A study of deep structure of volcanoes and crustal earthquakes by seismic observations

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Possible evidence for manmatic activity in the crust and the uppermost mantle beneath northern Japan has been obtained from seismic observations. Tomographic image of the 3-D seismic velocity structure clearly delineate low-velocity zones, which are continuously distributed from the upper crust just beneath active volcanoes to the upper mantle. They are inclined to the west and nearly parallel to the dip of the subducted oceanic plate. Exceptionally deep (22-40km), low-freqyebcy micro earthquakes have been found at 12 locations around the low-velocity zones beneath volcanoes, suggesting a close relation to the deep magmatic activity. At shallower depth (10-15km) in or around the low-velocity zones, there exist distinct S-wave reflectors. Observed amplitude spectral rations of reflected to direct S-wave show that the reflector has a strong velocity contrast to the surrounding medium and its thickness is in the order of 100m, which suggests that the reflector body is a thin layr filled with partially molten materials. A detailed seismicity study reveals that the cutoff depth for shallow crustal seismicity changes with location and becomes shallow beneath active volcanoes. These observations appear to shed some light on the state of magma at depths and its close relation to shallow seismic activity in subduction zones.