Mountain-sized chunk of rock hiding under Japan is channeling earthquakes

By Stephanie Pappas published about 3 hours ago

The Kumano pluton plays an important role in the subduction of Japan's southern coast.

An underground mountain-sized chunk of rock may be affecting paths of large earthquakes in southern Japan.

The dense igneous rock, known as the Kumano pluton, is lurking about 3.1 miles (5 kilometers) below the surface beneath Japan's Kii Peninsula. It sits in the crust of the continental Eurasian plate. Under this slab of continental crust, the oceanic Philippine plate is taking a dive toward the Earth's mantle, a process called subduction. New research suggests that the heavy pluton within the Eurasian plate changes the slope of that dive, forcing the Philippine plate down more steeply.

The pluton also sits near the epicenters of two large 1940s earthquakes, each of which traveled in opposite directions and did not rupture through the pluton itself.

"Ultimately, we don't really know why these earthquakes didn't overlap in the region of the pluton," said study co-author Dan Bassett, a marine geophysicist at New Zealand's GNS Science, an Earth science research service. "It does appear to be playing a really key role in nucleating these earthquakes and preventing them from joining up." (An earthquake's nucleation point is where it begins to rupture the crust.)

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