東北日本弧の部分溶融域と地震活動の3次元分布

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By using fine velocity structure in the sub-arc region of northeastern Japan, we here prcsent three dimensional mapping of magma source regions in the upper mantle. Distinctive mid-crustal S-wave reflectors and low-frequency microearthquakes have been observed from seismic studies in northeatern Japan. Partial melt zones and regions of -3 to -6% velocity perturbations as well as reflectors and hypocenters of microearthquakes are shown in three dimensions in this study. Partial melt exists locally within the low velocity regions that are distributed from the intermediate-depth of the mantle wedge to right beneath active volcanoes. In between the volcanoes and partial melting zones, S-wave reflectors and low-frequency microearthquakes are observed. Two major low-velocity regions exist below 〜60km depth and branch off at the uppermost mantle, and eventually underplate the Moho right beneath the volcanoes. Extensive volcanic activity at the volcanic front is due to the presence of the vast melting zones right beneath the Moho. Those melting zones may cause melting of lower crustal materials and produce felsic magma. Melt stays underneath the Moho, where crystallization fractionation may proceed. Magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors. The present results indicate that the diapir-like melting regions localized within the low-velocity regions cause volcanic activity in the northeastern Japan arc.
1997-04-25

東北日本弧の部分溶融域と地震活動の3次元分布

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