Seismic imaging of arc magma and fluids under the central part of northeastern Japan

A dense seismic network was temporarily deployed during 1997-1999 in the central part of northeastern Japan by Japanese university research groups. Using the local earthquake arrival time data recorded by the network, we have determined high-resolution three-dimensional images of P-wave velocity (V_p), S-wave velocity (V_s) and V_p/V_s ratio in this area. Our results show that low V_p, low V_s and high V_p/V_s areas are extensively distributed in the uppermost mantle along the volcanic front. In the lower crust, low V_p, low V_s and high V_p/V_s areas are visible but they are confined to the individual volcanic areas. In contrast, the upper crust of volcanic areas shows low V_p, low V_s and low V_p/V_s rather than high V_p/V_s. These observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas. Low V_p, low V_s and low V_p/V_s in the upper crust of the volcanic areas suggest the presence of H₂O (rather than melt) there. Deep low-frequency microearthquakes, perhaps caused by the rapid movement of fluids, are located mostly at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low V_p/V_s, suggesting that most of them are filled with H₂O.