrow fault are made of extremely fine sediment. ‘It’s the slipperiest clay you can imagine,’ said Rowe. ‘If you rub it between your fingers, it feels like a lubricant.’

A similar type of clay is present in other subduction zones in the northwest Pacific, suggesting that they may also be capable of generating huge earthquakes.

This story was published in the February 2014 edition of Geographical Magazine