ИСТИНА

Tsunamis, a kind of surface gravity waves in the ocean, are mainly generated by submarine earthquakes. The generation of tsunami is often considered as instant process. In reality, seismic motions of ocean-bottom are certainly a time-dependent process. While the instant (or impulsive) generation of surface gravity waves is well studied, the surface gravity waves that have dynamic origins, believed to have small amplitudes, are still less known. However, theoretical research has pointed out that in some extreme cases, this kind of waves may have rather large amplitude. Moreover, study of these waves may provide us some insights of information such as the character of the generating seismic source or the later coming tsunami. Recently, thanks to the development of observation system, surface gravity waves of dynamic origins are clearly recognized. During the 2011 Tohoku Earthquake, gravity waves that have amplitude of several centimeters and period of the order of a hundred seconds were observed by DONET stations near Kii peninsula, Japan, an hour before the arrival of the tsunami (Nosov et. al., 2015, Doklady Earth Sciences). We shall call these waves “tsunami forerunners”. According to the above paper, “tsunami forerunners” are generated by the long-period seismic surface waves. In this study, we tried to conduct numerical simulations of “tsunami forerunners” using GPS data from GEONET, to see if it is possible that this kind of surface gravity waves were generated by the seismic surface waves. 1Hz processed high resolution GPS data are needed for the simulation and because they are not available at the area of Kii peninsula, we chose the area most close. From the inland GPS data of the 2011 Tohoku Earthquake, we constructed the displacement field of the ocean bottom by extrapolation and supposing a single surface wave with a definite direction. Then we can get the ocean bottom velocity field, which is the only required boundary condition for the simulations. Our 3D numerical model implements the linear potential theory, whose only assumption is that the curl of the velocity field of the water is zero. From the simulation results, we can find surface gravity waves clearly. And the waveforms with amplitude of a few centimeters and period of the order of a hundred seconds are consistent with the observation. Therefore, our result confirms the dynamic origin of the “tsunami forerunners”.