Tomographic imaging of seismic velocity structure in and around the Onikobe volcanic area, northeastern Japan: implications for fluid distribution

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Abstract We estimated three-dimensional P- (Vp) and S-wave velocity (Vs) and Vp/Vs structures in and around the Onikobe volcanic area, northeastern Japan, by local travel time tomography. We used travel time data from source and receiver pairs located within and outside the study area, which plays an important role in obtaining the optimum ray coverage and in elucidating the deeper structure more accurately. Detailed information on deeper structures is essential for imaging the complete volcanic system from the magmatic source zone through areas of shallow hydrothermal circulation. More than 50 000 travel time data for the P-waves and 35 000 for the S-waves were used to image the velocity structure. Our results show the following dominant features: (1) two conduits in the upper crust with low Vp and low Vs indicative of H2O-rich fluid pathways: one lying beneath Naruko volcano, the other beneath the focal area of the 1962 Northern Miyagi earthquake (M6.5); (2) an underlying broad region in the lower crust with low Vp, low Vs and high Vp/Vs, suggestive of a zone of partial melt, from which the fluids in (1) are derived; and (3) low Vp/Vs areas near the surface of the Sanzugawa and Onikobe calderas, suggesting a diffuse vapor-saturated cap.

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