Future view of the research on prediction of volcanic eruptions and structure of volcano

Abstract

Research on prediction of volcanic eruptions has achieved a lot of results so far. For instance, it succeeds in detecting many precursory phenomena such as increase of seismic activity, deformation of volcanic body suggesting the accumulation and the movement of magma, geomagnetic changes caused by the rise of the temperature and anomalous change of volcanic gases. Knowing the structure of the volcano also enables us to understand the meaning of these anomalous phenomena. However, the research on prediction of volcanic eruption has left a lot of problems. For instance, eruption has not occurred in Iwate Volcano, though many precursory phenomena such as deformation of volcanic body and increase of seismic activity indicating magma intrusion were detected. In recent Asama Volcano, the correspondence of the seismic swarm and the eruption has changed to be unclear and the prediction of the eruption turned to be difficult. These problems originate in the difficulty of forecast of magma ascent. This paper proposes three aspects on these problems. It is firstly important to know the depth that magma has stopped. High accuracy determination of hypocenters led by accurate velocity structure is required. High accuracy determination of deformation source is also requested. Secondarily, it is important to know the density difference with magma and surrounding mediums. Magma will stop rising after it reaches a low density medium, while it continues to rise in a high density medium. Therefore, future experiments for the structure of volcano are requested to reach estimation of not only seismic velocity but even also density. Thirdly, the observation that can discuss the degassing from the magma should be required. According to the TDEM survey in Unzen Volcano, a high conductance zone trending W-E direction in the shallow part is found along the high seismic zone from the magma reservoir toward the summit. This evidence indicates the emission of volcanic gases into the shallow water saturated layer through the fault system in the western part of Unzen during the magma migration of the latest eruption. The research on prediction of volcanic eruptions is requested to answer the above mentioned problems, and new technique for geophysical survey is also requested.