Coseismic Surface Ruptures Associated with the 2014Mw 6.9 Yutian Earthquake on the Altyn Tagh Fault, Tibetan Plateau

Abstract

Field investigations reveal that the 2014 M w 6.9 Yutian earthquake on the left‐lateral strike‐slip Altyn Tagh fault (ATF) system, Tibetan Plateau, produced an ∼25‐km‐long surface rupture zone that contains conjugate Riedel shear faults. The coseismic surface ruptures occurred mainly along two parallel east‐northeast‐trending active left‐lateral strike‐slip faults. Rupture also occurred in a conjugate, west‐northwest‐trending zone along an active right‐lateral strike‐slip fault. The east‐northeast‐trending ruptures are concentrated in a zone of <500‐m wide and ∼25‐km long, and are characterized by Riedel shear structures including distinct shear faults (Y) with a maximum sinistral displacement of ∼1 m, right‐stepping en echelon cracks, and mole tracks. In contrast, the west‐northwest‐trending ruptures occur within a zone of up to 1.5‐km wide and ∼4‐km long in the jog area between the two parallel east‐northeast‐trending faults, and this zone is characterized by discontinuous shear faults with dextral displacements of <0.5 m, left‐stepping en echelon cracks, and mole tracks, all oriented oblique to the east‐northeast‐trending rupture zones at an angle of 30°–40°. The lengths and displacements of the coseismic surface ruptures measured in the field are comparable with those obtained from the empirical relationships between magnitude and coseismic surface rupture length and displacement. Our findings demonstrate that the coseismic conjugate Riedel faulting was controlled mainly by preexisting active faults of the ATF system, reflecting the present‐day tectonic stress field associated with the ongoing penetration of the Indian Plate into the Eurasian Plate.