The Tohoku region Pacific Ocean earthquake that occurred on March 11, 2011, resulted in extensive damage in a vast area ranging from the Tohoku to Kanto. However, at the time of the earthquake, no research vessel was confirmed to be investigating marine life in the seas adjacent to epicenter area, so physical data like water temperature and salinity, as well as ocean sound speed data required for the echo sounder, could not be acquired. Therefore, in this study, we compute the sound speed profile from the Argo system data(water temperature, salinity, pressure) that was being deployed near the area of the epicenter both before and after the earthquake, and attempt to detect the change in the sound speed field. In addition, the Tohoku region Pacific Ocean earthquake and the Sumatra earthquake in December 2004, which were accompanied by a giant tsunami, were also investigated. Results showed that in the main temperature front between the central thermocline and deep-sea isothermal layer, water temperature did not change very much. However, we confirmed the tendency for the underwater sound speed to suddenly increase dramatically after an earthquake.