Source of a Tsunami of Seismotectonic Origin

Abstract

Modern ideas are presented concerning the source of an earthquake and the seismotectonic source of a tsunami. The main physical processes taking place at a tsunami source are described. Estimation is performed of the role of secondary effects: of displacements of the bottom, occurring in its own plane, of the Coriolis force, of density stratification of the water. The Okada formulae are presented and the technique is exposed for calculating coseismic ocean bottom deformations caused by an underwater earthquake. The dependence of the properties of coseismic ocean bottom deformation at the tsunami source upon the earthquake magnitude and depth is analyzed applying the Okada formulae in the case of a rectangular fault. Formulae are presented that relate the maximum values of the ocean bottom deformation amplitude, the displaced volume, and the initial elevation energy to the moment magnitude of the earthquake. From the slip distribution, adopted from the SRCMOD database, the vector fields of coseismic ocean bottom deformations were calculated applying the Okada formulae for the sources of 75 underwater earthquakes that occurred during the period between 1923 and 2013. It was shown that horizontal deformation components of an inclined bottom, as a rule, provide an additional and noticeable contribution to the displaced water volume and to the potential energy of the initial elevation (the tsunami energy). The relationships were analyzed between the ocean bottom deformation amplitude, the displaced volume and the tsunami energy, and the moment magnitude of the earthquake; the respective regression dependences were plotted. The part of the earthquake energy transferred to the tsunami waves was shown to increase with its moment magnitude, but even in the case of catastrophic earthquakes it does not exceed 0.1 %. From HTDB/WLD and GHTD/NGDC data the peculiarities were investigated of the space–time distribution of tsunamis.