First contact to earthquake zone

[AndrewA]

Scientists have completed the first stage of an ambitious plan to drill down into an earthquake-generating region near Japan.

The project saw holes bored 1.4km into the sea floor, producing 3D images of stresses inside the quake zone.

The Nankai Trough produced major lethal earthquakes and tsunami during the last century.

The eventual aim is to place instruments 6km deep in the crust, possibly as an early warning system.

Findings from the initial phase of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) were presented here at the European Geosciences Union (EGU) annual meeting.

"Maybe it's a bit obvious after the 2004 tsunami why we're interested in doing this," said the project's co-chief scientist Harold Tobin.

"The Sumatra quake is a good example of the fact that the greatest quakes on the planet happen in subduction zones, where one tectonic plate is sliding beneath another.

"And virtually all of the big quakes, the ones of magnitude eight or nine or above, happen at sea; so we have to go to sea to study the plate boundaries, the actual faults, that generate those earthquakes," the University of Michigan researcher told reporters.

The Nankai Trough off the south-eastern coast of Japan, centre of the 1944 Tonankai and 1946 Nankaido earthquakes which both exceeded Magnitude 8, is structurally similar to the Sunda Trench where the 2004 tsunami originated, he added.

Fault in vision

The five months from last September saw eight holes drilled to various depths in the Nankai Trough, using the new Japanese research vessel Chikyu.

The drill bits travelled through the "megasplay zone", the region above the actual subduction path, which is riddled with faults.

Three-dimensional scans performed aboard Chikyu on cores taken from these boreholes reveal some of the stresses that the rock is placed under as the Philippines tectonic plate descends underneath Japan.

"We use this medical CT (computed tomography) scanner that scans through core samples instead of the human body," said co-chief scientist Masataka Kinoshita from the Japan Agency for Marine-Earth Science and Technology (Jamstec), which runs operations on behalf of the international Integrated Ocean Drilling Program (IODP).

"This gives 3D density images, just like in the human body we would see brain or bones or other things."

Most of the stress lines, as researchers anticipated, lie along the direction that the plates are travelling.

But one core showed strong stress lines at 90 degrees.

Mapping these stress lines could indicate which portions along the subduction zone are "locked" - where the descending plate snags and sticks to the one above - and which are loose and able to move freely.

That in turn would set some parameters for the likely scale of a tsunami, which is believed to occur in this region when the forces on the snagged upper plate grow too large and the plate snaps upward violently, releasing its accumulated energy into the water above.

The first set of boreholes also broke through into the "accretionary prism", where material such as mud, water and loose rock is carried downwards by the descending Philippines plate.

Some of the cores revealed evidence of past undersea landslides.

Deep plans

This first phase of NanTroSEIZE has only scratched the surface of where the scientists want to go.

"Later this year we will do some more shallow drilling," said Dr Tobin. "Then in 2010 or 2012 we aim to drill two deep holes down to about 6km.

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"We can then get cores through the fault zone where it branches down there; and we will also place instruments down inside the boreholes."

The Japanese government plans to run an undersea cable out to the Nankai Trough. That will allow data from seismometers, tiltmeters and other instruments inside the subduction zone itself to be carried back to the mainland and analysed in real time.

At the very least, this will provide unprecedented insights into the processes happening in a major earthquake zone.

At best, it will become a tool for forecasting quakes and tsunami.

The Japanese government puts the chances of a major event happening in the Nankai Trough over the next 30 years at more than 50%.

And eventually the findings here, or the technologies developed, could be applied in other similar regions, including the Sunda Trench which caused so much devastation back in 2004.