Seismic velocity structure of the subducting Pacific Plate in the Izu-Bonin Region (original) (raw)

1992, Journal of Geophysical Research

Although numerous studies have been made on the contrast between subducting slabs and surrounding mantle, little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones. The velocity structure within the slab can be expected to be heterogeneous, based on thermal and petrological considerations. In this study, observed travel time residual data from a spatially dense seismic network above deep earthquakes in the Izu-Bonin region are compared with three-dimensional ray tracing calculations. The data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations. The residual data can be explained by a model that has a velocity gradient within the slab. In the subducting Pacific plate, the velocity near the center of the slab is faster than that near the upper boundary, and gradually decreases toward the bottom of the plate. A model with a velocity decrease of 3%, as predicted by a thermal profile, explains the observed data. The subducting plate is heated by the surrounding mantle after subduction at the trench. Thermal models of the subducting plate [e.g., McKenzie, 1969; Hsui and Toksoz, 1979] show that the temperature within the slab increases from the central part toward both sides, resulting in a heterogeneous thermal distribution. The velocity structure within the slab can thus be expected to be heterogeneous. Heterogeneous velocity models of deep slabs have been developed using thermal models and observed travel time and amplitude data [Sleep, 1973; Creager and Jordan, 1984, 1986; Engdahl and Gubbins, 1987; Vidale, 1987; Fischer et al., 1988; Vidale and Gonzalez, 1988; Cormier, 1989; Iidaka et al., 1989c; Weber, 1990]. However, there have been few studies of heterogeneous velocity structure within the slab, due to the lack of dense seismic stations with suitable ray paths. However, the Japan arc is an exception because of the spatially dense local network. Beneath the Japan region, the heterogeneous velocity structure within the slab has been revealed by many seismological studies, including travel time residual analysis and converted phase analysis [Suyehiro and Sacks, 1979; Matsuzawa et al., 1986; Ando et al., 1989; Iidaka and Mizoue, 1991].