9:30 AM - 9:45 AM
[MIS24-02] Tsunami Source Model of the 1946 Showa Nankai Earthquake Estimated from Trace Information of Tsunami and Crustal Deformation
Keywords:The Nankai Trough earthquake , Showa Nankai earthquake and tsunami, Tsunami propagation numerical simulation
Tsunami associated with the 1946 Showa Nanaki earthquake, which occurred on December 21 1946, caused catastrophic damages along the Pacific coast of southwestern Japan. Satake (1993) and Baba et al (2005) have estimated tsunami source model from joint inversion of tsunami trace heights crustal deformation, and tsunami waveforms, concluding that the 1946 Showa Nankai event was a massive earthquake with a maximum slip of 5-6 m and moment magnitude of 8.1-8.3. However, these models are not considerable the three-dimensional subduction geometry of the Nankai Trough subduction zone and the reevaluated tsunami traces along the coast of Wakayama Prefecture (e.g., Ishibashi et al., 2016; Nakano et al., 2022). In this study, we reexamined the tsunami source model of the 1946 Showa Nankai earthquake by placing nine subfaults based on the three-dimensional subsurface structure model of the Nankai Trough subduction zone by Nakanishi et al. (2018) and applying an optimization algorithm to the latest trace information of tsunami height and crustal deformation.
Tsunami source estimation was conducted using an optimization algorithm developed by Imai et al. (2020) based on the principle of simulated annealing proposed by Kirkpatrick et al. (1983). Trace data were used for 334 points with the highest reliability (reliability rank A) from the Tsunami Trace Database (Tohoku University and Nuclear Regulation Authority) and 37 crustal deformation traces. For the tsunami traces, trace points obtained from the same settlement were aggregated to calculate the mean and standard deviation, and a total of 48 points, including 10 reevaluated tsunami traces, were adopted in this study. Tsunami forward simulation was conducted to further validate the estimated tsunami source model, and the results were compared with tide gauge recorded at tide stations of Uwajima, Sakai, Uchiura. Tide gauge records were digitized from analog records stored in the tsunami waveform image search system and corrected using the peaks and troughs of the theoretical tidal curve.
Tsunami source model with large slip area of 11 m off Kii Peninsula and slip area of 4.5m off Tosa was estimated from trace information of tsunami and crustal deformation. The moment magnitude was estimated as Mw 8.2±0.1. No previous models have indicated a large slip exceeding 10 m off the Kii Peninsula, suggesting that this result is obtained through the reevaluation of tsunami traces along the coast of Wakayama Prefecture. Additionally, simulated waveforms matched observed waveforms entirely, although the tsunami travel time of the simulations are several to several ten minutes earlier than observations. However, the estimated model exhibits a large geometric standard deviation, indicating that the slip amounts of subfaults may not be uniquely determined. Further verification is required including a reassessment of trace data.
This study was also supported by the project “Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai trough region” of the Ministry of Education, Culture, Sports, Science and Technology Japan (MEXT).
Tsunami source estimation was conducted using an optimization algorithm developed by Imai et al. (2020) based on the principle of simulated annealing proposed by Kirkpatrick et al. (1983). Trace data were used for 334 points with the highest reliability (reliability rank A) from the Tsunami Trace Database (Tohoku University and Nuclear Regulation Authority) and 37 crustal deformation traces. For the tsunami traces, trace points obtained from the same settlement were aggregated to calculate the mean and standard deviation, and a total of 48 points, including 10 reevaluated tsunami traces, were adopted in this study. Tsunami forward simulation was conducted to further validate the estimated tsunami source model, and the results were compared with tide gauge recorded at tide stations of Uwajima, Sakai, Uchiura. Tide gauge records were digitized from analog records stored in the tsunami waveform image search system and corrected using the peaks and troughs of the theoretical tidal curve.
Tsunami source model with large slip area of 11 m off Kii Peninsula and slip area of 4.5m off Tosa was estimated from trace information of tsunami and crustal deformation. The moment magnitude was estimated as Mw 8.2±0.1. No previous models have indicated a large slip exceeding 10 m off the Kii Peninsula, suggesting that this result is obtained through the reevaluation of tsunami traces along the coast of Wakayama Prefecture. Additionally, simulated waveforms matched observed waveforms entirely, although the tsunami travel time of the simulations are several to several ten minutes earlier than observations. However, the estimated model exhibits a large geometric standard deviation, indicating that the slip amounts of subfaults may not be uniquely determined. Further verification is required including a reassessment of trace data.
This study was also supported by the project “Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai trough region” of the Ministry of Education, Culture, Sports, Science and Technology Japan (MEXT).