Geophysical processes and biosphere: article

Аnnotation: Forecasting of the time of occurrence of the strongest earthquakes is one of the most important tasks of science. Appearing in the recent years, new methods of observing of the precursors of different nature can open avenues to achieve success in the field of medium-term seismic forecast. One such method is to observe the deformations of the Earth's surface using global positioning system (GPS). Such a region is the territory of South-Western China, where in 2008 occurred the catastrophic Wenchuan earthquake, which caused very large destruction of infrastructure and huge human losses. In the eastern part of the Qinghai-Tibet plateau, the Sichuan basin and the plateau of the Yun-Gui, since 1991, began to apply the GPS observations. The results of these observations have now been published and can be applied to study of the activity and segmentation of fault activity. Until 2004, the measurement network consisted of 10 main points and several temporary points. The 1991-2001 data of GPS observations show that in the Longmenshan fault zone was recorded offset to the northeast with a relatively low speed of 2.3 mm/year. As a result of the study of horizontal surface deformations by GPS method it was found that before the strongest earthquake of 2008 in the period 1999-2007 the focal area was characterized by abnormally low values of horizontal motion velocity. Apparently, it was due to the accumulation of stresses in the earth's crust. Retrospectively, it can be concluded that the results of GPS observations could be considered as a medium-term precursor of 2008 Wenchuan earthquake, which showed the location of the future seismic source position. And the length of the zone of relatively weak horizontal displacements exceeded 250 km, which according to statistics corresponds to the magnitude of the earthquake of about 8. During The Wenchuan earthquake of May 12, 2008, a total length of 240 km of seismic breakage system on the surface was timed to coincide with the Longmenshan fault zone of the North-Eastern stretch, located between the Eastern edge of the Qinghai-Tibetan plateau and the North-Western part of the Sichuan depression. Immediately after Wenchuan seismic event the distribution of velocities of horizontal movements has greatly changed. Horizontal deformation gripped much of the contiguous to seismoactive territory, while in the fault zone Longmenshan manifested tectonic surface reduction due to thrust movements of the wings of the fracture in the opposite direction, towards each other. At the same time, the amplitudes of horizontal displacements at the same distance on both sides of the branch of the main seismic rupture Yingxiu-Beichuan to the West are reduced slowly, and to the East - quickly. The GPS data allow to record the vertical coseismic surface deformation in the epicentral area of the seismic event. They are mainly expressed in the lowering of the surface by different amounts in the South-Eastern wing of the Longmenshan fault system in the Sichuan basin. Based on the analysis of the total seismic energy released during the earthquake, it is possible to conclude that the interval 1991-2007 was a clear stage of seismic calm before the strongest earthquake, and in 2008, at the time of the main shock and to date, a period of seismic activation is recorded, which can continue for several years. At the same time, abnormally low speed of horizontal displacements in the epicenter region of the preparing strongest earthquake can be considered as a medium-term precursor of its origin.