Structural analysis of the co-seismic surface ruptures associated with the Yushu Ms7.1 earthquake, Qinghai Province

The Yushu Ms7.1 earthquake in Yushu area, Qinghai Province, China, caused a 46km-long surface rupture, with a maximum horizontal left-slip displacement of 240cm and a maximum vertical displacement of about 60cm. The surface rupture distributed along the pre-existing Garzi-Yushu active fault.Four levels of ruptures can be classed on different scales and structural characters. They are level Ⅰ, Ⅱ, Ⅲ and Ⅳ from large scale to small one in sequence. Level Ⅰrupture, the surface rupture as a whole caused by this earthquake, generally strkes E119°S and comprises three level Ⅱ ruptures, i.e. Longbao Town Section, Jiegu Town Section and Changu Temple Section respectively from the northwest to southeast. A level Ⅱ rupture usually consists of a dozen of level Ⅲ ruptures. Level Ⅳ rupture is the smallest one which can be figured out on the ground in the field survey. Tens or hundreds of level Ⅳ ruptures constitute a level Ⅲ rupture. Different levels of rupture may have different mode in arrangement, with a left step among the Level Ⅱ ruptures and a right step mode among both level Ⅲ and Ⅳ respectively. However, different level ruptures have the similar character of the Riedel Shear structures and five basic shear planes, R, R’, Y, T and P, can be identified. The angles of internal friction of the initial fault plane is 26~44°according to Coulomb failure criterion.