Seismic investigation of the 26 March 2010 sinking of the South Korean naval vessel Cheonanham

A South Korean naval vessel sank on 26 March 2010. A seismic event with magnitude (ML) of 1.5 was observed at the time of vessel-sinking. Seismic waveforms were collected from three local stations. The event origin time is refined based on the P and S arrival times at the stations. The calculated event location and time are close to the reported vessel-sinking location and time, suggesting that the observed seismic event is associated with the vessel-sinking. The amplitudes of S waves are comparable to those of P waves. Seismic waves coupled from acoustic waves are observed, providing additional constraints of epicentral distance and source type. The coupled acoustic waves have a dominant frequency of ~32 Hz. The acoustic waves and high P=S amplitude ratios suggest an underwater explosion. The bodywave magnitude based on Pn is determined to be 1.46, which is consistent with the reported ML. P energy is dominant at around 8.5 Hz, with multiple frequencies of 17.7 and 34.6 Hz. The primary frequency suggests the water-column thickness in the source region to be 44 m, which agrees with the reported value.