By Jonathan Amos Science reporter, BBC News, Vienna

The 210m-long Chikyu is a brand new drill ship The scientists speak

It promises to be one of the grand scientific challenges of this decade.

Researchers are about to drill down into an earthquake zone at the Nankai Trough off the coast of Japan.

The project, which will cost hundreds of millions of dollars over the next 10 years, is being coordinated by the Integrated Ocean Drilling Programme.

It seeks to understand the causes of deadly quakes and tsunami by pulling up cores for study and by putting down sensors to monitor changes in the rock.

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) expects to get under way in September.

There is a strong need in the Japanese community to know what is going on under the sea floor Masataka Kinoshita, Jamstec

It is focused on a region of the sea floor that has been responsible for immense tremor events, including the 1944 Tonankai (Magnitude 8.1) and 1946 Nankaido (Magnitude 8.3) earthquakes.

"The place we are going to has a history of disastrous earthquakes and tsunami every 100 or 200 years; and these have resulted in the deaths of many people," explained Chief Project Scientist Masataka Kinoshita, from the Japan Agency for Marine-Earth Science and Technology (Jamstec).

"There is a strong need in the Japanese community to know what is going on under the sea floor," he told BBC News.

He was speaking here at the European Geosciences Union General Assembly meeting.

Real-time data

NanTroSEIZE is focused on an area of subduction, where a huge slab of Pacific oceanic crust is being pulled down under the continental rock on which Japan sits.

The drilling effort will attempt to cut directly into the faults that trace the grinding interface - the seismogenic zone.

The project has great relevance for Japan - but goes wider

"These are complex structures," said Cesar Ranero, from the Spanish research council, Icrea.

"They have areas that are very sticky - where the two plates are strongly mechanically coupled - and these are typically the areas where strong earthquakes nucleate.

"And they have other areas which are less sticky, but which may still be involved in the propagation of the quake. One of the goals of the project is to understand why an earthquake will rupture across a smaller or larger area, producing a smaller or larger earthquake."

Drilling operations will be conducted from a new 57,000-tonne ship, the Chikyu.

It will have to sit atop 2,500m of water, holding a steady position as the drill head cuts a series of holes up to 7km under the sea floor.

TECTONICS AROUND JAPAN (1) Philippine Sea Plate (2) Pacific Plate (3) North American Plate (4) Eurasian Plate The drilling (red dot) will cut to the interface of the Eurasian and Philippine plates How earthquakes happen

Core samples will be returned to the laboratory for study and the holes themselves will be instrumented to monitor changes.

Seismometers and tilt meters will follow how the rocks in the faulting zone are moving. The sensor package will also measure changes in temperature and pressure, and provide information to assess any geochemical reactions taking place.

"We'd like to know something about what fluids do at that position," said Nathan Bangs, from University of Texas at Austin, US.

"If there are a lot of fluids there that are under very high pressure, they can essentially act like lubricants and not allow stresses to build up. But if the fluids are not present, the rocks can build up big stresses that can eventually rupture as an earthquake."

Extreme environment

Data from the instruments will be stored by logging devices at nodes on the sea floor. Submersible vehicles will return year after year to retrieve the data packages, although it is hoped that eventually the nodes can be cabled, to bring the information straight on to shore and to the scientists' PCs.

The Integrated Ocean Drilling Programme (IODP) is aware that the venture is a risky one. The instruments will have to be built to survive searing heat, crushing pressures and corrosive fluids. They could fail at anytime.

There are places on the surface of the Earth that scientists can study the types of rocks being sought by the Nankai Trough experiment - but there is no substitute for seeing the true setting, researchers believe.

"To get those rocks up to the surface, they have been exhumed, which means, for example, that any fluids involved in an earthquake will have been lost," explained Paola Vannucchi, from the University of Florence, Italy.

"The process that brings these rocks to the surface from eight kilometres down is also a process of deformation, so you get one superimposed on the other. You don't get a pure signal."

As part of the IODP project, a second series of holes will be drilled off Costa Rica in Central America.

The scientists say the results from both experiments should be applicable to all major subduction zone earthquakes and related tsunami, as well as earthquakes generally.