Rupture processes of the 2021 and 2022 Fukushima-oki earthquakes: adjacent events on the complex fault system in the subducting slab

Abstract

The 2021 Mw 7.1 and 2022 Mw 7.4 Fukushima-oki earthquakes ruptured adjacent regions in the subducting slab, which gave us a good opportunity to better understand the rupture process of an intraslab earthquake and the fault system in a subducting slab hosting such large earthquakes. We developed source models of the two earthquakes by constructing fault models based on the distributions of relocated aftershocks and performing joint source inversion using strong motion, teleseismic and geodetic data. The results showed that the 2021 earthquake was initiated by the west-northwest dipping fault and that it then ruptured the east-southeast dipping fault. The rupture propagated to the southwest and up-dip directions. For the 2022 earthquake, the rupture primarily propagated to the north-northeast and up-dip directions on another east-southeast dipping fault, but a delayed rupture occurred around the hypocenter approximately 12 s after the rupture initiation. This was probably due to the complex fault system around the hypocenter. Our source models accurately reproduced observed data for both earthquakes, indicating that the fault geometry was appropriate. We found that the source faults of these earthquakes had similarities to faults in the outer-rise region, which suggests that the 2021 and 2022 earthquakes occurred on faults that originally formed in the outer-rise region and reactivated in the subducting slab. Such a fault system in the subducting slab was probably one of the factors that controlled the rupture processes of the two earthquakes. Graphical Abstract: [Figure not available: see fulltext.]