Underwater Explosion (UWE) Analysis of the ROKS Cheonan Incident SO GU KIM 1 and YEFIM GITTERMAN 2 Abstract—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 mag- nitude 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 fre- quency 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. Key words: Bubble pulse, bubble collapse, toroidal bubble, whipping, T-phase, N-wave. 1. Introduction The underwater explosion (UWE) incident vis-a `- vis the ROKS Cheonan occurred on March 26 2010 with an origin time of 12:21:57 (UTC) and location at 37.93°N and 124.60°E. The Multinational Civilian- Military Joint Investigation Group (MCMJIG) con- cluded that the UWE had a TNT-equivalent yield of 250 kg of explosives and occurred at a depth of 6–9 m with a seismic magnitude of 1.5 (MCMJIG 2010). This TNT-equivalent yield was in line with the yield of the North Korean CHT-02D torpedo. Anal- ysis was taken from the 3-component records of the BAR, DEI, and GAHB stations of KMA, INCN sta- tion of IRIS and the BRDAR station of KIGAM. The seismometers utilized for data collection are the STS- 2 (BB) for the BAR and INCN stations, the CMG-3T (borehole BB) for the GAHB station, the CMG40T-1 (SP) for the DEI station and short period (SP) and infrasound arrays for the BRDAR station (see Table 1; Fig. 1). The sampling rate was 100 sps, except for the INCN station with 40 sps. A shallow UWE was empirically confirmed by strong high-frequency features of acoustic phase signals at seismograms from the BAR station, observed approximately 30 s after P-wave arrivals and infrasound records also showed N-waves (SOR- RELS et al. 2002) clearly indicating a sonic boom. The main objective of this paper is to identify the source of the UWE vis-a `-vis the ROKS Cheonan incident via spectral analysis of the seismic waves related to the bubbling and reverberation effects of the UWE, including T-phase. 1 Korea Seismological Institute, 519-402 Joongsan Maul, Goyang, South Korea. E-mail: sogukim@hanmail.net 2 Geophysical Institute of Israel, P.O. Box 182, Lod 71100, Israel. Pure Appl. Geophys. Ó 2012 Springer Basel AG DOI 10.1007/s00024-012-0554-9 Pure and Applied Geophysics