Most underwater explosions show characteristics of a bubble pulse and reverberation effects. In order to specifically identify the cause of an underwater explosion, it is most important to find a bubble pulse and reverberation effects using spectral and cepstral analyses. For a very shallow underwater explosion, spectral analysis is preferable to cepstral analysis. Time-domain analyses show bubble pulses as well as positive polarities of the first P-wave arrivals on the vertical component, and frequency-domain spectral analyses also clearly reveal the bubble pulse and reverberation effects. The ROKS Cheonan sinking was a shallow underwater explosion that occurred near the surface showing a bubble jet characteristic resulting in splitting the ship into two pieces including a bubble pulse and reverberation effects. The findings of a bubble jet and a toroidal bubble deformation including a bubble pulse are highlighted for a shallow underwater explosion in this study. The ROKS Cheonan sinking took place off the Baengnyeong Island in the Yellow Sea of the Korean Peninsula at a depth of about 8 m in the sea depth of 44 m on March 26, 2010. The explosive charge weight was estimated at 136 kg TNT which is equivalent to one of the abandoned land control mines (LCM) that were deployed near the Northern Limited Lines (NLL) in the Yellow Sea in the late 1970s.

This paper focuses on the time scale of molecular cloud collapse by examining the correlation between virial temperature, effective temperature, molecular cloud mass, and lifetime. Additionally, it explores the connection between the mass of the collapsing cloud and the duration it takes for the collapse to occur from radius R to r. The findings are then compared with the data catalog VizieR, a 1D model of low-mass star formation, to further analyze the relation- ship between the evolution time of the molecular cloud, mass density, and cloud radius during the collapse process. The results highlight the intricate nature of the relationship between the parameters and properties of star-forming clouds.