Seismic imaging, crustal stress and GPS data analyses: Implications for the generation of the 2008 Wenchuan Earthquake (M7.9), China

Seismic imaging together with global positioning system (GPS) and crustal stress data analyses show that the Mw7.9 2008 Wenchuan earthquake occurred within a distinct area of high crustal stress (∼ 17.5 MPa) and high Poisson's ratio (7-10%) anomalies centered on the Longmen-Shan (Shan means Mountain in Chinese) tectonic fault belt. Low P-wave and S-wave velocities in the southwest (SW) segment contrast with high-velocity anomalies in the central portion (CP) and northeast (NE) segment within the uppermost ∼ 15 km depths along the tectonic fault belt, though a presumably ductile zone with low-velocity anomalies separates the CP and NE segment. The rupture initiated near the southwestern end of the CP at a zone of high Poisson's ratio (σ) which extends down into the lower crust. These low-velocity and high-σ anomalies immediately below the source hypocenter, together with the high crustal stress, indicate the presence of high-pressure fluids from the lower crust, which might have reduced the mechanical strength of the fractured rock matrix and triggered the earthquake. Our study suggests that the structural heterogeneity and high crustal stress played an important role in the nucleation of the Wenchuan earthquake and its rupture process.