Different surface ruptures and dynamics between the 2008 and 2014 Mw7.1 Yutian earthquakes in the western segment of the Altyn Tagh fault, Western Tibet

Thursday, 18 December 2014
Haibing Li1, Jiawei Pan1, Zhiming Sun2, Dongliang Liu1, Jiajia Zhang1,3, ChengLong Li1,4, Marie-Luce Chevalier1, Kang Liu1,5, Kun Yun1,3 and Zheng Gong1, (1)Institute of Geology, CAGS, Beijing, China, (2)Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, China, (3)Chengdu University of Technology, Chengdu, China, (4)China University of Geosciences Wuhan, Wuhan, China, (5)National Earthquake Response Support Service, Beijing, China
Two Yutian earthquanks (Ms7.3) happened at the eastern region of the West Kunlun Mountains in the western Tibet, on March 21, 2008, and February 12, 2014, respectively. The epicenters were 110 km apart in the west margin of Bayan Har block and the western Altyn Tagh fault where the fault strike changes. These two earthquakes have different kinematics characteristics, the former one showed normal with sinistral slip components, while the latter one shows sinistral with normal slip components. The two earthquakes show their own special seismogenic structures which reflect the different dynamic mechanisms.

The surface ruptures of the 2008 Yutian earthquake is ~32 km long along the NS-trending western Pingdingshan fault where the altitude is above 5400 m. The surface rupture zone is charactered by normal slip with local sinistral strike-slip. The maximum vertical offset is ~5 m and the left-lateral slip is ~1.5 m, the maximum width of cracking is ~10 m. The northern part of the surface rupture zone extended to NNE and linked with NEE-trending Ashikule fault (a branch fault of western Altyn Tagh fault), which not only controls the latest volcanic activity in western Tibet, and also is one of the west margin faults which control the Tibet eastward extrusion.

The 2014 Yutian earthquake shows that it produced ~25 km long surface rupture along two NEE-trending parallel faults, the Ashikule fault and the south Xor Kol fault. The surface rupture zone consists of a series of en-echelon tensional, shear-tensional to shear cracks, mole-tracks, and sag ponds at an altitude of 4600~5100 m. The maximum left-lateral slip and vertical offset are ~1 m and ~0.4m, respectively. The seismogenic fault of the earthquake is splay at the southwestern end of the Altyn Tagh fault. This fault combined with the Guozha Co and the Longmu Co faults constitutes a “new Altyn Tagh fault”.

These two Yutian earthquakes were caused by the India-Eurasia collision which fits the eastward escape of the Bayan Har block, which are the response of the different dynamics and deformation process in western Altyn Tagh. These surface rupture pattern supports the eastward block-like motion model whose deformation takes place mainly along the block boundaries delineated by mega-strike slip faults in the north Tibet.