Heterogeneous strain regime at the west of the Ogasawara Plateau in the Western Pacific Ocean from inversion of earthquake focal mechanisms

Abstract

The Ogasawara Plateau has a major control on the geology of the region between the Izu-Bonin and Mariana subduction zones. This region is important for understanding active collisional zones. We attempt to illustrate the stress field in the Wadati-Benioff zone of this region through seismic estimations. 49 events (M ≥ 3.5) are chosen for waveform inversion by the ISOLA software to distinguish the stress field. According to available focal mechanism solutions, the heterogeneous stress field parameters are determined for several depth ranges. Based on the epicentral distribution and seismic stress field, the subduction system at the west of the Ogasawara Plateau can be divided into three segments separated by 26.5°N and 24°N. At depths of 0–50 km in the outer-trench slope, the slab is characterized by a varying stress filed. The best-fit stress model with horizontal σ3 and vertical σ1 is related to the plate bending and preexisting faults. For the central segment, the solutions present a compressional stress regime in the outer-trench slope without preexisting faults, induced by the resistance of the buoyant plateau. The solutions for the depth range of 0–50 km in the forearc region, σ1 coincides with the plate convergence and σ3 dips steeply to the west in the northern and central segments, controlled by the friction effect and convergence of the plates. However, this depth range in the southern segment shows an extensional stress range, associated with subduction of older slab segment, relatively low rate of plate convergence and the morphological adjustment of the slab between different segments. At depths of 50–200 km, the inhomogeneous stress field exhibits a complicated extensional stress system with strike-slip focal mechanisms. At depths of 200–400 km, the seismicity presents a pronounced seismic gap and the stress field cannot be determined. The seismic gap may be attributed to the missing or tear of the subducting slab, which is caused by the subduction of the buoyant Ogasawara Plateau. The mechanism solutions are characterized by an inhomogeneous strain regime below 400 km, driven by gravity and strong contortion of the slab and resistance to penetration of the 660-km discontinuity. In contrast, there are little earthquakes below 400 km in the southern region of the Ogasawara Plateau.