Sergey V. Kolesov

The properties of co-seismic deformations of the ocean bottom as indicated by the slip-distribution data in tsunamigenic earthquake sources

In this study we consider 75 ocean-bottom earthquakes that occurred during the 1923–2013 period. Based on the slip-distribution data from the Finite-Source Rupture Model Database (SRCMOD) and Okada formulas, the vector fields of co-seismic bottom deformations were calculated. It is shown that, as a rule, the horizontal components of the sloping bottom deformation make an additional and essential contribution to the water displacement and the potential energy of the water-surface elevation that is similar in shape to the bottom surface displacement (the tsunami energy). On the basis of the analyzed relationships between the bottom deformation amplitude, the displaced water volume, tsunami energy, and the earthquake moment magnitude the corresponding regression dependencies were constructed. It is shown that the fraction of the potential energy of an earthquake that is converted into a tsunami increases with the increasing moment magnitude, but even during catastrophic earthquakes it is less than 0.1% of the total earthquake energy.

Residual hydrodynamic fields after tsunami generation by an earthquake

The linear theory of long waves was applied to study horizontal motions of the water layer in a rotating ocean which appear after tsunami generation by an earthquake. The structures of residual potential and eddy fields are analyzed on the basis of the analytical solution of a model axisymmetric problem for an ocean of constant depth. The estimates of the horizontal displacements of water particles, velocity of the eddy current, and energy of the geostrophic eddy are calculated for typical conditions of the tsunami source. Particular features of the residual fields related to the existence of stable stratification are considered. Static and dynamic numerical models are described that allow us to calculate the residual potential field and its evolution related to the realistic events. The field of residual horizontal displacements of water particles for the catastrophic earthquake near the coasts of Japan on March 11, 2011, is calculated and analyzed.

Relationship between pressure variations at the ocean bottom and the acceleration of its motion during a submarine earthquake

AbstractThe data provided by ten DONET deep-sea observatories, that on March 11, 2011, registered the Great East Japan Earthquake and tsunami, were used for investigation of the relationship between variations of the ocean bottom pressure and three-component accelerograms. Methods of cross-spectral analysis revealed the existence of a frequency range of “forced oscillations,” within which pressure variations are proportional to the vertical component of the acceleration. This proportionality is manifested by the magnitude-squared coherence (MSC) being close to unity and a phase lag (PL) practically equal to zero. The spectral analysis method showed the proportionality coefficient to be equal the mass of a water column of unit area at the installation point of the observatory or, approximately, to the product of the water density and the ocean depth. The observed boundaries of the frequency range of “forced oscillations” are revealed to correspond to the theoretical frequency values confining the manifestation of surface gravity and acoustic waves in pressure variations near the ocean bottom. The hypothesis is put forward that the small deviations of MSC from unity and of PL from zero observed by a number of stations within the range of “forced oscillations” are due to the contribution of horizontal movements of nearby submarine slopes. A theoretical analysis has been performed of the problem of forced oscillations of a water layer in a basin of varying depth. A formula is obtained that relates pressure variations at the ocean bottom to acceleration components of the bottom motion and the bottom slope. The pressure in the region of forced oscillations is shown to decrease exponentially with the distance from the moving segment of the ocean bed, so pressure variations, originating from movements of the bottom, are registered effectively by a gauge at the ocean bottom only within a radius less than 1–2 ocean depths. A cross-spectral analysis of pressure variations and of three-component accelerograms confirmed the hypothesis concerning the contribution of horizontal movements of nearby submarine slopes to pressure variations.

Numerical Simulation of Gravity Waves Excited in the Ocean by Low-Frequency Surface Seismic Waves Based on GPS Recordings

A numerical experiment for reproducing the generation of free gravity waves in the ocean by low-frequency surface seismic waves passing across the bottom is described. The dynamics of the bottom movement is reconstructed based on the real GPS data recorded during the disastrous Tohoku earthquake of March 11, 2011. Results of the numerical simulation show that horizontal movements of underwater slopes play a key role in the generation of free gravity waves.

Ocean-bottom pressure and seismic signals at tsunamigenic earthquake

Offshore observations make it possible to detect tsunamis in advance prior to their arrivals at the shoreline. For this purpose, ocean bottom pressure gauges are widely used. However, in near- and intermediate fields, ocean bottom pressure records usually exhibit a complicated interference of signals related not only to gravitational wave (i.e., tsunami), but also to hydroacoustic and seismic waves. Network of offshore observatories recently deployed in Japan is capable of providing high sampling rate of records of ocean bottom pressure and seismic (acceleration and velocity) signals. In the present study, by taking advantage of simultaneous measurements of pressure and seismic signals that were recorded during the recent tsunamigenic earthquakes, i.e., the 2011 Tohoku earthquake and the related 2013 October earthquake, we reveal particular features of these signals. The bottom pressure follows the bottom acceleration in the inter-mediate frequency band.