In-situ hydro-fracture stress measurement before and after the Wenchuan M_s8.0 earthquake of China

One week before the occurrence of Wenchuan M_s8.0 earthquake the maximum horizontal principal stress measured by hydro-fracturing technique in the earthquake Fault of Wenchuan Earthquake (Yingxiu-Beichuan fault) at ±400 m depth was 21～22 MPa,which was 8～10 MPa 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.