Variation in Occurrence Patterns of Large Earthquakes in the Southern Part of Hyuga-nada

Hyuga-nada is among the most seismically active regions in Japan. Numerous major interplate earthquakes of approximately magnitude 7 have occurred in the southern part of Hyuga-nada off southern Miyazaki Prefecture at approximately 30-year intervals. The Mj 7.1 earthquake of August 2024 revealed new details about the occurrence patterns of large earthquakes in this region. Aftershock analysis of the 2024 earthquake conducted in this study, combined with source process investigations from previous studies, revealed that the rupture area of the 2024 earthquake did not overlap with those of the October (Mj 6.9) and December (Mj 6.7) 1996 earthquakes. In contrast, comparison of tsunami and strong-motion observation data indicates that the 2024 earthquake exhibits remarkable similarities to the 1961 earthquake in both tsunami and strong-motion waveforms. Furthermore, the hypocenters of the 1931 and October 1996 earthquakes were spatially close, and comparable foreshock activity preceded each event. These observations collectively suggest that two distinct rupture zones exist in the southern part of Hyuga-nada, each of which accumulates slip-deficit and releases it in M 7-class earthquakes at ~60-year intervals, offset from one another by ~30 years during the past 100 years. The occurrence of M 7-class earthquakes at ~60-year intervals can be explained by applying a hierarchical asperity model to understand the slip budget. Based on this model, we interpret that the asperity responsible for M 7-class events accumulates slip-deficit corresponding to the estimated aseismic slip at the plate boundary—which accounts for approximately half of the total slip-deficit expected from the plate subduction rate—and releases this slip-deficit through M 7-class earthquakes at 60-year intervals. This hypothesis also explains why the Mj 6.6 earthquake of January 2025 had less than half the magnitude of the 7.0 earthquake that had been expected in previous studies. Only ~30 years had elapsed since the previous event, approximately half the ~60-year intervals, resulting in roughly half the accumulated slip-deficit. Meanwhile, the remaining slip-deficit that was not released by the M 7-class earthquakes accumulates in the background over much longer periods. This unreleased slip-deficit could lead to a large M 8-class earthquake similar to the 1662 Hyuga-nada earthquake in the future. To better understand the occurrence patterns of major earthquakes, it is essential to further analyze historical M 7-class earthquakes in southern Hyuga-nada, particularly the 1931 and 1961 events, and more accurately evaluate the slip budget at the plate boundary including offshore slow earthquakes. Additionally, comprehensive numerical simulations are needed to reproduce and verify the various slip phenomena reported in Hyuga-nada. These efforts will contribute to clarifying the spatial and temporal properties of seismicity in this region and to advancing long-term seismic hazard assessment for earthquake disaster mitigation.