The Role Of Fluids In The 2008 Ms8.0 Wenchuan Earthquake, China

Fluid intrusions into seismic zones play a fundamental role in the initiation of earthquakes and rupture processes, yet the mechanisms that control these interactions are enigmatic and widely debated. Here, we present a series of signatures for new seismic velocities (Vp, Vs) and Vp/Vs ratio imaging and stress simulations that reveal the impact of fluids, including partial melting, on the seismogenesis of the 2008 Wenchuan earthquake that occurred on the eastern Tibetan Plateau. We found that a prominent low-V and high-Vp/Vs body under the western side of the 2008 Wenchuan hypocenter and two gaps characterized by low-V and high-Vp/Vs inserted into its rupture process were clearly imaged. One of these gaps terminated the southwestward coseismic slip while the other weakened the northeastern rupture process. We interpreted both of these bodies as the result of fluid penetration associated with partial melting of the ductile flow from the lower crust of Tibet. Such ductile materials were pushed against the rigid basement of the Yangtze craton and accumulated there, maintaining high pore pressure. We presume that a system such as this, with high pore pressure and fluids in the fluid-pressurized regime, might have reduced the threshold for fault failure, transformed this zone into a potential fault risk front, and thus increased the likelihood of failure of the mainshock fault. We speculate that fluids invading the seismogenic layer of the Longmen-Shan fault zone played a crucial role in the initiation of the 2008 Wenchuan earthquake and its rupture process.