2:30 PM - 2:45 PM
[SSS08-13] Spatio-temporal variation of the aftershock activity following the 2021 and 2022 intraslab earquakes in off-Fukushima prefecture
Keywords:Intraslab earthquake, Aftershock distribution, Cluster analysis, DBSCAN
On February 13, 2021, a large earthquake (M 7.3) occurred in the Pacific slab offshore of Fukushima Prefecture, causing damages due to strong shaking across broad area in the Tohoku region. On March 16, 2022, about one year after the 2021 earthquake, another large intraslab earthquake (M 7.4) occurred to the north of the 2021 earthquake, causing severe damage again in Fukushima and Miyagi prefectures. The complexity of the fault structure of these two intraslab earthquakes was pointed out by several previous works. Kobayashi et al. (2023) defined multiple faults based on the aftershock distribution immediately after the main shocks and the focal mechanisms, to model the rupture processes of these two earthquakes.
The purpose of this study is to clarify the complex fault structure more comprehensively by analyzing the aftershock distribution over a long period since the first mainshock (M 7.3 in 2021), and to discuss the interaction between the two adjacent and successive large intraslab earthquakes.
The target earthquakes were those of M 2 or greater listed in the JMA catalog, after the 2021 mainshock occurrence until June 2023. The hypocenters were relocated by the double-difference method using waveform correlations. The relocated hypocenters were distributed along several planer structures, reflecting the distribution of faults activated after the first mainshock. We performed a cluster analysis using DBSCAN (Ester et al. 1996) to define dense clusters of hypocenters forming planer shape. We first divided the earthquakes into “the aftershocks of the 2021 earthquake”, the earthquakes after the first mainshock and before the second mainshock in 2022, and “the aftershocks of the 2022 earthquake”, those after the second mainshock. The cluster analysis was performed using these two data sets individually, to identify planer clusters from each of the aftershock activities. We then extracted earthquakes that occurred along all the planes identified in the first step for the entire period, to analyze the spatio-temporal distribution of seismic activities along all the identified fault planes.
The cluster analysis defined three planes from the aftershocks of the 2021 earthquake and three planes from the aftershocks of the 2022 earthquake. We found that the seismicity along one of the planes extracted from the 2021 aftershocks was reactivated after the 2022 earthquake, whereas the seismicities on the other two planes gradually ceased in time. We also found the seismicity on the plane including the hypocenter of the 2022 mainshock had been activated before the 2022 mainshock. These two planes suggesting the interaction between the 2021 and 2022 earthquakes are located near the hypocenter of the 2022 mainshock.
The plane including the 2022 mainshock hypocenter is consistent with one of the major fault planes identified by Kobayashi et al. (2023). The M 6.1 earthquake, the precursor to the M 7.4 earthquake, was also found to belong to the same plane. Most of the earthquakes in this fault plane occurred after the 2022 mainshock, but activity was also observed immediately after the 2021 mainshock. Activity in 2021 occurred on the southern side of the plane, close to the fault ruptured during the 2021 earthquake. The seismicity was particularly persistent until the 2022 mainshock occurrence within a small area around the hypocenters of the 2022 mainshock and the precursory. Activity within the area included an earthquake of M 5.2 that occurred more than a month after the 2021 earthquake.
This study reveals that seismic activity started and continued around the rupture initiation point of the 2022 M 7.4 earthquake immediately after the 2021 M 7.3 earthquake in the Pacific slab offshore Fukushima Prefecture. This would be an important observation for understanding the interaction of adjacent and subsequent M-7 class intraslab earthquakes.
The purpose of this study is to clarify the complex fault structure more comprehensively by analyzing the aftershock distribution over a long period since the first mainshock (M 7.3 in 2021), and to discuss the interaction between the two adjacent and successive large intraslab earthquakes.
The target earthquakes were those of M 2 or greater listed in the JMA catalog, after the 2021 mainshock occurrence until June 2023. The hypocenters were relocated by the double-difference method using waveform correlations. The relocated hypocenters were distributed along several planer structures, reflecting the distribution of faults activated after the first mainshock. We performed a cluster analysis using DBSCAN (Ester et al. 1996) to define dense clusters of hypocenters forming planer shape. We first divided the earthquakes into “the aftershocks of the 2021 earthquake”, the earthquakes after the first mainshock and before the second mainshock in 2022, and “the aftershocks of the 2022 earthquake”, those after the second mainshock. The cluster analysis was performed using these two data sets individually, to identify planer clusters from each of the aftershock activities. We then extracted earthquakes that occurred along all the planes identified in the first step for the entire period, to analyze the spatio-temporal distribution of seismic activities along all the identified fault planes.
The cluster analysis defined three planes from the aftershocks of the 2021 earthquake and three planes from the aftershocks of the 2022 earthquake. We found that the seismicity along one of the planes extracted from the 2021 aftershocks was reactivated after the 2022 earthquake, whereas the seismicities on the other two planes gradually ceased in time. We also found the seismicity on the plane including the hypocenter of the 2022 mainshock had been activated before the 2022 mainshock. These two planes suggesting the interaction between the 2021 and 2022 earthquakes are located near the hypocenter of the 2022 mainshock.
The plane including the 2022 mainshock hypocenter is consistent with one of the major fault planes identified by Kobayashi et al. (2023). The M 6.1 earthquake, the precursor to the M 7.4 earthquake, was also found to belong to the same plane. Most of the earthquakes in this fault plane occurred after the 2022 mainshock, but activity was also observed immediately after the 2021 mainshock. Activity in 2021 occurred on the southern side of the plane, close to the fault ruptured during the 2021 earthquake. The seismicity was particularly persistent until the 2022 mainshock occurrence within a small area around the hypocenters of the 2022 mainshock and the precursory. Activity within the area included an earthquake of M 5.2 that occurred more than a month after the 2021 earthquake.
This study reveals that seismic activity started and continued around the rupture initiation point of the 2022 M 7.4 earthquake immediately after the 2021 M 7.3 earthquake in the Pacific slab offshore Fukushima Prefecture. This would be an important observation for understanding the interaction of adjacent and subsequent M-7 class intraslab earthquakes.