[S22P-17] Response of the water layer to seismic waves in Toyama Bay during the Noto-Hanto earthquake on 1 January 2024
On 1 January 2024, a magnitude 7.6 earthquake (JMA) struck the Noto Peninsula and nearby regions. Immediately after the earthquake, noticeable sea-level oscillations were recorded in Toyama Bay before the leading tsunami wave arrived from the source area. Backpropagation analysis showed that the cause of these oscillations was a secondary source located in Toyama Bay. A similar situation was observed during the previous Noto Peninsula earthquake on 25 March 2007. After that earthquake, sea level stations near Toyama City also recorded “mysterious small tsunamis” that preceded the arrival of the leading tsunami wave. Nagao et al. (2010) showed that those “mysterious small tsunamis” might be caused by dynamic horizontal displacement of the steep underwater slopes during the passage of the seismic waves across Toyama Bay. This study aims to test whether the sea level oscillations recorded immediately after the recent 1 January 2024 earthquake could have been caused by seismic waves.
To test this hypothesis, we reconstructed the seafloor motion in Toyama Bay during the passage of seismic waves across it. The reconstruction was performed within the cylindrical wave approximation based on the K-net (NIED) seismometer records. Although the cylindrical wave approximation is quite coarse for this problem, it allowed us to accurately reconstruct the dynamic displacements of the seafloor near Toyama City. Based on the reconstructed bottom motion, we performed a numerical tsunami simulation. The simulation was performed on a regular grid with a spatial spacing of 10” obtained from the M7000 bathymetry. The tsunami model was based on the linear long-wave theory equations written for the flow velocity potential. Seabed motion was input into the model at each time step.
The modelling results are compared with Toyama WG records (NOWPHAS). It is shown that seismic waves do excite gravity waves in Toyama Bay, but the amplitude and frequency of these gravity waves are markedly lower than the amplitude and frequency of gravity waves recorded by Toyama WG. We also compared the role of horizontal and vertical components of bottom motion in the gravity wave generation. It was found that the horizontal component of seismic waves contributes significantly more to the generation of gravity waves than the vertical component.
Acknowledgements. We used wave gauge data observed by Ports and Harbours Bureau of MLIT.
Nagao, R., Inoue, S., & Ohmachi, T. (2010). Tsunami simulation considering horizontal dynamic displacement of the seabed. Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 66(1), 301-305.
To test this hypothesis, we reconstructed the seafloor motion in Toyama Bay during the passage of seismic waves across it. The reconstruction was performed within the cylindrical wave approximation based on the K-net (NIED) seismometer records. Although the cylindrical wave approximation is quite coarse for this problem, it allowed us to accurately reconstruct the dynamic displacements of the seafloor near Toyama City. Based on the reconstructed bottom motion, we performed a numerical tsunami simulation. The simulation was performed on a regular grid with a spatial spacing of 10” obtained from the M7000 bathymetry. The tsunami model was based on the linear long-wave theory equations written for the flow velocity potential. Seabed motion was input into the model at each time step.
The modelling results are compared with Toyama WG records (NOWPHAS). It is shown that seismic waves do excite gravity waves in Toyama Bay, but the amplitude and frequency of these gravity waves are markedly lower than the amplitude and frequency of gravity waves recorded by Toyama WG. We also compared the role of horizontal and vertical components of bottom motion in the gravity wave generation. It was found that the horizontal component of seismic waves contributes significantly more to the generation of gravity waves than the vertical component.
Acknowledgements. We used wave gauge data observed by Ports and Harbours Bureau of MLIT.
Nagao, R., Inoue, S., & Ohmachi, T. (2010). Tsunami simulation considering horizontal dynamic displacement of the seabed. Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 66(1), 301-305.