Deep Processes and Dynamic Responses Associated with the Wenchuan M_s8.0 Earthquake of 2008

Abstract

The Wenchuan Ms8.0 earthquake of 2008 occurred on the Longmen Shan tectonic zone which seems not very active from available data. And it is reported that no noticeable precursors have been observed before this event. It might imply that the crustal deformation at depth associated with this gigantic shock had no expression on the surface or shallow subsurface which was detectable. In this article, we attempt to analyze the processes of brewing and generation of this event at depth that can account for the reason of this devastating earthquake. Tectonically, it is obvious that the India‐Eurasia plate collision serves as the most important dynamic setting of major earthquakes in southwestern China. The Eastern Himalayan Syntaxis is moving rapidly in NNE direction into the Tibetan plateau, forcing deep material to flow toward east. Due to the obstruction of the NE trending Longmen Shan tectonic zone in the northwestern margin of the Sichuan basin, the partial flow turns to southeast. Longmen Shan is a conspicuous boundary of topography and crust structure, across which the elevation difference is as large as 3500±500 m, with crustal thickness 60±5 km in its west and 40±2 km in its east, respectively. Thus it must experience intense deformation and stress accumulation, though very slow without appreciable manifestation. Based on available data, it is inferred that material of lower crust and covering strata of upper mantle is flowing eastward along two deep slide surfaces, of which one is the layer of low‐velocity and low‐resistivity (20~25 km depth), and the other is the top of mantle asthenosphere. Hindered by rigid matter beneath the Sichuan basin, the flow of lower crust and covering strata of upper mantle material turns overthrusting at high angles at the Longmen Shan. On the other hand, three thrust faults visible west of the Longmen Shan extend downward to depth 20 km, where they converge into one active zone. Thus the Longmen Shan tectonic zone is a place where occurs the coupling between slippage on faults of shallow subsurface and material flow of lower crust and covering strata of upper mantle. Under the intense exchange of matter and energy, the source medium ruptures suddenly at a large scale to release a huge amount of elastic strain, resulting in the Wenchuan Ms8.0 earthquake.

Slip model of the 2008 M w 7.9 Wenchuan (China) earthquake derived from co-seismic GPS data

“…Many factors may cause the large residuals, such as the assumption of lateral homogeneity in the earth model, the simplicity of the fault geometry, topography and inelastic deformation. For example, the crustal thickness of the Tibet plateau on the NW side and the Sichuan basin on the SE side of the Longmenshan fault are about 70 km and approximately 40-45 km, respectively, with the mean crustal P velocity of approximately 6.25-6.30 km/s and approximately 6.45-6.50 km/s, respectively (Teng et al, 2008). Furthermore, the topographic difference between the Tibet plateau and the Sichuan basin is almost 3500 m, which may also influence the surface displacements.…”

Spatial distribution and focal mechanism solutions of the Wenchuan earthquake series: Results and implications

“…Zhou and He (2009) thought those features might be results of pore fluid in faults, but they cannot explain the new fact given in this paper that there is indeed almost no thrust-event in shallow depth. Zhang et al (2010) attributed them to products of differential uplift, which implies that the possibly existed molten material in the lower crust of Songpan-Garze terrain has contribution to local stress state (Royden et al, 2008;Teng et al, 2008;Hubbard and Shaw, 2009). Their explanation also contradicts with the facts obtained in this paper that the dip angle of the seismic zone with the main shock in it is small, there is no thrust-event occurred in shallow depth, and the stress state is characterized by a sub-horizontal compression.…”

Rapid afterslip and short-term viscoelastic relaxation following the 2008 MW7.9 Wenchuan earthquake

“…A low velocity layer (high-conductivity layer) has also been found at depth of 25-30 km, which is first gently dipping to northwest in the Longmenshan fault and then gradually extends horizontally in the northwestern Bayan Har block (Wang et al, 2003;Zhu, 2008;Teng et al, 2008). The southeast end of this low velocity layer terminates beneath the place located between the Longmenshan fault.…”

“…The material in the deep crust and upper mantle of the Bayan Har block moves southeastward as it is driven by the Indian Ocean Plate, but the movement is blocked by the relative strong Sichuan Basin in the Longmenshan fault. Under the continuous action as long as thousands of years and due to the interseismic block effect, the upper crust on the east side of the Bayan Har block undergoes lateral shortening deformation and right-lateral shear deformation parallel to the fault Teng et al, 2008;Wen et al, 2009). Du et al (2009) and Zhang et al (2008) presented a regional geodynamic model of the Wenchuan earthquake based on the comprehensive geological, geophysical and geodetic observations (see Figure 2).…”