Based on the finite-fault joint inversion of teleseismic body waves, onshore and offshore strong-motion waveforms, researchers led by Prof. ZHANG Wenbo and Dr. ZHENG Ao from the University of Chinese Academy of Sciences (UCAS) have reconstructed the rupture processes of the Fukushima-Oki earthquake doublet.
The study was published in Tectonophysics.
Subduction zone seismicity is mainly divided into interplate earthquakes that occur at the contact surface between two converging plates, and intraplate earthquakes that occur within a single plate. The Pacific Plate subducts beneath the Okhotsk Plate at a rate of 9 cm per year, forming the Japan Trench subduction zone. Seismic activity in this region has recently reactivated with the consecutive Mw 7.1 and Mw 7.3 earthquakes on February 13, 2021 and March 16, 2022, respectively. The epicenters of the two earthquakes were only a few kilometers apart and both were located within the subducting Pacific Plate. The pair of events with such similar timing, location, and magnitude constitute an intraplate earthquake doublet.
In this study, the researchers showed that the 2021 and 2022 earthquakes were both unilateral ruptures propagating to the southwest and northeast, respectively, with larger slips concentrated away from the rupture initiation area and closer to the top of the fault plane.
The aftershock distributions of both events also exhibit similar spatial patterns. Since the fault strikes of both earthquakes are parallel to the axis of the Japan Trench, the geometric characteristics of the seismogenic faults are consistent with the bending-related faults formed by bending deformation of the Pacific Plate prior to subduction.
Furthermore, the mainshock and aftershock rupture zones of the Fukushima-Oki doublet are concentrated in the upper plane of the double seismic zone within the subducting plate, indicating that the down-dip compressional stress regime was generated when plate underwent unbending deformation, further reactivating preexisting bending-related faults within the plate, and leading to the occurrence of the earthquake doublet.
In addition, fluids released by dehydration reactions of hydrous minerals in the subducting plate can effectively reduce the fault strength, making the shallow part of the fault is easier to generate large coseismic slips under the combined effect of buoyancy.
Seismic tomography imaging results show that the fluid content in the rupture initiation area of the two earthquakes is relatively low, but a small-scale structure enclosed by the two seismogenic faults forms a slab slice within the subducting plate, which is located just before the subduction front of a seamount in the Pacific Plate. The stress concentration caused by the subducting seamount makes this small-scale structure an unstable zone, and it is easier for the source rupture to nucleate and be triggered here.
Therefore, the heterogeneous fluid content and the effect of seamount subduction in the subducting plate are both important factors in the seismogenic environment of the Fukushima-Oki doublet, and they control the spatial distribution of fault slips and aftershock sequences.
This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, and the Fundamental Research Funds for the Central Universities.
Conceptual illustration of the seismogenesis of the 2021 and 2022 Fukushima-Oki earthquake doublet. (Image by UCAS)
