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An expedition is getting under way in the South Pacific to investigate one of the most seismically-active fault lines in the world.

Researchers are planning to study the Tonga Trench - a deep feature where the Pacific tectonic plate is being forced under the Indo-Australian plate.

The island nation of Tonga is regularly hit by tremors - most recently a 6.4 magnitude quake offshore last month.

The research expedition will last about one month.

The focus of the study will be an unusual zone on the seabed where undersea volcanoes are being dragged into the fault.

Scientists want a better understanding of how the submarine mountains affect the likelihood of earthquakes.

The volcanoes lie on the 4,000km-long Louisville Ridge and either act as a brake on the Pacific plate - or intensify the quakes which follow.

The area where they are pulled into the seabed suffers relatively fewer tremors than other stretches of the fault line.

The study - funded by Britain's Natural Environment Research Council (Nerc) - will carry out surveys and develop 3D models during seven transits of the region.

One of the lead scientists on the expedition, Professor Tony Watts of Oxford University, told ´óÏó´«Ã½ News:

"We want to know whether subducted seamounts are holding up earthquakes or whether they cause earthquakes.

"This is important to find out so that we can learn what controls earthquakes and make better assessments about where they may occur in the future."

Subduction zones like the Tonga Trench can trigger tsunamis - as happened off Japan last month and off Sumatra on Boxing Day 2004.

One recent study of an earthquake in Peru in 2001 showed that underwater mountains may have held up the quake for 40 seconds before rupturing.

A study of the Nankaido earthquake in Japan in 1946 successfully imaged a seamount that had been dragged 10km deep - and apparently limited the scale of the rupture and the tsunami risk.

According to Professor Watts, more data is needed on the deep structure of the Tonga Trench to understand the forces at work.

"We need to know whether the seamounts are more or less intact as they are carried into the trench or have been damaged or decapitated.

"If we find that there is a link between seamounts and earthquakes then imaging of the seafloor will put us in a much better position to understand future quakes and tsunamis."