[S-CG60] Shallow Fault Zone Structure and Seismic Hazard Assessment

[SCG60-03] Forecasting seismic rupture parameters from geology, geomorphology, and seismicity: An example from the 2016 Kumamoto earthquake

★Invited Papers

*Shinji Toda¹ (1.International Research Institute of Disaster Science, Tohoku University)

Keywords:active fault, Kumamoto earthquake, strong ground motion

The 2023 Gaziantep, Turkey earthquake of Mw 7.8 that brought unparallel damage to the areas due to its widespread strong ground motion. The earthquake was caused by dynamic rupture of ~400 km in total involving two or three fault segments. The disaster pushes us to think further development of “recipe” for predicting strong ground motions. Domestically, the 2016 Kumamoto, Japan earthquake enables us to validate if we had properly estimated the rupture scenario of the Futagawa-Hinagu fault zones. Here I present the validation retrospectively and then propose how we can better implement geologic, geomorphic, seismic information into the recipe model. The source fault model for the prediction of strong ground motions is characterized by outer, inner, and extra fault parameters (Irikura and Miyake, 2001). Regarding the outer fault parameters, the rupture extent of the Kumamoto earthquake was longer than one of the segments of the Fugatawa fault zone, which resulted in underestimate of the magnitude. Post-Kumamoto survey found that the length of the segment was longer than the previous estimate, which emphasizes the importance of detail active fault traces and precise recognition of the fault ends. In terms of inner parameters, an asperity was located closer to the place we measured maximum surface right-lateral slip of 2.2 m at Dozon, Mashiki Town. Perrin et al. (2016) which studied world-wide surface-rupturing earthquakes suggest that asperity tends to locate mature portion of a fault, which is roughly consistent with the case of Kumamoto. One of the extra fault parameters for prediction is hypocenter location. Recent stress triggering studies suggest that future epicenter tends to be high seismic spots, mostly related to raised seismicity due to a preceding large earthquake or seismic swarm. The hypocenter of the Kumamoto Mj 7.3 earthquake was located within the aftershock area of the preceding M6.5 earthquake. Finally, regarding the Probabilistic Seismic Hazard Assessment (PSHA), seven-year post-Kumamoto surveys has revealed the average inter-event time of the surface-rupturing earthquakes associated with the Futagawa fault is 2,000-3,000 years (e.g., Ishimura et al., 2021) and then the most recent event occurred about 2,000 years ago, which comes to ~5 % of 30-yr probability and it is much higher than ~0% of the previous estimate from fewer paleoseimic data.