Three-dimensional density tomography determined from multi-directional muography of the Omuroyama scoria cone, Higashi–Izu monogenetic volcano field, Japan

Scoria cone is one of the basic forms of volcanoes, and clarifying its detailed internal structure and magma movement during its formation is important for understanding the nature of eruptions and also for volcanic disaster mitigation. We conducted a multi-directional muographic survey of the Omuroyama scoria cone, Japan, in order to determine the three-dimensional density structure of the scoria cone. We used a nuclear emulsion detector optimized for multi-directional muography that was installed at 11 sites around the volcano. Muon tracks recorded on emulsion films were read with a high-speed automated readout system. We obtained the three-dimensional density structure by applying a linear inversion to the muographic images, and evaluated the uncertainties. High-density zones were detected in scoria cone. Based on these observations and detailed topographic and geological constraints, we infer that high-density zones are the central, highly welded vent of the scoria cone, three-directional radial dikes extending from the central vent, respectively. We also infer that an E-W-trending dike fed a small lava flow, and that a SSE-NNW-trending dike caused a small flank eruption and produced a crater on the flank of the cone. Our results visualize the three-dimensional internal structure of volcanoes with novel resolution, and thereby demonstrate that the formation process of volcanoes can be discussed in detail. Since it has been difficult to directly detect such clear visualization of the internal structures and eruptive processes in the past, multi-directional muography has resulted in providing one hope for understanding volcanic activity.