Time-dependent modeling of slow-slip events along the Nankai Trough subduction zone, Japan, within the 2018–2023 period

Abstract

Abstract According to a 2013 report by the Earthquake Research Committee of Japan, it was estimated that the probability of the occurrence of the next Nankai earthquake within the next three decades is 70–80%. Therefore, to realize hazard estimation, it is crucial to estimate the state of the plate interface in the Nankai Trough subduction zone. In this study, we focused on a transient from 2018 in the subduction zone of the Nankai Trough, southwest Japan, detected by the Global Navigation Satellite System (GNSS) network. Thereafter, we estimated the spatiotemporal evolution of the slip at the plate interface by subjecting the GNSS position time series to time-dependent inversion. The results obtained showed that a long-term slow-slip event (l-SSE) possibly occurred on the Kii Peninsula in 2020. The Kii-Channel l-SSE (Mw 6.3), with an irregular recurrence interval, was observed from 2019 to 2022. Additionally, the Central Shikoku l-SSE (Mw 6.5) was observed from 2019 to 2023, and there appeared to be a correlation between the Central Shikoku l-SSE and the northwestern Shikoku short-term slow-slip event (s-SSE). l-SSE occurred from July 2018 to August 2019 in the northern Hyuga-nada and Bungo channel, and in late 2018, another l-SSE occurred in southern Hyuga-nada before the May 2019 Hyuga-nada earthquake. Further, after the 2018–2019 events, the southern Hyuga-nada l-SSE occurred from mid-2020 to early 2021 and in January 2023 and is still ongoing, consistent with the expected recurrence interval. The seemingly transient slip off the Ohsumi Peninsula, detected after the Mw 5.7 earthquake of October 2022, continued until July 2023 with Mw 6.1. Furthermore, after the 2019 Tanegashima earthquake, an l-SSE occurred for approximately 4 years. The moment magnitude (Mw) of the afterslip of the Tanegashima earthquake was estimated to be 6.7, larger than that of the main shock (Mw 6.4). We also noted that the transient slip off Tanegashima included four possible s-SSEs that occurred in 2020, 2021, 2022, and 2023. Therefore, the sporadic aseismic slips along the Nankai Trough changed the stress state of the areas neighboring the aseismic slip zones in favor of the interplate slip. Graphical Abstract

Keywords

• Slow-slip event
• Hyuga-nada Sea
• Bungo Channel
• Central Shikoku
• Kii Channel
• Tanegashima