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°N and 32.5°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.

中文翻译：

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伊豆-波宁俯冲带内的非均质外层地震活动及其构造意义

结合现有的聚焦机制，我们选择了 13 个事件（M ≥ 3.5) 用于波形反演以确定伊豆-博宁俯冲带外上升区域的应力场。以震中分布、地震能量释放和外升区应力场为特征，俯冲带被划分为29°N和32.5°N的三段。外层地震活动在俯冲大洋板块上部（0～40km）呈现拉伸应力场。而外隆起区域内的最大震源深度趋向于向北增加，俯冲板片的深部（> 40 km）逐渐受到以前从未观察到的一般压应力状态的支配，这意味着旧太平洋板块沿伊豆-博宁海沟俯冲的特点是不均匀的弯曲变形。根据对海洋岩石圈弯曲弯曲的模拟，观察到伊豆-博宁俯冲带的有效弹性厚度（Te）沿海沟走向从 38 公里减小到 28 公里，表明板块刚度向北减小。Te的空间变化准确地解释了伊豆-博宁俯冲带内的异质外升地震活动，这是俯冲板块弯曲地层的能量释放。正如这里给出的结果所示，俯冲太平洋板块的不同弯曲变形与俯冲板块的非均质热年龄、变化的形态和耦合程度密切相关。据观察，伊豆-博宁俯冲带的有效弹性厚度（Te）沿海沟走向从 38 公里减小到 28 公里，表明板块刚度向北减小。Te的空间变化准确地解释了伊豆-博宁俯冲带内的异质外升地震活动，这是俯冲板块弯曲地层的能量释放。正如这里给出的结果所示，俯冲太平洋板块的不同弯曲变形与俯冲板块的非均质热年龄、变化的形态和耦合程度密切相关。据观察，伊豆-博宁俯冲带的有效弹性厚度（Te）沿海沟走向从 38 公里减小到 28 公里，表明板块刚度向北减小。Te的空间变化准确地解释了伊豆-波宁俯冲带内异质外升地震活动，这是俯冲板块弯曲地层的能量释放。正如这里给出的结果所示，俯冲太平洋板块的不同弯曲变形与俯冲板块的非均质热年龄、变化的形态和耦合程度密切相关。这是俯冲板块弯曲形成的能量释放。正如这里给出的结果所示，俯冲太平洋板块的不同弯曲变形与俯冲板块的非均质热年龄、变化的形态和耦合程度密切相关。这是俯冲板块弯曲形成的能量释放。正如这里给出的结果所示，俯冲太平洋板块的不同弯曲变形与俯冲板块的非均质热年龄、变化的形态和耦合程度密切相关。