Comparison between Observed Tsunami Heights and Numerical Simulation of the 1854 Ansei-Tokai Earthquake Tsunami in Gokasho Bay, central Japan

Abstract:

　Gokasho Bay is a blockade inner bay which has typical ria coasts and drowned valleys. It is located in the central Kii Peninsula and faces the Nankai Trough subduction zone. This Kumano-nada coastal area has been repeatedly striked by historical great tsunamis. For the 1854 Ansei-Tokai earthquake and its tsunami, there are comparatively many historical records including historical documents and oral traditions for tsunami behavior and damages along the coast.

　Based on these records, a total of 42 tsunami heights were measured by using a laser range finder and a hand level on the basis of spot elevation given by 1/2500 topographical maps. The average inundation height of whole bay area was approximately 4 - 5 m. On the whole, in the closed-off section of the bay, large values were obtained. For example, the average value in Gokasho-ura town area was 4 m, and the maximum run-up height along the Gokasho river was 6.8 m. Particularly in Konsa, located in the most closed-off section of the bay, tsunami heights ranged between 4 - 11 m, and were higher than those in other districts. It was comparatively high along the eastern coast and eastern baymouth.

　We simulate the distribution of the tsunami wave heights using numerical modeling, and compare the simulation results and above-mentioned actual historical data and results of our field survey. Based on fault models by Ando (1975), Aida (1981), and Annaka et al. (2003), the tsunami simulation was performed.

　After comparing the calculated results by three fault models, the wave height based on the model by Annaka et al. (2003) was found to have better agreement with observations. Moreover, the wave height values in a closed-off section of bay and at the eastern baymouth are high consistent with our survey data.