Stanford News

2/25/97

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Geophysicists catch eruption before it starts

Susan Owen has caught an erupting volcano in a net.

The volcano 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. A nearby group of campers stumbled out of their sleeping bags to watch fountains of lava spewing from a series of cracks more than a mile long.

The "net" is a network of 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 satellites to precisely calibrate its marker location. Similar networks of GPS stations are being deployed in California and Japan to monitor earthquake faults.

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 (afterward, the ground stretched 6 additional inches). Owen, a graduate student working with 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 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." She said that except for the scientists and tourists who tread on its surface ­ and the few local buildings that it has not already engulfed ­ Kilauea's lava flows are too slow to pose much of a danger to human lives.

However, she said, the lessons learned on Kilauea will apply to other volcanoes as well. For example, her preliminary models show that deep underneath, the movement may have been even more dramatic than the sudden crack at the surface. Based on data from GPS and other instruments, Owen calculated that the actual rift extends at a slant some 2.5 kilometers, or 1.5 miles, down beneath the surface. At that depth, some force ­ probably the pressure of hot flowing magma ­ pushed the rock a much greater distance than at the surface ­ conceivably as much as 4 meters, or 13 feet.

Segall said that lava is not the only hazard posed by a volcano like Kilauea. He uses GPS and other satellite technology to clock the movements of the earth in earthquake zones such as the San Andreas Fault. He and Owen are working with the USGS to study earthquakes on a fault 9 kilometers ­ about 6 miles ­ beneath Kilauea's south flank.

That fault has ruptured in two large earthquakes in the past 20 years; a tsunami generated by the 1975 quake killed several people camped on a beach. A magnitude 8 great quake shook Kilauea in 1868, and in ancient times the damage was far more devastating, Segall said in a session of the American Association for the Advancement of Science on Feb. 15. Geological records show that 100,000 and 200,000 years ago, giant waves swept past the Hawaiian Islands sloshing as high as 1000 feet.

Another great quake on Kilauea or a giant underwater landslide could be catastrophic not only for Hawaii but for other Pacific Basin shorelines struck by a tsunami, Segall said at AAAS. Now new technologies allow scientists to study these phenomena more closely ­ but so far not well enough to predict major events.

For the past several years, Segall, Owen and their colleagues have been using GPS markers to clock the slip of the Kilauea fault, as a giant chunk of the mountain shifts southward toward the sea at a rate between 7 and 10 centimeters a year. That is racing speed compared to most faults: the San Andreas creeps along at 3.5 centimeters ­ about 1.5 inches ­ per year. Kilauea slumps out into the sea and across the ocean floor in what many scientists believe may be a massive landslide. The group's findings on this movement were published in Science in 1995.)

Segall plans to expand the GPS network, with its precise hour-by-hour measurements, with another technology that measures movements every few days, all across the volcano's surface. That new technology, called single aperture radar interferometry. or SAR, is being developed by his Stanford colleague Howard Zebker.

One major focus of Owen's and Segall's research is to find out whether the earthquakes and landslides 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 that have kept Kilauea erupting from one fissure or another since 1983.

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By Janet Basu