2 TYPES OF DEEP SEISMICITY IN SUBDUCTING SLABS

The relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones (Kuril, NE-Japan, Izu-Bonin, Mariana, and Tonga) is investigated. When the temperature of the core of the slab at depths of 300 approximately 350 km is higher than the kinetic cut-off temperature of the olivine-spinel phase transformation, there is no seismicity maximum in the slab below that depth range. When the temperature of the core of the slab at 300 approximately 350 km depths is lower than the kinetic cutoff temperature, there is a depth range of relatively high seismic activity in the slabs. Moreover, estimated temperature in the depth range of relatively high seismicity is near the kinetic cut-off temperature. It is inferred that the phase transformation of meta-stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300 approximately 350 km in relatively low temperature slabs, when the phase transformation is the cause of deep earthquakes. In relatively high temperature slabs, however, it is considered that the phase transformation occurs in nearly equilibrium conditions and meta-stable olivine does not exist. Other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs.