Tokachi-dake volcano is one of the most active volcanoes in Japan, and magmatic eruptions occurred three times during the 20^th century. We reinvestigated the recent eruptive history and eruption style of this volcano on the basis of geological and petrological studies. Distinct whole-rock chemistry of proximal deposits from each source crater area allows us to correlate distal tephras with proximal deposits. The volcanic activity during the last 3,300 years can be divided into four stages, which has occurred from distinct craters areas. Stage I : Most explosive and voluminous eruptions had occurred after ＞10,000 years’ repose period to form the Ground crater. The eruption was initiated by scoria and pumice fallout followed by pyroclastic flows (3,300 cal yBP). This stage was terminated by effusion of lava flow. Stage II : Explosive eruptions had repeated three times to form a maar and multiple scoria cones at the northwestern flank of the edifice. The activities were followed by effusion of basaltic lava flows. These activities would occur around ca. 1,000yBP. Stage III : After several hundreds years' dormancy, magmatic activity started again with explosive eruption to form the Central cone, which was followed by effusion of lava flows from the cone. This possibly continued from 800 to 300 years ago. Stage IV : Since AD1926, magmatic eruptions have repeated near the Central cone. In AD 1962, explosive and voluminous eruption occurred to form new craters at the southern flank of the cone. Considering the similar temporal change of eruption style in each stage, from explosive to lava effusion, it could be considered that 1962 eruption was the initial explosive eruption of the new stage. Total amount of effused magma during 3,300 years is about 0.1km³ DRE (dense rock equivalent), and that of each eruption is less than 0.02km³ DRE. Thus, magma discharge rate of the volcano is quite small compared with other active volcanoes in Japan. However, mud flows had repeatedly occurred during the last 3,300 years, suggesting that serious hazard by mud flows should be considered in the volcano as in the case of 1926 eruption.

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Source mechanisms of harmonic tremors observed after B-type earthquake swarm (HTB) and those immediately after explosive eruption (HTE) at Sakurajima are estimated by inversion of root mean square (RMS) seismic amplitudes of 3 components at 5 stations. HTB and HTE had a similar source mechanism that indicates isotropic components of more than 50%. Source depths of the HTB and HTE correspond to the location of a gas pocket formed at uppermost part of the conduit. We infer that HTB is generated by resonance of the gas pocket formed after swarm of B-type earthquake and that HTE is related with resonance of the gas pocket of which top is partially collapsed by the explosion.

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