Role of fluids and seamount subduction in interplate coupling and the mechanism of the 2021 Mw 7.1 Fukushima-Oki earthquake, Japan

The frequent occurrence of large thrust earthquakes and subsequent tsunamis in a subduction zone are attributed to multiple factors, including structural heterogeneity, fluid saturation, and topographic variations of the subducting oceanic plate. To investigate these impacts on the generation of the 2021 Mw 7.1 Fukushima-Oki earthquake and interplate seismic coupling of the northeastern (NE) Japan subduction zone, high resolution three-dimensional (3-D) seismic structures of velocities (Vp, Vs) and Poisson's ratio (σ) were determined through a multi-parameter joint inversion method using 328,625 travel times of P- and S-wave receiver-source pairs from the earthquakes recorded by onshore seismic stations and ocean bottom seismometers. We found that more than 92.5% of M 6+ thrust earthquakes occurred in and around high-V zones, while less than 7.5% of the quakes were located in low-V patches. The high-V zones with high density M ≥ 6.0 thrust earthquakes (including the 2011 Mw 9.0 Tohoku mainshock) were thought to indicate strong interplate seismic coupling (asperities), while the low-V patches lacking large thrust-earthquakes reflected weak interplate coupling or decoupling. The present results revealed that seamount (including the Joban seamount chain and the bulgy Kitamachi volcanic batholith) subduction had a significant influence on seismography and the mechanical strength variation of interplate coupling in the forearc seismogenic zone. It was demonstrated that structural heterogeneities associated with fluids and seamount subduction played a fundamental role in interplate seismic coupling and large thrust-earthquake generation along the slab upper boundary in the NE Japan subduction zone.