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[SSS12-17] Spatio-temporal variation in seismic activity around the source areas of the 2021 and 2022 M7 intraslab earthquakes off Fukushima Prefecture
Keywords:Intraslab earthquake, Aftershock distribution, Cluster analysis, DBSCAN
After the 2011 Tohoku-Oki earthquake (hereafter referred to as the Tohoku-Oki earthquake), two M7-class intraslab earthquakes occurred within the Pacific slab off Fukushima Prefecture in 2021 and 2022. The first event, an M7.3 earthquake, occurred on February 13, 2021, followed by an M7.4 earthquake on March 16, 2022, in a closely adjacent area. These two M7 earthquakes, which were likely influenced by the Tohoku-Oki earthquake and its postseismic deformation, caused strong ground shaking over a wide area of the Tohoku region, resulting in significant damage. Following the Tohoku-Oki earthquake, intraslab seismicity increased along the downdip extension of the mainshock rupture (Kato and Igarashi, 2012). Within the Pacific slab off Fukushima, where the two M7 earthquakes occurred, seismic activity was notably more pronounced than in other intraslab regions surrounding the Tohoku-Oki rupture zone. This study aims to elucidate the interactions between interplate earthquakes and intraslab earthquakes, as well as interactions among intraslab events, by conducting a detailed analysis of the spatiotemporal evolution of intraslab seismic activity on and around the source faults of the two M7 earthquakes. To achieve this, we investigate the characteristics of seismic activity spanning more than a decade before and after the Tohoku-Oki earthquake, referencing fault-plane models of the two M7 events to characterize the activity of each fault plane.
The earthquakes analyzed in this study are those with magnitudes of M2 or greater listed in the Japan Meteorological Agency (JMA) unified earthquake catalog, covering the period from March 8, 2003, to July 10, 2023. The hypocenters were relocated using the Double-Difference method (Waldhauser and Ellsworth, 2000), incorporating arrival-time differences measured by waveform cross-correlation analyses. To identify planar seismic clusters, we applied a clustering analysis used by Osawa et al. (2024, SSJ) to the obtained hypocenter distribution. These planar clusters were interpreted as corresponding to fault planes that ruptured during major earthquakes. Fault-plane models were constructed by fitting planes to the hypocenters belonging to each planar cluster. Among the identified fault-plane models, four correspond to the source faults of the 2021 M7.3 earthquake, while two distinct models correspond to the source faults of the 2022 M7.4 earthquake. Additionally, a fault-plane model associated with the intraslab M6.5 earthquake that occurred off Fukushima on August 19, 2011—approximately six months after the Tohoku-Oki earthquake—was also identified based on a planar cluster formed by its aftershocks (Osawa et al., 2024, SSJ).
By analyzing the spatiotemporal variations in seismic activity before and after the Tohoku-Oki earthquake based on the obtained fault-plane models, we found no significant seismic activity preceding the rupture of the 2021 M7.3 earthquake on its source faults. In contrast, foreshock-like seismic activities were observed on two fault planes of the 2022 M7.4 earthquake. Among them, seismic activity on the fault plane containing the rupture initiation point of the M7.4 earthquake began immediately after the 2021 M7.3 earthquake. On the other fault plane (hereafter referred to as Fault 6), seismic activity was found to have begun after the 2011 Tohoku-Oki earthquake. The regions of enhanced seismic activity on Fault 6 after the Tohoku-Oki earthquake were concentrated along the down-dip and up-dip edges of the fault. These regions closely overlap with areas where aftershocks of the 2022 M7.4 earthquake were densely distributed. The concentration of seismic activity along the down-dip side of Fault 6 coincides with another planar seismic cluster identified from hypocenter distributions before the two M7-class earthquakes (from March 8, 2003, to February 13, 2021).
Around the upper edge of Fault 6, one of the two fault planes that ruptured during the 2022 M7.4 earthquake, swarm-like seismic activity began immediately after the mainshock. Since this seismic activity occurs near the intersection of Fault 6 and the plate interface, it is possible that the M7.4 intraslab earthquake triggered acceleration of aseismic slip on the plate interface, inducing swarm seismicity along the plate boundary.
The earthquakes analyzed in this study are those with magnitudes of M2 or greater listed in the Japan Meteorological Agency (JMA) unified earthquake catalog, covering the period from March 8, 2003, to July 10, 2023. The hypocenters were relocated using the Double-Difference method (Waldhauser and Ellsworth, 2000), incorporating arrival-time differences measured by waveform cross-correlation analyses. To identify planar seismic clusters, we applied a clustering analysis used by Osawa et al. (2024, SSJ) to the obtained hypocenter distribution. These planar clusters were interpreted as corresponding to fault planes that ruptured during major earthquakes. Fault-plane models were constructed by fitting planes to the hypocenters belonging to each planar cluster. Among the identified fault-plane models, four correspond to the source faults of the 2021 M7.3 earthquake, while two distinct models correspond to the source faults of the 2022 M7.4 earthquake. Additionally, a fault-plane model associated with the intraslab M6.5 earthquake that occurred off Fukushima on August 19, 2011—approximately six months after the Tohoku-Oki earthquake—was also identified based on a planar cluster formed by its aftershocks (Osawa et al., 2024, SSJ).
By analyzing the spatiotemporal variations in seismic activity before and after the Tohoku-Oki earthquake based on the obtained fault-plane models, we found no significant seismic activity preceding the rupture of the 2021 M7.3 earthquake on its source faults. In contrast, foreshock-like seismic activities were observed on two fault planes of the 2022 M7.4 earthquake. Among them, seismic activity on the fault plane containing the rupture initiation point of the M7.4 earthquake began immediately after the 2021 M7.3 earthquake. On the other fault plane (hereafter referred to as Fault 6), seismic activity was found to have begun after the 2011 Tohoku-Oki earthquake. The regions of enhanced seismic activity on Fault 6 after the Tohoku-Oki earthquake were concentrated along the down-dip and up-dip edges of the fault. These regions closely overlap with areas where aftershocks of the 2022 M7.4 earthquake were densely distributed. The concentration of seismic activity along the down-dip side of Fault 6 coincides with another planar seismic cluster identified from hypocenter distributions before the two M7-class earthquakes (from March 8, 2003, to February 13, 2021).
Around the upper edge of Fault 6, one of the two fault planes that ruptured during the 2022 M7.4 earthquake, swarm-like seismic activity began immediately after the mainshock. Since this seismic activity occurs near the intersection of Fault 6 and the plate interface, it is possible that the M7.4 intraslab earthquake triggered acceleration of aseismic slip on the plate interface, inducing swarm seismicity along the plate boundary.