**4. The seismotectonics analysis of the Wenchuan earthquake**

As the Indian plate pushes strongly toward the Qingzang Plateau, different blocks in southwestern China have different movement directions (Fig.12), The northwestern block of Sichuan mainly moves towards the east. In the movement process, due to the Minshan block lock-up →earthquake→geo-stress release (stick-slip) cyclic process, the middle segment of the Longmen Mountain Range structural belt is more active than the southern and northern segments.

Both the geophysical survey and the geological analysis strongly suggest that the three fractures in the southern part of the Minshan block converge into an intracrust low velocity layer and form a slip plane (Fig.2). The stress accumulation in the southern part of the Minshan block caused a high stress concentration along the Yingxiu-Beichuan and Guanxian-Anxian fractures, and they became a rupture point that probably could be fractured at anytime. According to the hypocenter parameters of the 5.12 Wenchuan Earthquake (epicenter at Niumiangou with depth of focus 14 km), the initial focus is located at the front range fracture. At the surface, the bottle-neck's lock-up of the Minshan block is small in the front and large in the tail; vertically, it is small in the lower part and big near the surface (Fig.5). Because of the restriction in the northern and southern boundaries, together with the blockage in the east, the dextral strike-slip action of the block requires thrusting movement at the southern boundary to provide enough space. Thus, the movement of the Minshan block is mainly characterized by vertical thrusting in combination with a small strike slip. The transient variation of stress and the sudden release of tremendous amounts of strain energy forced movement in the Yingxiu-Beichuan fracture (just above the focus) and induced large scale rupture toward the northeast. Because of the obstacle (at the block's east boundary) around Beichuan County, the consequent progressive failure released large amounts of strain energy. After the thrusting movement occurred along the southern boundary of the Minshan block, the lock-up effect was lost instantly. The powerful pushing from the west was immediately transmitted to the eastern (Motianling) block, which was under lower levels of stress because of the barrier action from the Minshan block. This caused continuous aftershocks in the Motianling block and the northern segment of Longmen Mountain Range which up to then were fairly calm. Because of the eastward movement of the Minshan block, the stress level at the southern segment of the western boundary fracture changed significantly. The stress adjustment caused relative movements of both sides along the boundary, which led to continuous aftershocks along the Yuzixi – Lixian-Chibusu and the '√'shape distribution of aftershocks (Fig.12) (Zhao et al. (1994b); Chen et al. (1994a); Chen et al. (1994b); Hua et al. (2009); Hu et al. (2008)). The secondary geological hazard anomaly zone has similar shape too.

Fig. 12. Regional geostress directions in southwestern China

Minshan block is mainly characterized by vertical thrusting in combination with a small strike slip. The transient variation of stress and the sudden release of tremendous amounts of strain energy forced movement in the Yingxiu-Beichuan fracture (just above the focus) and induced large scale rupture toward the northeast. Because of the obstacle (at the block's east boundary) around Beichuan County, the consequent progressive failure released large amounts of strain energy. After the thrusting movement occurred along the southern boundary of the Minshan block, the lock-up effect was lost instantly. The powerful pushing from the west was immediately transmitted to the eastern (Motianling) block, which was under lower levels of stress because of the barrier action from the Minshan block. This caused continuous aftershocks in the Motianling block and the northern segment of Longmen Mountain Range which up to then were fairly calm. Because of the eastward movement of the Minshan block, the stress level at the southern segment of the western boundary fracture changed significantly. The stress adjustment caused relative movements of both sides along the boundary, which led to continuous aftershocks along the Yuzixi – Lixian-Chibusu and the '√'shape distribution of aftershocks (Fig.12) (Zhao et al. (1994b); Chen et al. (1994a); Chen et al. (1994b); Hua et al. (2009); Hu et al. (2008)). The secondary

> position of arrested material movement and geostress concentration

> direction of maximum main geostress

Lanzhou

Chengdu

Kunming

geological hazard anomaly zone has similar shape too.

displacement direction

Lasa

direction of plate

of the fault

Fig. 12. Regional geostress directions in southwestern China

movement

Indian plate According to historical earthquake records, since 186 B.C the spatial distribution of earthquake in northwestern Sichuan was constrained by the Minshan Block. The earthquake record in the Minshan block demonstrates a higher magnitude and frequency than along its southern boundary, the middle-segment of the Longmen Mountain Range structural belt. Historically, there was no earthquake with a magnitude over 7.0. The occurrence of the magnitude 8.0 earthquake in Wenxian in 1879 was the consequence of stress accumulation, which led to the migration of seismicity from north to south. Earthquakes with a magnitude larger than 6.0 occurred several times along the boundaries and in the interior of the Minshan Block, and earthquakes larger than 5.0 magnitude are frequent. Except for the 5.0 magnitude earthquakes that occurred in Beichuan, Dayi and Mianzhu counties in 1999, the seismic activity along the southern boundary was historically not frequent. In other words the stress accumulation that can cause a magnitude 8.0 earthquake takes a long time to develop, more than 2000-4000 years.

Fig. 13. The structural framework and focal mechanism solution for the Wenchuan earthquake and its aftershocks(from Hu Xingping et al 2008 modified)

#### **5. Conclusion**

On the basis of the presented analysis, we can draw the following conclusions: (1) the Minshan block is an important tectonic element with larger rigidity than that of the surrounding areas; (2) This unique tectonic framework caused a bottle-neck effect in the Minshan block. Exactly the effect which causes stress concentrations along the boundary and in the interior of the block which lead to frequent earthquakes; (3) Before the nineteenth century, the earthquakes were concentrated on the northern, western and eastern Minshan block boundaries. A magnitude 8.0 earthquake occurred in the Wenxian county in 1879 and triggered another round of large-scale reverse faulting and strike-slip movements. Against this background, the movement of the southern boundary of the Guanxian-Anxian fracture induced the 5.12 Wenchuan Earthquake; (4) The fracture just above the focus, the Yingxiu-Beichuan fracture, was forced to act strongly; (5) The release of the Minshan block lock-up lead to a high stress level in eastward direction causing continuous aftershocks in the northern segment of the Longmen Mountain Range structural belt and in the Motianling block. (6) Further aftershocks will occur in the middle and northern segment of Longmen Mountain Range structural belt and the southern segment of the Mounigou Valley fracture.

### **6. Acknowledgements**

The research has been supported by National Natural Science Foundation of China (NSFC), (Grant No. 2008CB425801, 41072231, and by the China Geological Survey Bureau (Grant No.1212010914010). We also particularly thank Niek Rengers for comments and suggestions to earlier versions of the manuscript.

#### **7. References**

On the basis of the presented analysis, we can draw the following conclusions: (1) the Minshan block is an important tectonic element with larger rigidity than that of the surrounding areas; (2) This unique tectonic framework caused a bottle-neck effect in the Minshan block. Exactly the effect which causes stress concentrations along the boundary and in the interior of the block which lead to frequent earthquakes; (3) Before the nineteenth century, the earthquakes were concentrated on the northern, western and eastern Minshan block boundaries. A magnitude 8.0 earthquake occurred in the Wenxian county in 1879 and triggered another round of large-scale reverse faulting and strike-slip movements. Against this background, the movement of the southern boundary of the Guanxian-Anxian fracture induced the 5.12 Wenchuan Earthquake; (4) The fracture just above the focus, the Yingxiu-Beichuan fracture, was forced to act strongly; (5) The release of the Minshan block lock-up lead to a high stress level in eastward direction causing continuous aftershocks in the northern segment of the Longmen Mountain Range structural belt and in the Motianling block. (6) Further aftershocks will occur in the middle and northern segment of Longmen Mountain Range structural belt and the southern segment of the Mounigou Valley fracture.

The research has been supported by National Natural Science Foundation of China (NSFC), (Grant No. 2008CB425801, 41072231, and by the China Geological Survey Bureau (Grant No.1212010914010). We also particularly thank Niek Rengers for comments and suggestions

Aki, K.(1981) A probabilistic synthesis of precursory phenomena, in Earthquake Prediction:

Chen Chunzai, Jin Youyu(1997) Application of fractal theory in metallogenic prognosis,

Chen G, Ji F, Zhou R(2007) Primary Research of Activity Segmentation of Longmenshan Fault Zone Since Late-Quaternary. Seismology and Geology 29(3): 657-673. Chen Z, Lin B, Bai W, et al. (2009) The mechanism of generation of May 12, 2008 Ms8.0 Wenchuan earthquake. Chinese J. Geophys 52(2):408-417.(in Chinese) Chen S, Deng Q, Zhao XL, et al. (1994a) Evolution history, deformed mechanism and

Chen S, Deng Q, Zhao XL, et al. (1994b) Evolution history, deformed mechanism and

Cheng GW, He XB, Chen GR, et al. (2010) Change in Sediment Load of the Yangtze River after Wenchuan Earthquake. Journal of Mountain Science 7:100-104. Cheng GW, Wang XD, He XB et al. (2008) Outburst Risk of Barrier Lakes in Sichuan, China.

Gu Gongshu(1983) Index of China Earthquakes, Beijing: Science Publishing House, pp307.

P. G. Richards, pp. 566-574, AGU, Washington, D.C.

Mineral resources and geology, 11(4), 272-276.

An International Review, Maurice Ewing Ser., vol.4, edited by D. W. Simpson and

earthquake of middle segment nappes and related structures(1). Seismology and

earthquake of middle segment nappes and related structures(2) Seismology and

**5. Conclusion**

**6. Acknowledgements**

**7. References**

to earlier versions of the manuscript.

geology 16(4): 404-412.

geology 17(4): 404-412.

Journal of Mountain Science 5:189-193.