Analyses of global seismic waveform data for the 2008 devastating Wenchuan Earthquake have indicated where the rupture started and how it expanded. Here we reveal that the rupture process of this earthquake was strongly controlled by structural heterogeneities of the upper crust. We made a tomographic inversion of 200,000 local travel time data, according to which the rupture was confined in a presumably brittle layer of high P- and S-wave velocity anomalies within the upper ~ 15 km depths. The rupture propagated northeastward through the southwest (SW) segment of the fault and then jumped to the northeast (NE) segment with little slip at the intervening, presumably ductile zone characterized by low P and S velocities with high Poisson's ratio. The rupture initiated near the southwestern end of the SW segment within a body of high Poisson's ratio, which extends down into the lower crust, suggesting upward migration of deep-seated aqueous fluid to trigger the earthquake by reducing the mechanical strength of the brittle layer.