Segmentation of the Izu-Bonin and Mariana plates based on the analysis of the Benioff seismicity distribution and regional tomography results

We present a new model of P- and S-velocity anomalies in the mantle down to 1300 km depth beneath the Izu-Bonin and Mariana (IBM) arcs. This model is derived based on tomographic inversion of global travel time data from the revised ISC catalogue. The results of inversion are thoroughly verified using a series of different tests. The obtained model is generally consistent with previous studies of different authors. We also present the distribution of relocated deep events projected to the vertical surface along the IBM arc. Unexpectedly, the seismicity form elongated vertical clusters instead of horizontal zones indicating phase transitions in the slab. We propose that these vertical seismicity zones mark zones of intense deformation and boundaries between semi-autonomous segments of the subducting plate. The P- and S-seismic tomography models consistently display the slab as prominent high-velocity anomalies coinciding with the distribution of deep seismicity. Based on joint consideration of the tomography results and the seismicity distribution we propose a scenario of the subduction evolution in the IBM zone during the recent time. We can distinguish at least four segments which subduct differently. The northernmost segment of the Izu-Bonin arc has the gentlest angle of dipping which is explained by backward displacement of the trench. In the second segment, the trench stayed at the same location, and we observe the accumulation of the slab material in the transition zone and its further descending to the lower mantle. In third segment, the trench is moving forward that causes steepening of the slab. Finally, for the Mariana segment, despite the backward displacement of the arc, the subducting slab is nearly vertical. We propose that it might be due to the high density of the slab which is responsible for turning any inclined subduction to the vertical position. Between the Izu-Bonin and Mariana arcs we clearly observe a gap which is traced down to about 400 km depth.