**Tectonic Background of the Wenchuan Earthquake**

Yunsheng Wang, Runqiu Huang, Yonghong Luo, Hongbiao Xu, Shitian Wang, Liangwen Jiang and Yusheng Li *State Key Lab. of Geohazard Prevention, Chengdu University of Technology, Chengdu China* 

#### **1. Introduction**

16 Will-be-set-by-IN-TECH

214 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

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**5. References**

Since the occurrence of the Wenchuan Earthquake on May 12, 2008, several papers on its seismotectonics analysis have been published (Lei et al. 2009; Yue, 2010; Chen et al. 2009; Tang et al. 2009; Zhu et al. 2009). Although their opinions were not uniform, most authors believe that the source of the earthquake was the Central Longmen Mountain Range fracture with a length of about 230km. However, this hypothesis cannot explain the following phenomena: 1) the focus, of which the depth was provided by the State Seismological Bureau, cannot be projected to the central fracture but is located on the front range fracture; 2) the epicenter is not the center of the isoseismal contour; 3) the surface rupture along the front range fracture and its thrusting displacement are similar to the surface rupture and displacement along the central fracture, and 4) the area in which the aftershocks (Ms≥4.0)occurred have a shape in the form of a '√'.

The ambiguity results from the complicated tectonic environment of northwestern Sichuan. People generally pay more attention to the large boundaries of the triangular block in northwestern Sichuan (SCNWTB in Fig 1), but neglect the effect of the Minshan block (MSB) in the geostress conditions. The present authors think the "bottlle neck" geostress concentration in the Minshan block is the main controlling factor for the occurrence of the Wenchuan earthquake. The Minshan block is a sub-block of the northwest Block of Sichuan, it is bordered by Longmen Mountain Range fractures ((2) in Fig 1)in the south, the Huya fracture (5) in the east, the Maqin-Lueyang fracture in the north and the Mounigou Valley fracture (6) in the west. The block has been recognized earlier as an important tectonic element in the northwestern region of Sichuan (Tang et al. 2009; Jiang et al. 2004; Zhao et al. 1994a; Tang et al. 1991; Qian et al. 1999; Zhou et al. 2000). It not only includes the Minshan uplifted block in a narrow sense, but also the middle segment of the Longmen Mountain Range structural belt (Fig.1). Although the positions of the eastern and western block boundaries are still controversial, their existence and their recent activation are widely accepted. Analysis of seismological setting and the deformation data shows that the Minshan block has been activated by the north Mounigou Valley fracture ((6) in Fig 1) in the west, the Huya fracture (5) in the east, the back range fracture in the south and the Maqin-Lueyang fracture (1) in the north.

The whole middle segment of the Longmen Mountain Range structures has been strongly pushed along the southern boundary of the Minshan block since the Mesozoic. The PaleoPeng(zhou)-Guan(xian) complex has been thrusted over the lower Pleistocene, especially since the Quaternary. During this overthrusting, nappes in Longmenshan structural belt have been formed. The landform of the block shows a great contrast in elevation and relief. As a result of the uplifting of the Minshan block, strong erosion caused the deposition of a thick Quaternary accumulation in the Chengdu plain and formation of the Longquan mountain. Due to the repeated Minshan block lock-up→earthquake→geo-stress release (stick-slip) mechanism, the middle segment of the Longmen Mountain Range structural belt is more active than the southwestern and northeastern segments. The earthquake history of northwestern Sichuan reveals that the migration of seismicity in northwest Sichuan is around the Minshan block, and the seismic activities on the northern margin of the Minshan block created the conditions for the Wenchuan earthquake.

The aim of the Chapter is to present a new idea through the tectonic background analysis of Wenchuan earthquake, so that people pay more attention to the important role of the Minshan Block activities in the northwestern seismic tectonics.
