It is well-known that strong shallow earthquakes are usually accompanied with series of aftershocks which can be explained by non-complete removal of strains, collected in the center during the main shock. But in some cases after some time interval the repeated strong earthquake happens in the same place. There are different versions for such repeated earthquakes. It is obvious, that all existing versions require presence or accumulation of strain in the active zone after the main shock which stress out the actuality of development of theoretical and methodological basics for assessment of strains on the basis of experimental data on plates motion. The importance of geodynamic research of remained strain at the Earth crust after strong earthquakes by way of calculation of remained strain on the basis of data plates is noted. It is shown, that utilization of GPS system for determination of shifts of plates could lead to error caused by shifts of GPS stations due to various causes.The consideration of the case where the non-stability of atmospheric pressure is a prevailing factor is reasoned. The related formulas for calculation of strain of the Earth crust on the basis of GPS measurements in the triangular network are given.