11:45 〜 12:00
[SSS04-11] Quantifying interplate seismic coupling from fault modeling of the 2024 Hyuganada earthquake using tsunami and GNSS data
The asperity model is a conceptual model that describes areas where the interplate friction is high, causing strong seismic coupling and supposed to slip seismically (e.g., Lay & Kanamori, 1980; 1981). Accurately quantifying the seismic coupling of the asperity along the plate interface is important to estimate the hazard associated with interplate earthquakes in subduction zones (e.g., Sherill & Johnson, 2022). On 8 August 2024, an M 7.1 earthquake occurred in the Hyuganada region, the westernmost part of the Nankai Trough subduction zone (hereafter, the 2024 earthquake). In the Hyuganada region, some major interplate earthquakes have occurred in the past (e.g., Hatori, 1971) and the epicenter of the 2024 earthquake is located close to the rupture areas of past earthquakes. When the 2024 earthquake occurred, tsunamis up to a few centimeters were recorded by the deep-ocean tsunami observation networks, N-net and DONET of NIED. Tsunami data from the deep-ocean networks, which are free from the complex coastal site effects, contain unique information about the area of the fault (e.g., the extent of the tsunami source). Also, due to its slow propagation velocity, the tsunami data analysis is much less affected by the assumption of the rupture propagation across the fault than seismic waves, resulting in the reliable constraint of the fault horizontal location. Utilizing these advantages, in this study, we estimate the fault model of the 2024 earthquake using the deep-ocean tsunami data and examine the spatial relationship between the rupture areas of the 2024 and past earthquakes to discuss the seismic coupling in this region.
In addition to the offshore tsunami data, we use the onshore geodetic data from the GNSS at GEONET of GSI. The joint inversion showed the maximum slip of 2.4 m near the Global CMT centroid and the average slip of 1.2 m. To examine the resolving performance and limitation of each dataset, we additionally conducted the inversions using either tsunami or GNSS data. If we use only the tsunami data, the fault up-dip extent was almost the same as the joint inversion model, while the down-dip extent of the fault was estimated to be much broader than the joint inversion. This is because the crustal deformation due to the fault slip near the down dip is partially overlapped with land, that does not excite tsunami. We also found the inversion using only the onshore GNSS data constrained the fault down-dip extent well, but the fault up-dip limit was estimated farther off the coast than the joint inversion, suggesting the up-dip limit was not constrained well. The joint analysis is needed to reasonably constrain both up-dip and down-dip extents of the fault.
The main slip region of the 2024 earthquake corresponds to the southern half of the tsunami source area of an M 7.0 earthquake in 1961 (Hatori 1968), suggesting the 2024 earthquake may have ruptured a part of an asperity of the earthquake in 1961. Considering the average and maximum slip amounts of the 2024 earthquake and the recurrence interval of about 60 years, the slip deficit accumulation rate in this asperity is estimated to be approximately 2 cm/yr on average. Using the observed plate convergence rate in this region (5–6 cm/yr, DeMets et al. 2010), the seismic coupling ratio (e.g. Scholz & Campos 2012) is estimated to be less than 40 % on average on this asperity, while the coupling ratio along the plate boundary in the middle part of the Nankai Trough, corresponding to the rupture zones of the past megathrust earthquakes, was nearly 100 % (e.g. Kimura et al. 2019). One possible interpretation for the low seismic coupling ratio could be the slip deficit was partially released due to the aseismic events such as slow slip events (e.g., Okada et al., 2022)
In addition to the offshore tsunami data, we use the onshore geodetic data from the GNSS at GEONET of GSI. The joint inversion showed the maximum slip of 2.4 m near the Global CMT centroid and the average slip of 1.2 m. To examine the resolving performance and limitation of each dataset, we additionally conducted the inversions using either tsunami or GNSS data. If we use only the tsunami data, the fault up-dip extent was almost the same as the joint inversion model, while the down-dip extent of the fault was estimated to be much broader than the joint inversion. This is because the crustal deformation due to the fault slip near the down dip is partially overlapped with land, that does not excite tsunami. We also found the inversion using only the onshore GNSS data constrained the fault down-dip extent well, but the fault up-dip limit was estimated farther off the coast than the joint inversion, suggesting the up-dip limit was not constrained well. The joint analysis is needed to reasonably constrain both up-dip and down-dip extents of the fault.
The main slip region of the 2024 earthquake corresponds to the southern half of the tsunami source area of an M 7.0 earthquake in 1961 (Hatori 1968), suggesting the 2024 earthquake may have ruptured a part of an asperity of the earthquake in 1961. Considering the average and maximum slip amounts of the 2024 earthquake and the recurrence interval of about 60 years, the slip deficit accumulation rate in this asperity is estimated to be approximately 2 cm/yr on average. Using the observed plate convergence rate in this region (5–6 cm/yr, DeMets et al. 2010), the seismic coupling ratio (e.g. Scholz & Campos 2012) is estimated to be less than 40 % on average on this asperity, while the coupling ratio along the plate boundary in the middle part of the Nankai Trough, corresponding to the rupture zones of the past megathrust earthquakes, was nearly 100 % (e.g. Kimura et al. 2019). One possible interpretation for the low seismic coupling ratio could be the slip deficit was partially released due to the aseismic events such as slow slip events (e.g., Okada et al., 2022)