Investigation of the Lithospheric Plate Boundary Zone within the Sakhalin Island Based on Satellite Geodesy Data

— Modelling of the movements at the interface of the Amur and Okhotsk plates within the Sakhalin Island was performed using repeated satellite measurements on the Sakhalin Island and the nearest continental zone for the period of 2016–2021, as well as previously published data. When modelling fault-block kinematics, well-known relations were used to calculate reverse movements for buried rectangular dislocations in an elastic medium, which were implemented in the TDEFNODE software package. In the process of modelling the movements, the measured horizontal components of GNSS (Global Navigation Satellite System) velocities, the boundary and the mutual kinematics of the Amur and Okhotsk plates relative to the North American Plate according to the NNR-MORVEL56 model were used as the input data. This approach revealed persistent deviations in the direction of the simulated displacements of the Earth’s surface from the observed ones, which can be explained by the discrepancy between the a priori specified kinematics of the blocks and the observed movements. To eliminate the systematic discrepancy, it was necessary to allow the possibility of updating the mutual kinematics of the blocks. The repeated calculations, with the same input data but in a problem formulation that allowed refinement of block kinematics, led to suppression of systematic discrepancies between the model and measured displacements while retaining the random scatter. The movement parameters of the Amur and Okhotsk plates, which were refined during the modelling, show typical slight differences from the movement parameters of the corresponding large lithospheric plates (the Eurasian and North American plates), from which they are separated into independent blocks in modern constructions. The calculated locking coefficients in the Sakhalin segment of the interplate boundary reach maximal values at depths of 20–30 km. The obtained locking pattern at the plate interface is compared with the sources of the largest earthquakes in the last 30 years in the considered area, viz., Neftegorsk (May 27, 1995) and Uglegorsk (April 4, 2000) earthquakes, which are found to be associated with zones of maximal locking and a high locking gradient, both in the dip and strike directions.