A STUDY ON CHARACTERISTICS OF THE M7.0 LUSHAN EARTHQUAKE AND THE M8.0 WENCHUAN EARTHQUAKEIN IN ,

Fang DU1 , Youjin SU2 , Xiaoyan ZHAO 3, Feng LONG 4

The Longmenshan fault zone is the important boundary zone of the Bayan Har block on the eastern margin of the . It is a large active fault zone that the length and width are about 500km and 40～50km. It shows the properties of a dextral strike-slip and a thrust motion. The M7.0 Lushan earthquake happened in the southern segment of the Longmenshan fault zone in Sichuan on the 20 April 2013. Five years ago, the M8.0 Wenchuan earthquake of magnitude occurred in the north segment of the Longmenshan fault in Sichuan on 12 May 2008. According to the data of the telemeter seismographic network in Sichuan province and the current correlative information from internal and overseas, we analyze characteristics of the earthquake rupture, type of earthquake dislocation, aftershocks distribution of the M7.0 Lushan earthquake and the M8.0 Wenchuan earthquake. Fault rupture propagation and mechanism are different between the M7.0 Lushan earthquake and the M8.0 Wenchuan earthquake. The fault rupture of the M7.0 Lushan earthquake showed the features of a bilateral rupture propagation, a small scale and a blind thrust-type. The fault rupture of the M8.0 Wenchuan earthquake showed the features of unilateral rupture propagation, a large scale. The M8.0 Wenchuan earthquake is a thrust earthquake with a small amount of right-lateral strike-slip component. The energy of the middle segment of the Longmenshan fault is fully released by the M8.0 Wenchuan earthquake. The M8.0 Wenchuan earthquake occurred not only to release strain energy in the middle segment of the Longmenshan fault but also to release strain energy triggered along the northern segment of the Longmenshan fault. The M ≥ 6.0 aftershocks of the M8.0 Wenchuan earthquake are concentrated in the middle segment of the Longmenshan fault from 12 to 17 May 2008. The M≥6.0 aftershocks of the M8.0 Wenchuan earthquake are concentrated in the northern segment of the Longmenshan fault from the beginning of 18 May 2008. The largest aftershock is the M6.4 event occurred in the north segment on 25 May 2008. They show that the strain energy of the middle segment of the Longmenshan fault is fully released by the M8.0 Wenchuan earthquake and the strain energy of the northern segment triggered to release. The M7.0 Lushan earthquake only produces a finite rupture of the south segment the Longmenshan fault. The focal mechanisms show that a north-east trending thrust fault with a low angle is the seismogenic fault which may not reach the surface. Although fault surface rupture trace is not obvious, discontinuous surface deformation is clearly visible. The long axis direction of the aftershocks distribution on the northeast to southwest is consistent with the fault strike of the Longmenshan fault and the isoseismal trend. The aftershocks distribution concentrated area is located on the northwest side of the south segment trace of the Longmenshan front fault.

1 Prof., Earthquake Administration of Sichuan Province, Chengdu 610041, China , [email protected] 2 Prof., Earthquake Administration of Yunnan Province, Kunming 650224, China , [email protected] 3 Eng., Earthquake Administration of Yunnan Province, Kunming 650224, China , [email protected] 4 Eng., Earthquake Administration of Sichuan Province, Chengdu 610041, China , [email protected]

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The epicenter distance between the M7.0 Lushan earthquake and the M8.0 Wenchuan earthquake is about 90 km. The distance between two locations of the aftershock-intensive areas is 50km. Although the M6.2 Dayi earthquake occurred in Sichuan in 1970 between the M7.0 Lushan earthquake and the M8.0 Wenchuan earthquake, the rupture length of the M6.2 Dayi earthquake is limited. They show that the unruptured segment of 50km between the M7.0 Lushan earthquake and the M8.0 Wenchuan earthquake may be dangerous with another rupture. The risk of rupture also increase on the unruptured southwest segment of the M7.0 Lushan earthquake.

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Strike-slip fault. Normal fault Reverse faultt. Guangzhou Fig. 1200km 600km Figure 1. Distribution map of the M8.0 Wenchuan earthquake and the M7.0 Lushan earthquake

About 5 years apart, the M8.0 Wenchuan earthquake and the M7.0 Lushan earthquake occurred one after another along the Longmenshan fault are relatively independent events. There are difference of the types of focal mechanism solutions, rupture processes, the surface ruptures and the aftershocks activity of the two events. Although they are connected with each other on same seismogenic structure, they are two relatively independent events.

REFERENCES

Chuanyou LI, Xiwei XU, Weijun GAN, et al. (2013) “Seismogenic Structures Associated With The 20 April 2013 MS7.0 Lushan Earthquake,Sichuan Province”, Seismology And Geology, 35(3):671-683,doi: 10.3969 /j.issn.0253－4967.2013.03.020. Qidong DENG, Shefa CHEN, Xiaolin ZHAO, et al. (1994) “Tectonics, seismisity and dynamics of Longmenshan Mountains and its adjacent regions”, Seismology and Geology, 16(4):389-403. Xiwei XU, Guihua CHEN, Guihua YU,et al. (2010) “Reevaluation of surface rupture parameters of the 5•12 enchuan earthquake and its tectonic implication for Tibetan uplift”, Chinese Journal Geophysics,53(10): 2321-2336,doi: 10.3969/j.issn.0001-5733.2010.10.006.

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