桜島火山昭和火口で発生する火砕流の特徴

Abstract

Eruptions at the Showa crater become active since 2008 and are sometimes accompanied with small pyroclastic flows. It is important to understand the mechanism of generation of the pyroclastic flow for volcanic disaster prevention. We research characteristics of inflation and precursory earthquakes before eruptions, amplitude of air-shock and deflation after eruptions accompanied with the pyroclastic flow. In the case of eruptions accompanied with the pyroclastic flows, precursory earthquake swarm and long suspension of inflation (more than 30 min) before eruption and larger deflation after eruption are observed. It is considered that ascent magma before occurrence of pyroclastic flow is longer stay in conduit and erupt much volcanic ejecta than normal eruptions. The pyroclastic flows may be caused by fall down of much volcanic ejecta composed of high density magma. The possibility of the occurrence of the pyroclastic flow is high when swarm of precursory earthquakes and long suspension of inflation are observed.