Usu volcano, Hokkaido, Japan, burst into eruption on August 7 (09h12m local time), 1977, after 32 years’ dormancy since Showa-Sinzan lava dome was formed. In the present paper, focal mechanism solutions of 759 earthquakes (M : 2.8-4.3), which swarmed associating with the 1977-1978 eruptions of the volcano, are analyzed for the periods of October 1977, March and August 1978, and January and June 1979, using P-wave first motions at 8-12 seismometric stations. The results show that the focal mechanisms were almost equal to each other for each seismic cluster and each locality throughout 20 months, irrespective of the occurrence of major explosions (July to September 1978) and sudden increase in seismic activity and crustal deformations (the end of January 1978). The distribution of the principal stress axes can not be explained by the tectonic stress in this region. The spatial distribution of the compressional stress axes may suggest the intrusion of inclining magma body beneath the volcano. Remarkable uplift and northeastward deformation of the volcano may support this model.
