Mantle beneath the Japanese arc

Seismic velocity structure of the mantle beneath Japan has been determined on the basis of teleseismic explosion, long-period surface wave, and dΔ/dt data. A marked difference in structure is found across the deep seismic plane which dips from the Pacific Ocean side towards the continent. The mantle on the continental side of the seismic plane has such low velocity and Q (≈80) as to require a partial melting in addition to a possible compositional change. The low velocity seems to be associated with the underlying earthquake activity. It is found that the lower bound of the low velocity zone is relatively abrupt; this also provides a favorable evidence for the partial melting. A clear later phase branch observed at 15° < Δ < 20° suggests a rapid velocity increase at depths 375 to 400 km. This rapid velocity increase can be attributed to the onset of the olivine-spinel phase change of (MgFe)_2SiO_4, if the temperature at 400 km depth is about 1300 °C. This low temperature may call for a hydrous state in the upper mantle in order to cause the partial melting. The earthquake energy release has a notable maximum around 400 km depth in Japan region. The coincidence of this depth with that of the rapid velocity increase suggests that the olivine-spinel phase change is in some way related to the generation of deep focus earthquakes