Development of the very long-range cosmic-ray muon radiographic imaging technique to explore the internal structure of an erupting volcano, Shinmoe-dake, Japan

Abstract. Muography offers us a tool to observe hazardous erupting volcanoes remotely. However, practical muographic observations of volcanoes from a distance are difficult; therefore, various observations have been performed in the vicinity (< 1.5 km) of volcano peaks to suppress background noise and enhance images. In this study, we created a muographic image directly beneath the caldera floor of the erupting Shinmoe-dake volcano in Japan by locating our muography telescope 5 km from the peak. The Shinmoe-dake volcano began to erupt on 19 January 2011 and, in less than 1 month, the ejected lava almost completely filled the caldera and completely changed the topography of the caldera floor. The resultant image shows a low-density region underneath the western part of the newly created caldera floor, which indicates the existence of a void there. After the volcano became less active in February 2011, infrequent eruptions might have left a void beneath the caldera floor, which may trigger a collapse in the future. We anticipate that our novel muography will be a practical tool for monitoring and predicting eruption sequences in the near future.