Seismic Observation of the 26 March 2010 Sinking of the South Korean Naval Vessel Cheonanham

A South Korean naval vessel, Cheonanham, sank at ~2.5 km southwest from Bakryeong Island (37.929°N, 124.601°E) in 21:22 local time (12:22 UTC) on 26 March 2010. Only 58 people out of the 104 crew members were rescued from the incident, and the other 46 sailors were dead or missing in the incident. Three plausible causes of the sinking were raised: (1) striking by an explosive source (torpedo or mine), (2) shear breakage due to strain accumulation by fatigue, and (3) collision with a sunken rock. The incident was recorded as an M1.5 event at local seismic stations. We analyze local seismic records and investigate the source properties. The event location is determined not only by a usual location method, but also using low-frequency horizontal polarization analysis. The determined event location agrees with the reported sinking location. The S-wave amplitudes are found to be comparable to the P-wave amplitudes. Seismic waves coupled from shock waves are observed, which allows us to constrain the epicentral distance and explosion-source feature. The coupled shock waves have a dominant frequency of ~32 Hz. The shock waves and high P/S amplitude ratios suggest an underwater explosion that is responsible for the vessel sinking. The spectral contents of P waves allows us to constrain the source depth in the water. We infer the depth of the explosion in the sea by comparing the observed spectra with synthetic spectra. We compare the seismic features with those from nuclear explosions.