ИСТИНА

Ten DONET seafloor observatories were in operation during the disastrous 2011 Tohoku event. All of the observatories recorded the main seismic event (05:46:24 UTC, Mw=9.0, USGS), the first strong aftershock (06:15:40, Mw=7.9, USGS) and the tsunami waves that arrived more than an hour later [1,2]. All DONET observatories also recorded free gravity waves, excited by oscillations of the nearby underwater slopes during the passage of a seismic surface wave [3,4]. Records of these waves provide a unique opportunity for verification of numerical models of the ocean gravity waves generation by seismic bottom movements. Indeed, thanks to the DONET bottom seismometers records, we are able to reconstruct the real bottom movement in the observatory setting area, and the DONET bottom pressure gauges records, located at the same points as the seismometers, allow us to verify the correctness of numerical simulation of the water layer response to this bottom movement. The objectives of the present work are (1) a description of the numerical model of the ocean wave generation by seismic bottom movements and (2) verification of this model using the DONET recordings of the 2011 Tohoku event. For simulation we used combined 2D/3D numerical model [5]. The calculation domain was divided into two parts: the deep-water part and shallow-water part. The separation was performed along the fixed isobath. In the deep-water part the problem was dealt with within the framework of the potential theory of a compressible liquid. In the shallow-water part, the liquid was assumed to be incompressible, and we made use of the linear theory of shallow water. In both modules the equations involve the same sought function F – velocity potential. For this reason, our model is called CPTM – Combined Potential Tsunami Model. The most important element of our model is the use of a sigma-spherical coordinate system for the deep-water module [6]. The sigma-spherical coordinate system maps the calculation domain with a complex form (confined by the uneven bottom) onto a rectangular region. Our model was verified with the use of the DONET records during the 2011 Tohoku event. Based on the records of the DONET bottom seismometers and the nearest ground-based GPS stations the bottom movement was reconstructed within the plane wave approximation in the entire computational domain [4,7,8]. Then a simulation was performed using the CPTM model. The simulation results were compared with the records of the DONET bottom pressure gauges. Good agreement between the synthetic and observed pressure confirmed the correctness of the model and allowed us to perform a series of numerical experiments aimed at identifying the generation mechanisms of the observed gravity waves. The results of the numerical experiments showed that (1) horizontal, rather than vertical, bottom movements play a key role in their generation, (2) the amplitude of the excited gravity waves is determined by the amplitude of the dynamic horizontal bottom motions, while the contribution of horizontal static bottom displacements is insignificant, and (3) the amplitude of the excited gravity waves depends on the relative orientation of the slope and the propagation direction of the seismic surface waves. This work was supported by the Russian Foundation for Basic Research, Project 19-05-00351. References. 1. Matsumoto, H. & Kaneda, Y. (2013). Some features of bottom pressure records at the 2011 Tohoku earthquake - Interpretation of the far-field DONET data. Proceedings of the 11th SEGJ International Symposium, Yokohama, Japan, 18-21 November 2013, 493-496. https://doi.org/10.1190/segj112013-124 2. Matsumoto, H., Nosov, M. A., Kolesov, S. V. & Kaneda, Y. (2017). Analysis of Pressure and Acceleration Signals from the 2011 Tohoku Earthquake Observed by the DONET Seafloor Network. Journal of Disaster Research, 12 (1), 163-175. https://doi.org/10.20965/jdr.2017.p0163 3. Nosov, M. A., Sementsov, K. A., Kolesov, S. V., Matsumoto, H. & Levin, B. W. (2015). Recording of gravity waves formed in the ocean by surface seismic waves during the earthquake of March 11, 2011, off the coast of Japan. Doklady Earth Sciences, 461 (2), 408-413. https://doi.org/10.1134/S1028334X15040121 4. Sementsov, K. A., Kolesov, S. V., Nosov, M. A., Karpov, V. A., Matsumoto, H. & Kaneda, Y. (2017). Generation of free gravity waves in the ocean by packet of surface seismic waves, Memoirs of the Faculty of Physics, Lomonosov Moscow State University, 4, 1740504–1740504 (in Russian) 5. Nosov, M. A. & Kolesov, S. V. (2019). Combined numerical model of a tsunami. Mathematical Models and Computer Simulations.— Vol. 11, no. 5. — P. 679–689. 6. Phillips, N. A. (1957). A coordinate system having some special advantages for numerical forecasting. Journal of Meteorology, 14, 184-185. https://doi.org/10.1175/1520-0469(1957)014<0184:ACSHSS>2.0.CO;2 7. Graizer, V. (2010). Strong motion recordings and residual displacements: what are we actually recording in strong motion seismology? Seismological Research Letters, 81 (4), 635-639. https://doi.org/10.1785/gssrl.81.4.635 8. Fukao, Y., Sandanbata, O., Sugioka, H., Ito, A., Shiobara, H., Watada, S. & Satake, K. (2018). Mechanism of the 2015 volcanic tsunami earthquake near Torishima, Japan. Science Advances. 4, https://doi.org/0.1126/sciadv.aao0219