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We were always skeptical of the Cheonan skeptics; useful pieces in Nature and Asia-Pacific Journal: Japan Focus summarize the contradictory positions held by this eclectic group. Critics were motivated by President LMB’s effort to exploit the issue politically and weakness in the joint investigation process. They seemed better at raising red flags about the government’s report than outlining coherent alternatives, engaging in classic fallacies (particularly arguments from ignorance; that if North Korean involvement could not be definitively proven, it must be false). Some of the alternatives, such as collision with an American ship that subsequently returned to port undamaged, had a short shelf life.
Some did venture into the science, including political scientist JJ Suh and physicist Seunghun Lee. Suh and Lee cast doubt on the claim that the Cheonan was destroyed by an external explosion. Their main finding, however, was that materials testing failed to establish a link between the recovered torpedo and the explosion. The Russian investigation, never released, also dissented from the official report and apparently claimed that the Cheonan was sunk by a discarded mine. This theory—among a bunch of others that were subsequently discredited—was buried in a particularly conspiratorial analysis by Japanese journalist Tanaka Sakai, also from the Asia-Pacific Journal.
Now a new scientific paper by So Gu Kim and Yefim Gitterman in the journal Pure and Applied Geophysics raises another line of attack and also comes to the conclusion that the Cheonan was probably sunk by a discarded mine. But this time, the claim has a lot more scientific grunt work behind it; Kim heads Korea’s Seismological Institute and Gitterman is at Israel’s Geophysical Institute.
The science hinges on the parameters linking seismic waves, acoustic waves and bubble frequency and how those can be used to infer the magnitude of underwater explosions; the abstract is below. The team derives a magnitude for the explosion that was substantially smaller than that of the joint investigation and almost precisely equivalent to the yield from land control mines (LCMs) that were discarded in the area in the 1970s. The article speculates that the detonation may have been triggered by the mine cables being dragged out with the mines during the period when the Cheonan was experiencing navigational difficulties.
Pure and Applied Geophysics is clearly a serious scientific journal; this is not an unreviewed web posting. Whatever the merits—to be established by further review—you can bet that Hankyoreh was all over it; their coverage of the story is here; an interview with Kim can be found here.
Abstract
So Gu Kim and Yefim Gitterman, “Underwater Explosion (UWE) Analysis of the ROKS Cheonan Incident,” Pure and Applied Geophysics 2012, DOI: 10.1007/s00024-012-0554-9 at http://www.springerlink.com/content/c861379488372070/
The underwater explosion (UWE) resulting in the sinking of the South Korean warship, ROKS Cheonan occurred on March 26 2010. Raw data was analyzed from several 3-component stations—Baengyeong-do Korea Meteorological Administration (KMA) station (BAR), Ganghwa KMA station (GAHB), Incheon Incorporated Research Institutions for Seismology (IRIS) station (INCN), the short-period station—Deokjeok-do KMA station (DEI), as well as from the seismo-acoustic array Baengyeong-do Korea Institute of Geoscience and Mineral Resources (KIGAM) station (BRDAR). The ROKS Cheonan incident has been investigated by both the Multinational Civilian-Military Joint Investigation Group (Ministry of National Defense, 2010) and Hong (Bull Seism Soc Am 101:1554–1562, 2011). Their respective methods and conclusions are also presented in this study. One of the main differences between their findings and ours is that we deducted that the fundamental bubble frequency was 1.01 Hz with a subsequent oscillation of 1.72 Hz. Also, in contrast to findings by the MCMJIG and Hong, our analysis shows the first reverberation frequency to be 8.5 Hz and the subsequent one to be ≈25 Hz. The TNT-equivalent charge weight (seismic yield) and seismic magnitude were estimated from an observed bubble frequency of 1.01 Hz and the analytical model of a bubble pulse. From the data analyzed, we deducted that the seismic yield would be about 136 kg of TNT, which is equivalent to the individual yield of a large number of land control mines (LCM) which were abandoned in the vicinity of the ROKS Cheonan incident by the Republic of Korea (ROK) Navy in the 1970s (Ministry of National Defense 2010). Also, whereas both the MCMJIG and HONG estimated the local magnitude at 1.5, our findings came to the conclusion of a local magnitude of approximately 2.04 based on the bubble frequency of 1.01 Hz measured on the vertical component of BAR station data considering the empirical relationship between charge weight (TNT yield) and underwater explosion magnitude. Strong high-frequency signals collected at the 3-component BAR station approximately 30 s after P-wave arrivals and infrasound records at BRDAR clearly indicate powerful acoustic phases and N-waves caused by a relatively shallow UWE. T-phases are also observed on seismograms and spectra at 15–17 Hz on the DEI, GAHB, and INCN stations.
