Frequency-dependent amplification of the Sanriku tsunamis in Ryori Bay

Abstract

In the present study, the local tsunami amplification observed in Ryori Bay, located on the Sanriku coast of Japan, was investigated using numerical simulations. Large-scale tsunami propagation simulations and tsunami inundation simulations for the bay were systematically conducted to estimate and model the 2011, 1933, and 1896 tsunamis that occurred off the Sanriku coast and which resulted in large run-ups. The simulation results, which are moderately consistent with observations, presented larger run-up heights and inundations for the 1933 and 1896 tsunamis (which followed relatively small earthquakes) compared to those of the 2011 tsunami (which followed a larger earthquake). Furthermore, the frequency analysis indicated that the former two tsunamis comprised higher predominant components. A tsunami inundation simulation using parametrized synthetic waveforms was conducted to identify the contributing factors associated with the large amplification and run-ups. The results indicated that the predominant components are significantly amplified in the bay and the initial decrease in the water surface elevation prior to the primary waves of the two tsunamis leads to an increase in their run-up heights. Furthermore, the simulated waveforms of the tsunamis revealed that the 1933 and 1896 tsunamis had their wavefronts changed into a steep wavefront, i.e., a bore-like wave, during their wave developments in the bay, attributed to shoaling, narrowing bay width, and the nonlinear effect of the wave. These results, therefore, indicate that bores which are known to generate large run-up heights were generated in the bay during the two tsunamis.

Machine learning-based tsunami inundation prediction derived from offshore observations

“…While the large standard deviation of misfits occurs sporadically in relatively small areas, clustered large values of more than 1.5 m are located at Ryori Bay. Yamanaka et al 30 suggested that the tsunami height inside the Ryori bay is most likely amplified by the bay resonance. In our case, the expected flow depths in this area are also high, as shown by the mean flow depth of the target of up to ~10 m (Fig.…”

Short-wave run-ups of the 1611 Keicho tsunami along the Sanriku Coast

“…Shimozono et al (2014) demonstrated that the short-wave components in the 2011 tsunami were responsible for the extreme run-up height observed in the Sanriku Coast. Yamanaka and Nakamura (2020) indicated that the substantially large run-up heights in a bay during the 1896 and 1933 tsunamis were characterized by short-wave components. By comparing the observations for the 1611 tsunami with simulation results obtained from a numerical experiment, we investigate the type of wave component most suitable for reproducing the large variation in tsunami height observed around Koyadori.…”

“…For the numerical experiment, we introduced a parametric waveform as a product of sinusoidal and Gaussian functions (Shimozono et al 2014;Yamanaka and Nakamura 2020):…”

Impulsive Tsunami and Large Runup Along the Sanriku Coast of Japan Produced by an Inelastic Wedge Deformation Model

“…Several studies (e.g., Shimozono et al, 2012Shimozono et al, , 2014Yamanaka & Nakamura, 2020;Yamazaki et al, 2018) showed that the rugged Sanriku coast, characterized by steep terrains and narrow and long bays, can greatly amplify the impulsive (short-period) tsunami to cause large runup, although local amplification patterns can be highly nonlinear (e.g., Rogers & Mei, 1978). The amplification of long-period tsunami on the Sanriku coast due to long-wavelength seafloor uplift south of ∼39°N in the 2011 Tohoku earthquake was found to be significantly less (e.g., Yamanaka & Nakamura, 2020). This frequency-dependent amplification on the Sanriku coast may explain why the short-wavelength tsunami of the 1896 and 1933 Sanriku earthquakes produced similar runup to the 2011 Tohoku earthquake despite their different magnitudes.…”

“…In this study, we will show that such impulsive tsunami signals can be a direct result of short-wavelength seafloor uplift produced by inelastic wedge deformation. Several studies (e.g., Shimozono et al, 2012Shimozono et al, , 2014Yamanaka & Nakamura, 2020;Yamazaki et al, 2018) showed that the rugged Sanriku coast, characterized by steep terrains and narrow and long bays, can greatly amplify the impulsive (short-period) tsunami to cause large runup, although local amplification patterns can be highly nonlinear (e.g., Rogers & Mei, 1978). The amplification of long-period tsunami on the Sanriku coast due to long-wavelength seafloor uplift south of ∼39°N in the 2011 Tohoku earthquake was found to be significantly less (e.g., Yamanaka & Nakamura, 2020).…”