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Seismic evidence points to N-test
The Yomiuri ShimbunSeismic waves of varying magnitude were detected at a number of locations shortly after North Korea's announced nuclear test Monday. In South Korea, the magnitudes ranged from 3.58 to 3.7, while the United States said the magnitude was 4.2. In Japan, one calculation showed a magnitude of 5.1. Magnitude differences are likely caused by the differences in calculation methods and the location of observation spots. If those different magnitudes estimated by Monday's blast are translated into energy levels, there is more than a 100-fold difference among the measurements. But the force of the blast in North Korea was seen as much smaller than the atomic bombs dropped on Hiroshima and Nagasaki, which had yields of 15 to 20 kilotons. Prof. Katsuyuki Abe at Tokyo University's Earthquake Research Institute said, "Estimated from magnitudes reported in nuclear tests conducted in the past in the United States and former Soviet Union, the power of the blast this time probably was 0.5 to 3 kiloton, only a fraction of the power of the same type of bomb dropped on Hiroshima." Another expert, however, said, "If there is much space in the mine shaft where the test was conducted, the energy [of the blast] may have been absorbed, showing low-magnitude readings." In either case, the blast this time was small if it was a nuclear detonation. But the scale is still more than dozens of times greater than the most powerful conventional weapons. The seismic waves detected differed from naturally occurring earthquakes. The Meteorological Agency first detected primary waves, or P waves, which are weak but fast-traveling waves, at two observation spots on the Okinoshima islands, Shimane Prefecture, about 660 kilometers away from the epicenter. P waves were detected at one time also at eight observation spots in Nagano, which are equipped with high-precision monitors. In earthquakes, P waves are followed by secondary, or S, waves, which cause more powerful shaking. But S waves are only faintly detected in underground nuclear tests. This also was true in this seismic event. Natural tremors also result in complicated P waves due to fault slippage. But this time, as shock waves are believed to have been generated from one point at one precise moment, the blast likely caused only simple P waves. Takeshi Matsumoto, assistant professor at Kyushu University's Institute of Seismology and Volcanology, said, "Judging from the results of the analysis of seismic waves, [Monday's] tremors were no doubt those caused by an artificial explosion." Although a number of experts said Monday's blast was small for a nuclear explosion, Yuji Yagi, assistant professor at Tsukuba University, concluded the power of the blast was equivalent to 25.72 kilotons of TNT. Yagi's analysis did not rely on magnitude readings. Instead, he used data on seismic waves obtained from overseas research institutes and worked out energy levels observed from the explosion. As the testing is believed to have been conducted in an underground mine shaft, Yagi said he estimated the focus of the seismic event was 1.5 kilometers below the surface. The amount of 25.72 kilotons of TNT is close to the figure observed in a nuclear test conducted by Pakistan in May 1998. Yagi said, "It's still necessary to analyze data in detail, but the blast may have been quite powerful." Posted by: john 2006-10-10 |
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