Characterized Source Models of Four M7-class Hyuganada Earthquakes

Abstract

On August 8, 2024, the M_J 7.1 Hyuganada earthquake occurred at the southwestern edge of the Nankai Trough subduction zone. Near the source region of this earthquake, M_J 6.6 earthquake occurred on January 13, 2025 and two M 7-class earthquakes (M_J 6.9 and M_J 6.7) occurred in 1996. For the prediction of strong ground motions of the next M 8-9 Nankai Trough earthquake, the characterized source models of the four M 7-class earthquakes were estimated in this study. The empirical Green’s function method was used to synthesize the strong motions from the characterized source models, which were composed of strong motion generation areas (SMGAs). Two SMGAs for the M_J 7.1, M_J 6.9, and M_J 6.7 earthquakes and three SMGAs for the M_J 6.6 earthquake were estimated respectively, using the simulated annealing algorithm. The strong motion records over a frequency range of 0.2-10 Hz observed at N-net on the seafloor and K-NET, KiK-net, and F-net on land were used for the estimation. The SMGAs estimated for the four earthquakes showed little overlap with a subducted seamount. The Brune stress drops of the SMGAs near the leading edge of the subducted seamount were higher than the others. There was little overlap in the location of the estimated SMGAs for the four M 7-class earthquakes. Because the three SMGAs of the 2025 M_J 6.6 earthquake were scattered to fill in the gaps between the SMGAs of the other three earthquakes, the duration of the strong motion records of the M_J 6.6 earthquake was the longest among the four earthquakes. The locations of the SMGAs were then compared with the existing slip models of the three (M_J 7.1, M_J 6.9, and M_J 6.7) earthquakes estimated by long-period waveform and/or geodetic data. As a result, the SMGAs in proximity to the rupture initiation points were located near the main slip area, while the other SMGAs were found to be located at the periphery or outside of the main-slip area. This outcome suggests a frequency-dependent rupture process, a phenomenon that has been observed in M 7-class Miyagiken-oki earthquakes and the 2011 M_w 9.0 Tohoku earthquake in the Japan Trench. The relationships between the seismic moment and the short-period spectral level, which is defined as the constant value of the acceleration source spectral amplitudes at frequencies higher than the corner frequency, for the M_J 7.1, M_J 6.9, and M_J 6.6 earthquakes were identical to the empirical relationships for the M 7-8 class interplate earthquakes in the Japan Trench and the Kuril Trench. It was pointed out in previous studies that the relationship for the 2011 M_w 9.0 Tohoku earthquake was almost identical to the extrapolation of this empirical relationship. The findings in this study have the potential to contribute to strong ground motion predictions of the M 8-9 Nankai Trough earthquakes.