Quaking fault lies far underground

By Jan TenBruggencate
Advertiser Science Writer

A newly discovered fault zone 20 miles deep under Kilauea volcano is changing scientists' understanding of the way the volcano works.

"It's a new view of the deep structure of the volcano," said University of Hawai'i seismologist Cecily Wolfe, an assistant professor with the School of Ocean and Earth Science and Technology.

Wolfe and associates studied a zone of frequent earthquakes that extends horizontally in a region that reaches from just south of the center of Kilauea to 6 miles farther south. The region is in Earth's mantle, deep below the structure of the Big Island and much deeper than the bottom of the ocean.

People have known for decades that there were deep earthquakes, but most believed those were along the magma conduit that feeds molten rock from deep in the Earth to the surface at Kilauea, she said.

The new emerging picture is that there is a fault zone that is not part of the magma conduit, but which may react to pressure from magma moving within the Earth, she said. Wolfe and fellow researchers Paul Okubo of the Hawaiian Volcano Observatory and Peter Shearer of the Scripps Institute of Oceanography published their findings in yesterday's issue of Science magazine.

The researchers have not yet clearly outlined the shape of the fault zone. It is about 6 miles long from north to south, but they don't know its east-west dimension, and don't clearly understand how the underground rock is behaving in the perimeter area where the quaking fault meets the area that is not experiencing earthquakes.

"Right now we don't know what's happening at the edges," she said.

Researchers are finding, too, that these deep fault zones are more common than previously believed. These react both to the movement of molten rock and to the stresses of a comparatively young volcanic island on the underlying rock structure.

While the new work increases knowledge about the insides of the volcano, it "means we know less about the magma conduit than we thought," Wolfe said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.