Imaging of Mantle Diapir by Seismic Waves

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We carried out temporary seismic observations in two areas, Nikko area in Tochigi Prefecture and Onikobe area in Miyagi Prefecture, where anomalous seismic structure has been revealed. Based on the analyzes of data by these dense seismic observations or other stationary observation networks, we obtained the following results.Estimation of 3D seismic attenuation structure of the crust and upper mantle shows the existence of the inclined low-Q bodies in the crust and upper mantle. Spatial distribution of the low-Q bodies roughly coincides with that of seismic low-velocity bodies imaged by the travel time tomography.Low-velocity, low-Q bodies detected in the crust of Nikko and Onikobe areas are closely correlated in space with active volcanoes or caldera structure distributing in these areas. Mid-crustal S-wave reflectors have been detected in these areas, and they are also closely related in space with active volcanoes and caldera structure.Seismic low-velocity, low-Q bodies in the mantle wedge corresponding to the mantle diaptr, deep low-frequency microearthquakes and midcrustal S-wave refllectors reflect the root of arc volcanoes, and it is closely related with the occurrence of shallow inland earthquakes above it. Based on these obsservations we have constructed a qualitative model to explain the observed pattern of shallow seismic activity.