An index of submarine volcanic explosivity concerning tsunami generation has been proposed on the basis of the relationship between phreatomagmatic explosion and resultant initial waveform of tsunamis. In the generation process of tsunamis due to phreatomagmatic explosion, the seawater touches magma of high temperature, after which the water evaporates in an instant with explosive increase of volume. We assume the value of this index specifically to simulate tsunamis caused by submarine volcanic eruption in Kagoshima Bay, where submarine explosion has been observed. Furthermore, tsunamis due to subsidence of a caldera near a volcanic mountain in the same bay are numerically simulated using an initial tsunami profile assumed without discussion about the emission process of magma out of a chamber through submarine volcanic eruption.

Numerical simulation of tsunamis in two bays in Kagoshima Prefecture was performed: Shibushi Bay and Kagoshima Bay. The former, which faces the Hyuganada connected to the Pacific, has been attacked by Hyuganada earthquake tsunamis, as well as Nankai earthquake tsunamis. In the present study, the tsunamis due to both 1968 and 1970 Hyuganada earthquakes were simulated to study the properties of tsunamis around the southern part of Kyushu including the inside of Shibushi Bay. On the other hand, although the bay mouth of the latter is not directed to the sources of these tsunamis, tsunamis have been generated by submarine volcanic eruption in the bay. We supposed the value of scale index for submarine eruption concerning tsunami generation and then simulated tsunamis due to submarine volcanic eruption in Kagoshima Bay.