SHIMIZU, Japan (AFP) - A Japan-led team of seismologists set off on Friday on a mission to drill deep beneath the seabed in a search for the origin of earthquakes.
The scientists weighed anchor on Japan's deep-sea drilling vessel Chikyu, heading for a spot in the ocean off the Kii peninsula, south-western Japan, and a fracture in the Earth's crust known as the Nankai Trough.
Experts have warned the trough, which marks the place where the Philippine Sea plate slides under the Eurasian plate, is the likely source of a monster earthquake sometime in the near future.
Japan's government last year unveiled a worst-case scenario, warning a big quake in the area could kill over 320,000 people, dwarfing the March 11, 2011, quake-tsunami disaster.
In its four-month mission, the latest stage of a multi-year project that began in 2007, the team plans to drill 3,600 metres down and take samples from the crust.
They will also be readying for another trip next year in which they hope to get 5,200 metres down, to the spot where the action actually happens.
"It would be unprecedented to drill directly into a seismogenic zone, the area believed to release great energy and cause crusts to slide along fault lines and trigger tsunami," said Tamano Omata, a researcher for the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
Scientists want to plant sensors - such as seismometers, deformation-measuring devices and thermometers - in the zone, that will form part of a system called Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), which is linked directly to onshore monitors.
"We expect to become able to monitor how the crusts move immediately before a quake hits," Mr Omata said.
Shinichi Kuramoto, deputy director of JAMSTEC's Center for Deep Earth Exploration, said recent research has shown mild earthquakes, in which the two crusts slip gently past each other, have occurred frequently over stretches of the Nankai Tough in the past five years.
He said it was possible these were precursors to a mega-quake.
"Directly drilling into and observing the place that may release a big quake would be a big step towards understanding the seismological mechanism," he said.
The 56,752-ton Chikyu - "Earth" in Japanese - has been anchored in central Shimizu port, and was open to foreign press this week ahead of the mission.
The vessel, built in 2005 at a cost of US$500 million (S$634 million), is equipped with a 121-metre drill tower that can descend 7,000 metres below the seabed, nearly three times as deep as its predecessors.
It depends on satellite location systems with pinpoint accuracy that allow its captain to know exactly where the ship is in relation to the Earth's crust.
Seismically-active Japan experiences 20 percent of the world's major earthquakes every year.
Building standards are high and its people are well-practised at taking cover when quakes strike, meaning damage and death tolls are often much lower than in other parts of the world.
But its proximity to major tectonic faults means the risk is ever-present.
On March 11, 2011, the northeastern region was hit by a 9.0 magnitude quake, which triggered a huge tsunami.
More than 18,000 people were killed when the waves swept ashore. They also swamped cooling systems at a nuclear plant in Fukushima, sending reactors into meltdown and sparking the world's worst atomic accident in a generation.