Coseismic Deformation of the Earth’s Surface in the Area of the Catastrophic 2008 Wenchuan Earthquake According to GPS Measurements

Abstract

The occurrence time prediction of the strongest earthquakes is one of most topical tasks of seismology. Appearing in the recent years, new observation methods for 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 Earth’s surface deformation using the 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. This GPS system has been in operation since 1991 in the eastern part of the Qinghai–Tibet Plateau, the Sichuan Basin, and the Yunnan–Guizhou Plateau. The results of these observations have already been published and can be applied to study of the activity and segmentation of faults. Until 2004, the measurement network covering the Longmen Shan fault zone and the adjacent territory consisted of ten major points and some additional ones. The 1991–2001 data of GPS observations show that in the Longmen Shan fault zone was recorded offset to the northeast with a relatively low rate of 2.3 mm/year. As a result of studying the surface horizontal deformation using GPS observations, it was established that the focus area in 1999–2007 (prior to the strongest earthquake of 2008) was characterized by anomalously low rates of horizontal movements. This was probably related to the accumulation of stresses in the crust. It can be concluded retrospectively that the results of GPS observations could be considered as a medium-term precursor of the 2008 Wenchuan earthquake, which showed the location of the future seismic source position. The length of the zone of relatively weak horizontal displacements exceeded 250 km, which statistically correspond to a magnitude about 8 earthquake. The seismic rupture system (with a total length of ~240 km, originating during the 2008 Wenchuan earthquake) was confined to the NE-trending Longmen Shan fault zone, which occurs between the eastern margin of the Qinghai–Tibet Plateau and the northwestern part of the Sichuan Basin. Immediately after the Wenchuan event, the pattern of distribution of rates of horizontal movements has greatly changed. The horizontal deformations spanned most of the territory adjacent to the seismic fault zone, and the Longmen Shan Fault Zone exhibited the tectonic reduction of the surface at the expense of reversed movements of fault wings toward each other. The amplitudes of horizontal movements at equal distance on both sides of the branch of the Yingxiu–Beichuan main seismic rupture to the west are reduced slowly, and to the east – quickly. The vertical coseismic surface deformation, registered using satellite geodesy in the epicentral area of the seismic event, were mainly expressed in the submergence of the surface by different amounts in the southeastern wing of the Longmen Shan fault zone in the Sichuan Basin. It can be concluded on the basis of an analysis of the total seismic energy release during the earthquake that the period of 1991–2007 was an evident stage of seismic calm period prior to the strongest earthquake, whereas a period of seismic activity has been registered since 2008 (at the moment of the major shake) up to the present and may last several more years. The extremely low rate of horizontal displacements in the epicentral area of the preparing strongest earthquake can be considered as a middle-term precursor of its occurrence place.