ИСТИНА

Small-amplitude free gravity waves in the ocean were registered by DONET ocean-bottom pressure gauges just after the passage of surface seismic waves during the 2011 Tohoku earthquake. The amplitude and period of these free gravity waves amounted to 3 cm and 150 s, respectively. Based on theoretical estimates we assume that generation mechanism of these free gravity waves is related to horizontal movements of ocean-bottom slopes during the passage of low-frequency Love and Rayleigh waves. To verify our hypothesis we made numerical simulation of these free gravity waves inside the DONET stations region. For our simulation we used combined 3D/2D numerical model, which consists of two modules: deep-water module (based on the linear potential theory) and shallow-water module (based on the linear long wave theory). Dynamics of bottom movement during the passage of surface seismic waves was reconstructed with the use of DONET ocean-bottom seismometers data. Earthquake epicenter was located quite far from DONET stations (approximately 800 km), so we assumed that surface seismic waves were plain inside the simulation domain. Their profiles were calculated by double integration of DONET accelerograms. The results of our numerical simulation are in good agreement with the observed values. We made two different simulations for two different types of bottom boundary conditions: (1) only vertical movements and (2) only horizontal movements of ocean bottom. The results of these simulations confirmed that horizontal movements of ocean-bottom slopes play the key role in the free gravity waves generation whereas the role of vertical movements is negligible. We made another simulation to find, whether residual (coseismic) bottom deformation generates free gravity waves with the observed amplitudes or not. The results of this simulation showed that it is necessary to take into account dynamics of bottom movement in the far-field. DONET data were transfered and processed according to Implementing Agreement Between Faculty of Physics of Lomonosov Moscow State University and Japan Agency for Marine-Earth Science and Technology on Early tsunami detection methods on the base of real-time seafloor observatory network (2013). Keywords: gravity waves, surface seismic waves, linear potential theory, 2011 Tohoku tsunami, bottom slopes