Reexamining the Seismological Implications of the Present‐day Stress State of the Yingxiu‐Beichuan Fault after the Wenchuan Earthquake

AbstractThe present‐day stress state of the Yingxiu‐Beichuan fault after the Wenchuan earthquake was re‐estimated using measured in‐situ stress data obtained after the Wenchuan earthquake. The results reveal that the gradient coefficients of principal stresses versus depth decrease from south to north along the Yingxiu‐Beichuan fault, revealing that the stress level decreases from south to north. The consistency between the present‐day stress levels and surface ruptures generated during the earthquake indicates that the accumulated tectonic stress beneath the Yingxiu‐Beichuan fault before the Wenchuan earthquake was relieved in form of surface ruptures. This resulted in the stress remaining high in the southern section of the Yingxiu‐Beichuan fault but relatively low in the northern section. Abnormal high pore pressure conditions and an extremely low frictional coefficient play important role in the interpretation of the stress field adjustment and seismic events observed after the Wenchuan earthquake along this fault, according to the estimation results using the Coulomb frictional‐failure theory incorporating frictional coefficients ranging from 0.4 to 1.0. To accurately estimate the seismological hazard of the Yingxiu‐Beichuan fault by analyzing fault instability using the Coulomb frictional‐failure theory, much attention should be focused on the pore pressure conditions and the evolution state of the frictional coefficient under the present‐day stress state.