Constraints on eruption processes and source conditions of explosive caldera-forming events using volcanogenic tsunamis: insights from the Krakatau and Kikai eruptions

Caldera-forming eruptions are catastrophic volcanic events that pose one of the great natural hazards on earth. The 1883 eruption of Krakatau in Indonesia (VEI 6) and the 7.3 ka Kikai eruption (VEI 7) of Japan are the representative of young marine caldera-forming eruptions. Although these eruptions must have significantly and devastatingly affected the development of coastal human activities and environments around the volcanoes, they still remain speculative and controversial, particularly with respect to the effects of seawater on eruption processes, dynamics, and evolution of such large-scale marine eruptions. In this presentation impacts of volcanogenic tsunamis during these two caldera-forming eruptions are discussed based on geological evidences and results of numerical studies. Then eruption processes and source conditions are estimated. A key issue is that which tsunami generation mechanism is more plausible for each eruption; caldera collapse, phreatomagmatic explosion, or pyroclastic flow? Tsunamis generated by pyroclastic flows can be evaluated using two types of two-layer shallow water model which can simply describe the effects of interaction between pyroclastic flow and seawater. One of the models is for denser flow than seawater, and another is for lighter one. Tsunamis generated by caldera collapse and phreatomagmatic explosion can be evaluated using simple plunger models. Parameter studies were conducted under the various initial conditions for different tsunami mechanisms and for both eruption. For the Krakatau eruption, computed wave heights of tsunamis are broadly consistent with historical wave data in coastal areas, including a tide gage record at Batavia, when the flow has a volume of 20 km3 and a mass flux of 108 m3/s. The result indicates that a voluminous pyroclastic flow entering sea would be a plausible mechanism. On the other hand, for the Kikai eruption, a caldera collapse is a possible mechanism for a huge tsunami and its timescale can be estimated less than 6 hours by comparing numerical results with limited geological records. It is suggested that volcanogenic tsunamis would give quantitative constraints on eruption processes and source conditions of explosive marine eruptions.