Detailed mapping of the surface rupture of the 12 February 2014 Yutian Ms 7.3 earthquake, Altyn Tagh fault, Xinjiang, China

Abstract:

Large-scale detailed mapping plays a key role in revealing the rupture characteristics and mechanisms of strong earthquakes. Relatively few studies have been performed on the surface ruptures of large earthquakes in central and western Tibet due to its remote nature and high elevation. Based on high-resolution unmanned aerial vehicle (UAV) photography, we mapped the coseismic surface rupture of the 2014 Yutian Ms 7.3 earthquake. Along the western Altyn Tagh fault system, the earthquake produced ~37 km of surface rupture along the South Xor Kol fault (southern section S1), Xor Kol fault (central section S2) and Ashikule fault (northern section S3). Section S1 has a 16-km-long surface rupture with an average sinistral offset of 52±25 cm, while section S3 has a 14.2-km-long surface rupture with an average sinistral offset of 36±21 cm. A compilation of 5308 cracks yields an average crack width along the southern section of 85±71 cm; the average width along the central section is 39±21 cm; and the average width along the northern section is 61±44 cm. In addition, the average opening across rupture zone is 3.4±2.9 m along the southern section; 4.3±3.6 m along the central section; and 1.7±1.6 m along the northern section. Evidently, the average crack width and opening decrease towards bends and steps along the fault. A global synthesis of surface rupture distributions corresponding to strike-slip earthquakes indicates that the rupture zone is wider near the complex parts of fault geometries (such as bends, steps and fault bifurcations) than along straight sections, suggesting that the fault geometry has an obvious control on the surface rupture width. The Yutian earthquake also produced a large number of gravity-driven slides on alluvial fans with gentle slopes. The friction efficiency of the water-bearing salt layer beneath fans could decrease the sliding threshold and trigger instability under surface shaking. These distributed deformations and gravity-driven slides reflect the coupling between the rupture propagation and fault geometry and indicate that the rupture may have propagated in two directions along the Ashikule fault after passing through a step. Therefore, the investigation of coseismic surface rupture provides important observational constraints on the dynamic rupture process.