Undersea Mountain Shapes Fault

Aug. 7, 2006 — Earth scientists have discovered and imaged a buried mountain violently gouging its way into the Earth five miles under southwestern Japan.

The plucky little undersea mountain has ridden the oceanic crust of the Earth there right down into the Nankai Trench subduction zone, where the Eurasian Plate overrides the Philippine Plate.

But instead of being sheared off, the undersea mountain — also called a seamount — has been plowing a furrow in the underbelly of the Eurasian Plate. It has the potential to eventually influence the frequency and power of earthquakes and therefore tsunamis generated there, say scientists.

"We are trying to figure out what the conditions are along that fault," said geologist Nathan Bangs, describing the research to create detailed seismic "sonograms" of the tilted plane along which the two plates meet. “In the process we found this smaller seamount."

The seamount is plowing into a wedge of wet ocean-floor sediments, "bulldozing" them onto the underside of the Eurasian Plate in the subduction zone where it meets the Philippine Plate, Bangs explained.

Bangs, a geophysicist at the University of Texas in Austin, is the lead author of a paper reporting the seamount's rowdy behavior in the August issue of the Geological Society of America Bulletin.

"When a seamount goes through you expect it to do some very significant damage," said Bangs, which is exactly what they have seen in this case.

About 15.5 cubic miles of material has been plowed from the wedge by the half-mile thick seamount.

All that material appears to be on its way into the subduction zone, where it could have major implications for how the two plates slip and move past each other — and therefore the kind of earthquakes they produce, said Bangs.

"It's one of the less understood processes that go on at subduction zones," said Demian Saffer, who builds computer simulations of subduction zones at Pennsylvania State University.

Seamounts are thought to do one of two things, Saffer explained. On one hand they could gunk up the smooth working of the "subduction factory" and therefore increase resistance and create a more jerky, quake-prone subduction zone.

On the other hand, seamounts help break rocks up and drag down wet material — as might be the case at Japan's Nankai Trench fault — which makes the subduction zone weaker, more lubricated and perhaps less able to produce large quakes.

Sorting out which is the case has implications far beyond Japan, said Bangs. The infamous Sumatra subduction zone that let loose the monstrous Dec. 2004 mega-quakes and Indian Ocean tsunami is also affected by seamounts, he said. "It is a similar situation and a similar problem."

It's that sort of danger that keeps people like Saffer and Bangs busy trying to figure out how these subduction zones really work.

Providing funding for a lot of this work is the 21-nation, 10-year, $1.5 billion Integrated Ocean Drilling Program (IODP), which is already underway with plans to drill right down into the Nankai Trench fault, Bangs said.

In fact it was the search for good drilling sites that led Bangs and his colleagues to stumble onto the seamounts, he said. The IODP plans include creating a Nankai Trench fault-observatory similar to the San Andreas Fault Observatory at Depth (SAFOD) in California.