Fig. 1

Reconstructed average density distribution for each muon path line in Mt. Asama. The average density distribution is plotted as a projection on the cross-sectional plane. The distribution of the average density was reconstructed by applying the range-energy relationship for cosmic-ray muons through rock to the path length (L) calculated from the surface shape of the mountain for each arriving angle. The high attenuation region (yellow to red) coincides with the position and shape of the lava mound emplaced in the 2003 crater floor during the 2004 eruption (between white and black dashed lines) with dense lava. A low-density region below (blue) is interpreted as the fall-back of lava in the conduit. The fall-back is consistent with the intermittent collapses of the crater floor that occurred between 22 October 2004 and 11 January 2006. The thickness of the lava mound has been reconstructed from Airborne SAR scanning of the crater floor surface (performed on 11 January 2006).