Crustal P-wave velocity structure of the Longmenshan region and its tectonic implications for the 2008 Wenchuan earthquake

The P-wave velocity structure of the crust in the Longmenshan region has been imaged by seismic travel time tomography using local and regional first P-wave arrivals recorded from 2000 to 2008. The tomographic model provides a way to analyze the deep tectonics of the Longmenshan fault belt and the tectonic implications for the 2008 M (s)8.0 Wenchuan earthquake. The P-wave velocity images indicate that the initial rupture site and focal depth of the Wenchuan earthquake, together with the direction of rupture propagation, closely relate to the crustal structure of the Longmenshan region. The Pengguan massif to the west of the Longmenshan fault belt is characterized by high velocity anomalies, suggesting that the crust has a strong strain strength that can accumulate large stresses over a long period. The M (s)8.0 Wenchuan earthquake is located at the southwestern end of the Pengguan massif and the western edge of the Sichuan Basin. The collision between the Pengguan massif and the Sichuan Basin becomes the primary reason for the occurrence of the M (s)8.0 Wenchuan earthquake. To the north of Wenchuan, the occurrence and propagation of rupture benefit from low velocity anomalies along the Longmenshan fault belt; whereas to the south of Wenchuan, the brittle rupture can occur with more difficulty in relatively weak crust with low velocities. This may be one of the reasons for the absence of aftershocks to the south of Wenchuan, and the rupture induced by the M (s)8.0 Wenchuan earthquake propagating from the north to the south along the Longmenshan fault belt. The deep geodynamics of the M (s)8.0 Wenchuan earthquake may occur due to the discrepancy of crustal structures on the two sides of the Longmenshan fault belt. Ductile deformation and crustal flow can easily occur in the weak middle-lower crust beneath the Songpan-Garze orogenic belt. The eastward movement of the Tibetan Plateau is obstructed by the rigid lithosphere of the Sichuan Basin, and then the thickening of the middle-lower crust and vertical deformation occur in the crust of the Longmenshan fault belt. In addition, the down-warping of the Moho and the basement thrusting onto the range front induced crustal deformation and strain accumulation, which provided the potential energy to trigger the occurrence of the M (s)8.0 Wenchuan earthquake.