Numerical simulation of tsunamis generated by caldera collapse during the 7.3 ka Kikai eruption, Kyushu, Japan

The relationship between tsunamis and scales of caldera collapse during a 7.3 ka eruption of the Kikai volcano were numerically investigated, and a hypothetical caldera collapse scale was established. Wave height, arrival time, and run-up height and distance were determined at some locations along the coastline around Kikai caldera, using non-linear long-wave equations and caldera collapse models using parameters showing the difference in geometry between pre- and post-collapse and the collapse duration. Whether tsunamis become large and inundations occur in coasts is estimated by the dimensionless collapse speed. Computed tsunamis were then compared with geological characteristics found in coasts. The lack of evidence of tsunami inundation at Nejime, 65 km from the caldera, suggests that any tsunamis were small; indicating that the upper limit of dimensionless caldera collapse speed was 0.01. On the other hand, on the coast of the Satsuma Peninsula, 50 km from the caldera, geological characteristics suggests that tsunamis did not inundate, or that even if tsunamis inundated the area, the traces of a tsunami have been eroded by a climactic pyroclastic flow or the tsunami itself and they have not been left. In numerical computations, when a dimensionless caldera collapse speed is more than 0.003, tsunami can inundate this area.