Heterogeneous outer‐rise seismicity within the Izu–Bonin subduction zone and its tectonic implications

Abstract

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.

Tectonics of the Solomon Sea Basin from Vertical Gravity Gradient and Seismic Data

“…Large-scale normal faults induce the formation of horsts, which further contributes to the high VGG. In the forebulge region, a series of normal faults were caused by the flexural deformation of the subducting Solomon Sea plate (Zhang et al, 2014;Zhou et al, 2015;Zhou and Lin, 2018;Gong et al, 2019bGong et al, , 2021. Similar to that of the AA' section, the forebulge region in the BB' section also presents an increasing VGG towards the trench as a whole.…”

“…This shows certain limitations in analyzing the structure of the basin by VGG, which can only reflect fault activity on a slightly greater structural scale. In the forebulge region, a series of large-scale normal faults, which are inclined towards the trench and induced by the flexural deformation of the subducting plate (Zhang et al, 2014;Zhou et al, 2015;Zhou and Lin, 2018;Gong et al, 2019bGong et al, , 2021, do not have a good spatial correspondence with the VGG, presenting a continuously increasing trend towards the trench. This may be related to the large-scale structural characteristics of the trenches.…”

“…For the active forebulge region, hypocenters and focal mechanisms are usually used to constrain structural characteristics (Garcia-Castellanos et al, 2000;Ammon et al, 2008;Gong et al, 2021). As shown in Fig.…”

Heterogeneous morphology of the Solomon Sea slab along the New Britain Trench and its dynamic implications from the analysis of seismicity and gravity anomalies

“…At depths of 0–50 km in the outer trench slope region, the stress axes present a pure normal‐faulting regime. The NE horizontal tensional stress is nearly parallel to the subduction direction of the plate, which means that it should be induced by the bending of the subducting plate at the trench (Gong et al, 2021; Gong, Jiang, Xing, Xu, & Xu, 2019; Zhang et al, 2014). At depths of 0–50 km in the fore‐arc region, the stress field is characterized by a typical compressional stress environment.…”