In order to understand magma supplying system to volcano, a detailed study on the spatial distribution of S wave reflectors around volcano is fundamental and effective. Distinct S waves reflected from a midcrustal seismic velocity discontinuity have been detected beneath Nikko-Shirane volcano in the southernmost part of the northeastern Japan arc. NMO record sections presently acquired through a dense seismic network temporarily set up in this region clearly show the existence of some other S wave reflectors distributing in this region. A detailed travel time analysis of the reflected S waves by inversion shows that two of these unusual S wave reflectors are distributed over an area of 15×15 km² south of Nikko-Shirane volcano at depths ranging from 8 to 15 km and an area of 15×30 km² north of the volcano at depths ranging from 12 to 16 km, respectively. The reflector located to the south of the volcano has a conical shape becoming shallow toward the summit of Nikko-Shirane volcano. The reflector north of the volcano is slightly shallowing toward northeast direction. These reflectors are connected with the extremely low velocity zone, which is estimated from a travel time tomography. Especially, the reflector south of the volcano comes in touch with the top of the low velocity zone, which suggests that magma is supplied to the volcano through two ways. A proposed model for magma supply system by Takahashi et al. (1995) based on petrological constraints, in which two type magmas are mixed just before eruption, supports the present interpretation.

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March 1996 eruption of Hokkaido-Komagatake was phreatic explosions discharging ash of over 30,000 ton. Volcano explosivity index of the eruption was 1. All of the ash and lapilli discharged during the eruption were derived from pre-existing andesitic lavas and pyroclastics of the volcano. No juvenile materials was identified. Glassy particles found in the ash sample were derived from hydrothermally altered part of the volcanic edifice grown in historic age. The discharged ejecta are characterised with their abraded external shape and abundant amount of fine ash. Cumulative volume-time relation of the ejecta within 3,000 years reveals current capacity of major plinian eruption 0.1-0.3 km³ in volume.

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