One week before the occurrence of Wenchuan Ms8.0 Earthquake the maximum horizontal principal stress measured by hydro-fracturing technique in the seismogenic fault zone of Wenchuan Earthquake (Yingxiu-Beichuan fault) at 400±m depth was 21∼22MPa, which was 8∼10MPa higher than the values measured in the nearby footwall. The repeated measurements after the great earthquake at the same locations indicate that the maximum and minimum horizontal principal stress in the fault belt decreased by 29% and 23% respectively; while in the footwall the stresses did not change after the earthquake. The analysis of the measured result indicates that anomalous high stress in an active fault belt provides a reliable basis for determining the area of high seismic risk, and it is a warning sign of generation and occurrence of strong earthquakes. Therefore the in-situ stress measurements have quite important function and significance in the strong earthquake prediction research.
In-situ stress status is the most important character of crust. All kinds of geological phenomena (include the occurrence of shallow earthquakes) inside and outside crust and all concomitant physical phenomena are associated with in-situ stress. Pregnant and occurrence of strong destructive earthquakes are the result of strong tectonic stress, and also an express of strong crust movements. The strength pushed the movement which was existed in crust, will gradually increase at special conditions, and destroy the crust at any fragile place, lead to vibration, it is the cause and process of earthquake (Li 1977). Therefore, Professor Li Siguang suggests to study and prediction earthquake by measuring in-situ stress status. However, it is impossible to predict accurately when and where a strong destructive earthquake may occur. And it's difficult to fulfill the aspirations of measuring the in-situ stress in the pregnant seismogenic zone just before earthquake.