Structural interpretation of the coseismic faults of the Wenchuan earthquake: Three‐dimensional modeling of the Longmen Shan fold‐and‐thrust belt

Abstract

[1] The 2008 M w 7.9 Wenchuan earthquake is a result of ongoing India-Tibet collision and reflects the growth of the Longmen Shan fold-and-thrust belt. In this paper, we construct a 3-D structural model of the geometry of the coseismic faults and related structures of the Wenchuan earthquake by integrating geological investigations, relocated aftershocks, and seismic reflection profiles. In the 3-D structural model, the differences between the southern and northern segments of the rupture are highlighted. The structural transition zone between the two segments contains a major geometric segment boundary, reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system, which appears to have localized significant damage from Anxian to Beichuan. Within the northern segment, we identify a transverse structure across which the Beichuan fault plunges under the Tangwangzhai syncline. This boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the Longmen Shan thrust belt. In the southern segment, our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography. On the basis of this association, the southwestern segment of the Longmen Shan, south of the Wenchuan earthquake, is likely active and presents a significant earthquake hazard, despite the lack of historical earthquakes in this region. This study illustrates the importance of building 3-D models to study active faulting and folding, as well as to assess earthquake hazard.

Structural Setting of the 2008 Mw 7.9 Wenchuan, China, Earthquake

“…Based on fault-related folding theories (Suppe, 1983;Shaw et al, 2005), we suggest that a shallowing of the dip of the Beichuan thrust to a detachment between ∼16-20 km is consistent with the earthquake data, the surface fault constraints, and the location and geometry of the synclinorium. While other solutions for the fault geometry at depth are likely permissible, we suggest that this interpretation is compatible with tomographic observations of a low-velocity zone at depths of 16-26 km beneath the range (Huang et al, 2009), as well as the fact that very few earthquakes have occurred beneath those depths (Li et al, 2010).…”

“…10). In many cases, such as in the Zhongba thrust sheet, Cenozoic structures have clearly reactivated older Mesozoic fault systems (Burchfiel et al, 1995;Jia et al, 2006;Li et al, 2010). Exhumation rates imply that the present relief of the mountain belt has largely formed since the Miocene (Kirby et al, 2002).…”

“…The Longmen Shan and Sichuan basin systems are zones of active reverse and oblique-slip faulting (Densmore et al, 2007;Liu-Zeng et al, 2009;Xu et al, 2009;Li et al, 2010) and contain a number of faults that may be capable of generating large earthquakes. These include along-strike sections of faults that ruptured in the Wenchuan earthquake, the Wenchuan and Range Front faults, and possibly structures within the basin.…”

Preseismic deformation in the seismogenic zone of the Lushan M S7.0 earthquake detected by GPS observations

“…The GPS coseismic displacements mainly reflect the elastic deformation of the upper crust, and therefore, the observed eastward movement of the Bayan Har block, which accelerated overtime, might be affected by deep tectonic deformation. There are many tectonic models of the Longmen Shan fault zone (Hubbard and Shaw, 2009;Li et al, 2010;Yao et al, 2012), however, it is difficult to define how the deep tectonics affect crustal deformation clearly due to the current limitations in the accuracy and density of the GPS observations. In subsequent work, we will study the observed crustal deformation in combination with the deep tectonics.…”

The Evolution of Stress and Strain around the Bayan Har Block in the Tibetan Plateau

“…Yutian earthquake is caused by a normal fault, which implied extension setting [21,22]. Wenchuan earthquake indicated the intense activation of Longmen Shan thrust-fold belt, which is a typical compressive tectonic environment [23][24][25]. Kunlun Shan earthquake in the north of the block and Yushu earthquake in the south of the block suggested the strike slip of Kunlun Shan fault and Yushu-Xianshuihe fault [26][27][28].…”