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First direct measure of volcano's blast.

Since Japan's Unzen volcano awoke in 1990 from a 200-year repose, lava has oozed from a vent on its eastern slope, forming an unstable dome that looms menacingly over towns below. Periodically, part of the dome shears off or collapses, releasing a cascade of debris with explosive force.

Volcanologists yearn to measure directly the energy released during such volcanic events. But such close-up, derailed observations pose extreme danger, and the fury unleashed by a dome collapse can turn expensive instruments into scorched, shattered hulks.

Now, using a simple, rugged device designed to gauge military munitions and other explosives, Japanese scientists have achieved the first direct measurement of the energy released during a volcanic blowout. Volcanologist Hiromitsu Taniguchiof the Science Education Institute in Osaka and geologist Keiko Suzuki-Kamata of Kobe University report their findings in the Jan. 22 GEOPHYSICAL RESEARCH LETTERS.

The researchers measured the shock wave created by a dome collapse on June 8, 1991, and then calculated the pent-up energy required to generate it - the equivalent of about 12,000 tons of TNT. Previously, volcanologists relied on more approximate measurements. In one widely used method, researchers locate a chunk of debris and calculate the energy required to hurl it from the volcano to its landing place.

To make their measurements, Taniguchi and Suzuki-Kamata set up three meters within the volcano's destructive range. The pressure-sensitive part of the meter consists of a hollow chamber about two inches wide, covered with a thin lead plate. The sensor is mounted on a sturdy pole driven into the ground.

The researchers calculated the June 8 shock wave at 75 meters per second at the source on the basis of how severely the wave deformed the lead plate as it passed the meter at Taruki-daichi, a town 2,700 meters northeast of the lava dome. That's powerful enough to knock over a person standing in Taruki-daichi, they report.

The new method of measuring may reduce inaccuracies, says Richard B. Waitt, a volcanologist at the U.S. Geological Surveys Cascades Volcano Observatory in Vancouver, Wash. "It's a far more direct means of [making measurements]," he says. "This allows some calculations as to what the volcano is capable of"

Unzen has proved capable of quite a lot. The blast that the Japanese researchers measured came just five days after a massive flow of hot ash and debris from a dome collapse killed 43 people in Kita-kamikoba, a town directly in the firing line of the volcano's east-lacing vent.

Direct measurements of volcanic blasts may provide a means of checking the theoretical models some volcanologists have created to explore the physics of crumbling lava domes, says volcanologist Jonathan H. Fink of Arizona State University in Tempe, who helped develop such a mathematical tool,

"It's interesting that the number these [researchers] came up with - 75 meters per second as the maximum velocity -- is well within the range that we would calculate based on the model," Fink comments. - D. Pendick
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Author:Pendick, Daniel
Publication:Science News
Date:Feb 13, 1993
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