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

日本北方地壳和上地幔中的地幔活动可能是由地震观测获得的。三维地震速度结构的层析成像图像清楚地描绘出低速带，这些低速带从上地壳到上地幔连续分布。它们向西倾斜，几乎与俯冲洋板块的倾角平行。在异常深（22- 40 km）的火山低速带周围的12个地点发现了低频微震，表明与深部岩浆活动有密切关系。在低速带内或其周围较浅的深度（10- 15 km），存在明显的S波反射。反射横波与直达横波振幅谱比的观测结果表明，反射体与周围介质有很强的速度对比，反射体厚度约为100 m，反映了反射体是一个充满部分熔融物质的薄层。详细的地震活动研究表明，地壳浅层地震活动的截止深度随位置而变化，在活火山下方变浅。这些观察似乎揭示了岩浆在深处的状态及其与俯冲带浅层地震活动的密切关系。

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.