Ground deformation cycles in a magma‐effusive stage, and sub‐Plinian and Vulcanian eruptions at Kirishima volcanoes, Japan

Abstract

AbstractThe dynamics of explosive eruptions, such as Plinian, sub‐Plinian, Vulcanian, and Strombolian ones, is one of the most fascinating subjects in volcano physics. During the early period of volcanic activity at the Shinmoe‐dake Volcano in 2011, various kinds of activities, such as sub‐Plinian eruptions, a magma effusion, and Vulcanian eruptions, occurred sequentially. A different kind of tilt motion accompanied each activity. Here we clarify the characteristics of the tilt motions and their time sequences, presenting a new explanation of the triggering mechanism of a Vulcanian eruption. The sub‐Plinian and the Vulcanian eruptions at the Shinmoe‐dake Volcano were preceded by inflations at shallow depths near the summit. The inflation‐deflation cycles were also recorded during the magma‐effusive stage, with a typical period of 1 h, synchronized with volcanic tremors or long‐period events. Almost all Vulcanian eruptions were preceded by trapezoidal inflations, whose durations systematically lengthened as time progressed, and were followed by various time sequences of tilt motions, which became increasingly more complicated throughout the frequent Vulcanian eruptions. We have found clear linearity with a constant gradient of 0.45 between the logarithm of the preceding duration versus elapsed time for each substage. These observations can be consistently explained based on the assumption that a Vulcanian eruption is induced by a catastrophic rupture of the strongest closed, solid magma frame due to magma degassing overpressure, and the degassing from magma declines exponentially with time.