Heterogeneous Outer-Rise Seismicity Within The Izu-Bonin Subduction Zone And Its Tectonic Implications

Combining with available focal mechanisms, we chose 13 events (M >= 3.5) for waveform inversion to determine the stress field at the outer-rise region within the Izu-Bonin subduction zone. Indicated by the distribution of the epicentres, seismic energy release, and stress field within the outer-rise region, the subduction zone is divided into three segments separated by 29 degrees N and 32.5 degrees N. The outer-rise seismicity presents an extensional stress field in the upper part (0-40 km) of the subducting oceanic plate. Whereas, the maximum focal depth within the outer-rise region tends to increase northwards, and the deep part (>40 km) of the subducting slab is gradually governed by a general compressional stress regime that has never been observed before, which implies that the subduction of the old Pacific Plate along the Izu-Bonin trench is characterized by a heterogeneous bending deformation. According to modelling the flexural bending of the oceanic lithosphere, it is observed that effective elastic thickness (Te) of the Izu-Bonin subduction zone decreases from 38 to 28 km along the trench strike, indicating a decrease of plate rigidity northwards. The spatial variations of Te exactly explain the heterogeneous outer-rise seismicity within the Izu-Bonin subduction zone, which is the energy release of the bending formation of the subducting plate. As shown in the results presented here, the varied flexural deformation of the subducting Pacific Plate is closely correlated with the heterogeneous thermal age, the varied morphology, and the coupling degree of the subducting plate.