Tsunami Simulations of the 1933 Showa-Sanriku Earthquake using Well-Mapped Normal Faults along the Japan Trench

Abstract:

To assess the risk of tsunamis from outer-rise events, we proposed 33 models for normal faults along the Japan Trench based on seismic observations and surveys. The 1933 Showa-Sanriku earthquake, a large (M 8.4) outer-rise event, occurred in the area of the fault models created in this study. If our modeling procedure is reasonable, one of the fault models should explain the 1933 tsunami. Therefore, we compared the calculated and observed tsunami heights from this event in the tsunami database compiled by Tohoku University and the Nuclear Regulation Authority. We used only the observed tsunami data considered most reliable (category A) for the comparisons. Tsunamis from the 33 fault models were calculated by solving the non-linear shallow water equations on a finite difference scheme. The interval of the computational grid points was 18 arc-second. The observed data were compared with calculated values only if their respective locations were less than 1 km apart. The total number of points to compare was 410-537 depending on the magnitudes of the fault models. Using these datasets, we calculated K and κ of Aida (1978) for all the calculated tsunamis in this study to find the best-fit fault model for the 1933 Showa-Sanriku earthquake. K is the geometric average of the ratio between the maximum tsunami heights. κ is the geometric standard deviation indicating variance of K. The observed data were reproduced best by the model of fault ID 10 (L = 218km, Mw = 8.3) with K = 0.85 and κ = 1.64. We also applied our procedure to evaluate several previously proposed fault models of the 1933 Showa-Sanriku earthquake and estimated the corresponding values of K and κ. We found K = 0.54 and κ ＝ 3.10 for the fault model of Kanamori (1971), and K = 0.60 and κ ＝ 2.05 for the model of Abe (1978). Uchida et al. (2016) proposed a model with a single fault plane for which we obtained K = 1.14 and κ ＝ 1.83, a model consisting of two fault planes for which we obtained K = 1.31 and κ ＝ 1.65. Our fault model ID 10 was comparable with the previously proposed fault models for the 1933 Showa-Sanriku earthquake.