ИСТИНА

Deformation processes in subduction zones, the features of their temporal evolution and spatial variations, their correlation with the seismicity are the key issues for understanding the mechanisms for the strongest earthquakes preparation. Compared to seismological observations, satellite geodetic observations (GPS) reflect not only fast, but also slow processes in the earth's crust and asthenosphere, including that portion which is not exhibited seismically, but contributes significantly to the accumulation of the deformation potential or to its release through aseismic motion (creep), viscoelastic relaxation in the asthenosphere, etc. The longterm satellite geodetic observations that have been collected globally over the past decades have made significant adjustments to the pre-existing understanding of the dynamics of many seismically active regions. Comparison of the of surface displacement velocity with the plate convergence rate in the subduction zones discovered mismatch between the observed motions and the hypotheses of complete mechanical coupling in the interplate contact zones, which is broken only during the seismic jumps. For quantitative relation between the above velocities, dislocation models of deformation of elastic or viscoelastic medium are used, which provide the basis to solve the problem of determining the geometry of the interplate coupling in the subduction zone from observations of surface displacements. Such an inverse problem has a stable physically feasible solution with discretization adequate to the observation density and with regularization by the solution norm minimization while retaining the condition of statistical agreement between the residuals of the observational equations and the errors of the raw measurements. Analysis of temporal variations of interplate coupling in the contact zone requires separation of effects from various mechanisms generating deformations in subduction zones: in addition to plunge of the oceanic plate slab and slipping along the asperities the observable surface displacements exhibit coseismic jumps, postseismic transient processes, instrumental variations of various periodicity (diurnal and seasonal). Separation of all these effects is based on the regression analysis of time series by characteristic features in the time dependence of each listed mechanism. Application of the described approaches to observations over the Japanese satellite geodetic network provided by the Geospatial Information Agency of Japan made it possible to identify the specific spatial-temporal deformations preceding the strongest subduction earthquakes. The localization of the focal zone of one of the strongest earthquakes within the Japan subduction zone in 2011 is characterized by the maximum gradient of the interplate coupling in the direction of the strike of the contact zone. In addition, this event was preceded by local temporal variations in the zone of the maximum coupling gradient, as well as variations involving the entire subduction zone, which makes it possible to distinguish the instrumentally recorded pre-seismic phase of the seismic cycle.