Stanford News

5/27/97

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Geophysicists catch eruption before it starts

Susan Owen has caught an erupting volcano in a net.

Owen, a graduate student in geophysics at Stanford, reports on that feat on Wednesday, May 28, at the spring meeting of the American Geophysical Union in Baltimore. The volcano in question is Kilauea, where a 3 a.m. eruption on Jan. 30 opened a new rift on the south flank of Hawaii's most active volcanic slope. Fountains of lava spewed from a series of cracks more than a mile long.

The "net" is a network of continuously recording data-collecting stations, placed along the flanks of the volcano on each side of the rift by a team of scientists from Stanford and the U.S. Geological Survey's Hawaiian Volcano Observatory. Each station includes a marker anchored to the rock and an antenna that takes readings from Global Positioning System (GPS) satellites to precisely calibrate its marker location.

On Jan. 29 and 30, some of the markers on Kilauea went on the move. Over a period of eight hours before the eruption, the ground beneath the Napau crater slowly stretched and expanded 20 centimeters ­ almost 8 inches ­ before finally splitting open in a lava-spewing rift. The GPS instruments tracked that motion as the stations ­ and the apparently solid rock beneath them ­ were pushed apart by the surge of hot magma.

It took Owen's analysis of the raw GPS data to show how that stretching evolved, both before and after the eruption when the ground stretched 6 additional inches. Owen, who works with Stanford Associate Professor Paul Segall of geophysics, said she wasn't even tempted to hop on a plane and join the vulcanologists in Hawaii watching the fountains of fire. From a geophysicist's standpoint, the view was better from her office half an ocean away in the basement of Stanford's Mitchell Hall, where she analyzed 19 hours of GPS measurements and plotted out how the stations moved at 40-minute intervals.

A day after the eruption started, Owen had a picture of how the ground expanded before the rift. In the future, she said, Stanford and the Hawaiian Volcano Observatory should be able to analyze the GPS data almost as quickly as they come in ­ to watch the stretching and widening that leads to a rift, almost in real time.

Could this method be used to warn people of dangerous lava flows? "We won't be able to tell the exact time that an expansion will lead to an eruption," Owen said. "We should learn enough to tell people to get out of the way."

Episode 54

At the AGU meeting, Owen and C.R. Thornber of the Hawaiian Volcano Observatory both will present papers describing the Jan. 30 event. Officially, it is Episode 54 in the eruption that has been going on almost continuously at Kilauea since 1983. Owen's paper describes the results of data collected by 13 GPS receivers positioned on the flanks of the volcano.

Some of the instruments had been placed to measure the fluctuations of Kilauea's summit. Inside that summit, a chamber holds hot magma that has risen from deep below the surface. Normally, magma flows underground from that chamber to a lava pond that feeds the active Pu'u O'o crater in the volcano's east rift zone. The GPS instruments recorded a slow deflation of the summit magma chamber ­ until Jan. 30, when the chamber suddenly emptied, and its surface contracted more than 10 centimeters (4 inches). Pu'u O'o temporarily stopped erupting, while several miles away, lava rushed to flow out of a new fissure in the Napau Crater.

The new eruption lasted only a day, however. By March, magma had returned to the summit chamber and the main eruption at Pu'u O'o resumed.

Other receivers in the GPS network recorded how the new rift was formed. As the ground beneath them stretched, stations on either side of the rift were carried away from each other. Owen reports that in the first two days of Episode 54, the receivers moved apart a total of 36 centimeters (14 inches). The movement began just as other instruments recorded a harmonic tremor, a low hum within the earth that often signals the movement of magma. More than half the movement on the mile-long rift occurred before the fissure opened and began spewing lava.

Owen has developed mathematical models to explain this movement and earlier movements recorded along Kilauea's south flank. For the past several years, Segall's research group and their colleagues have been monitoring this region because of evidence that a major earthquake fault lies 9 kilometers ­ about 6 miles ­ beneath the surface. They use continuously recording GPS markers to clock the slip of this horizontal fault, or "decollement," as a giant chunk of the mountain shifts southward toward the sea, at a rate between 7 and 10 centimeters (3 to 4 inches) a year.

The scientists want to know whether the earthquakes are triggered by the same force that makes Kilauea an active volcano: the surge of molten magma from the earth's mantle. Alternatively, small and large earthquakes may trigger new surges of magma and volcanic activity. Either way, actions down at the fault zone may influence the magma flows near the volcano's surface.

A rift unlocked

Owen's data show that both of these models are consistent with the events of Jan. 30. Based on GPS and other measurements taken between 1992 and 1996, it appears that the extension of the south flank of the volcano has occurred mostly well beneath the surface, in a region 3 to 9 kilometers (2 to 6 miles) deep.

The upper few miles were "locked" ­ and that locked region was the one that split open at the Napau Crater. The rift created by the Jan. 30 event extends at a slant some 2.2 kilometers, or 1.5 miles, down from the surface. The opening ranges in width from 1.8 to 2 meters, more than 6 feet.

The rift's opening was dramatic, but harmless. The stretching and cracking of the earth occurred relatively slowly, not fast enough to trigger the propagation of damaging seismic waves like an earthquake's. However, in the past, there have been several large or great quakes centered at Kilauea; the latest, in 1975, inundated a nearby beach with a tsunami and killed several people who were camping there.

How dangerous is the volcano itself? Owen said scientists and tourists tread on its surface, and there are some local buildings in its path. But for the most part, Kilauea's lava flows are too slow to pose much of a danger to human lives or property.

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By Janet Basu

FOR MORE ILLUSTRATIONS FROM OWEN'S SCIENTIFIC PRESENTATION, SEE THE WORLD WIDE WEB: http://pangea.stanford.edu/~owen/jan30AGU.html