Muography for structural characterization of volcanoes: a case study at Mount Unzen, Japan

SUMMARY Destabilization of volcanic edifices can generate debris avalanches with catastrophic impacts on their environment. We present the first high-resolution muography of Mount Unzen, Japan, conducted to characterize the structure of lava lobes formed on the volcano’s summit and flank during the 1990–1995 eruption. A multi-wire-proportional-chamber-based muon tracking system was operated for 203 d. The obtained high-resolution muographic image shows the internal density structure of Mount Unzen with a spatial resolution of 12 m. Mean densities were respectively measured as 2470 and 2290 kg m−3 for the base rock and a fracture zone, and both were consistent with the results of prior drilling and sampling experiments. The mean density of lava lobes was measured significantly lower value of 1570 kg m−3, indicating post-eruptive structural weakening. A comparison between the time-series of muographically measured density-lengths and daily precipitation records suggest that rainfall-induced gravitational destabilization did not occur during the observational period. This work demonstrates that long-term (multi-year) muon monitoring of the lava lobes can provide valuable complementary information for volcanic stability assessments.