What Really Caused the ROKS Cheonan Warship Sinking?

This paper is concerned with the sinking of the Korean naval warship (ROKS Cheonan) and the reported spectra of the seismic signals recorded at the time of the incident. The spectra of seismic signals show prominently amplitude peaks at around 8.5？Hz and its harmonics. These frequencies were explained with the vibrations of a water column due to an underwater explosion. This explanation is highly doubtful and concerns about its validity have already been raised in the scientific community. In this work an alternative explanation is presented: it is shown that the recorded seismic spectra are consistent with the natural frequencies of vibrations of a large submarine with a length of around 113？m. This finding raises the possibility that the ROKS Cheonan sunk because of the collision with a large submarine rather than the explosion of a torpedo or an underwater mine. 1. Introduction Several years have passed since the incident of the Korean naval warship (ROKS Cheonan) sinking that occurred on the 26th of March 2010 [1, 2]. The Cheonan warship was split largely into two parts as shown in Figure 1 and sank off Korea’s west coast near Baekryeong Island in the Yellow Sea. The seismic signals were recorded at the time of the incident in many stations including Baekryeong Island. The analysis of the seismic signals followed soon after the incident as reported in [3]. Based on the official government report of a summary of its investigation on 20 May 2010, the warship was sunk by a North Korean torpedo fired by a midget submarine (JIG’s report [1] printed in September 2010). Nevertheless controversy over the scientific legitimacy of the report has been aroused among scientists [4–8]. The doubts raised by each of the reference are summarized in Appendix A. Figure 1: (a) ROKS warship; (b) top view. Sketch of the part damaged during the incident. This work was motivated from the report of [3, 9] showing the spectra of the seismic signals generated from the Cheonan incident: the spectral Figure 7 in [3] shows very interesting spectral peaks at around 8.5？Hz and its harmonics. The author in [3] has explained these characteristic frequencies as those of the hydroacoustic reverberation waves generated by an underwater explosion. This explanation is based on the theory in the report of [10] that underwater explosion generates reverberation waves in the column of seawater with characteristic odd harmonic series of a fundamental frequency by , where is the wave speed in water (~1.5？km/s) and is the water column depth (for ？Hz, ？m). However contrary to this prediction,