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The Honolulu Advertiser
Posted on: Thursday, March 7, 2002

Scientist disputes Kilauea slide danger

By Jan TenBruggencate
Advertiser Science Writer

The southern flank of Kilauea volcano slipped seaward for a day and a half in November 2000, hundreds of times faster than normal.

Kilauea volcano has been erupting steadily since Jan. 3, 1983, primarily from the Pu'u 'O'o vent on the volcano's east rift zone. A 2000 movement of a large flank area is being termed a "silent earthquake," which is in scientific dispute as a signal of danger.

Advertiser library photo • Jan. 19, 1998

The movement never shook the seismometers that cover the volcano and which, until recently, were the only way to tell there was movement underground.

This was not the precursor of a catastrophic quake that some network and print media have described during the past couple of weeks, said geophysicist Peter Cervelli, of Stanford University and the Hawaiian Volcano Observatory at Kilauea. But is does provide clues about a whole new way of understanding underground movement of volcanoes.

The November 2000 movement was detected by satellite-based positioning equipment that geologists installed from 1995 to 1999.

"It was the first time such motion has been measured, due to the improved continuous GPS network," said Hawaiian Volcano Observatory chief Don Swanson.

"It was not associated with any quakes. Some scientists are calling it a silent earthquake," although it is not properly a quake at all, he said.

GPS stands for Global Positioning System. It is primarily used for navigation, but modern equipment can measure movement of as little as a tenth of an inch, making it useful for measuring not only movement over the ground, but movement of the ground.

A GPS receiver on the ground picks up signals from several orbiting GPS satellites, and is able to calculate its position on Earth. The Hawaiian Volcano Observatory has about 15 GPS receivers around the Big Island volcanoes.

The slippage occurred on a fault line that lies nearly three miles below the surface, roughly below Halape, directly south from the Kilauea caldera.

It may be the same fault region that shook disastrously in 1975, involving a massive surface earth movement and a locally confined tsunami, or sea wave, that killed two campers at Halape . A huge quake in the 1860s was also probably a south flank shift, Cervelli said.

Although there have been arguments by some geologists that the southern side of Kilauea may be poised to slide into the sea, causing a tsunami that could devastate the state and possibly the Pacific rim, Cervelli said the fault line that caused the "silent quake" of November 2000 is probably not the place that might happen.

"In my opinion, this structure that we have imaged is not particularly likely to fail catastrophically. My feeling is that the biggest threat there is earthquakes," he said.

The side of Kilauea is constantly moving, generally slipping seaward at a rate of about 3 inches a year. But during 36 hours in November 2000, it speeded up, sliding at 3 inches a day—slower than a snail's pace, but nearly 400 times the normal rate of movement.

Among the interesting features of the silent quake is that it occurred eight to 10 days after a downpour that lasted for days and dumped about 3 feet of rain on the south side of Kilauea. The rain caused lots of damage on the surface, wiping out roads and bridges. It also appears to have caused some changes below the surface.

Cervelli suggests that the water pressure caused by that rain may have helped prompt the earth movement.

"Raise the water table very fast by adding this excess fluid pressure and you essentially pry open faults," he said.

He and co-researchers believe that it took the eight to 10 days, from the rainfall to the earth slippage, for the "pressure pulse" caused by the heavy rain to reach the fault area. One view is that the increased water pressure simply allowed the earth on one side of the fault to move more easily with respect to the other side.

Swanson and Cervelli said this is certainly not the first such silent quake. It's just the first that has been identified here, thanks to GPS technology.

"Ours was the first one on a volcano, the first one on Kilauea and the first on this time scale," Cervelli said.

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