Evidence for possible deep-seated magmatic activity beneath northeastern Japan can be obtained from seismic observations. Tomographic inversions of P-wave velocity data show low-velocity zones distributed in the crust and upper mantle beneath active volcanoes. In or around these zones, anomalously low-frequency micro-earthquakes, perhaps caused by magmatic activity, are found at depths of 25-40 km; distinct S-wave reflectors, corresponding to the upper surfaces of magma bodies, are found at depths of 10-18 km. Large crustal earthquakes also occur around the low-velocity zones. These observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs.