Jianganlin surface fracture induced by 2013 Lushan M_S7.0 earthquake

A WNW-trending Jianganlin surface fracture induced by the Lushan M_S7.0 earthquake was found by the fieldwork team of Geophysical Exploration Center, China Earthquake Administration, during the period of emergency investigation into the earthquake. The geometry, striation and slip features of the surface fracture were inspected in detail，and different understandings of the fracture and its significance to seismogenic structure research are discussed. The Jianganlin surface fracture is located at the mountain slope having an elevation of 1529—1578 m and dipping 30°—40° to the west of Group 2, Jianganlin village of Shangli town region, Ya’an City, Sichuan Province of China. The surface fracture with a strike of 120° consists of three segments stretching near linearly in the range of about 80 m long, each with a length of 6—10 m, and their maximum width being 40 cm. The fracture surface is smooth, dipping 62°—85° southwestward. Most part of the fracture passes through clay. There are two sets of fresh striations on the fracture surfaces, on which the striations with larger slip-angle are usually seen above the range of 10—20 cm below ground surface, while in deeper part the large slip angle striations are covered by smaller slip-angle ones. A tree root with a diameter of about 4 cm was cut and offset due to the rupture passing through. The hanging wall of the fracture rose 15 cm and shifted 10 cm toward SE. Besides, some strata were inversed on the hanging wall. The trench exposes that the black humus layer on the hanging wall rose about 15 cm relative to the black humus layer on the footwall. A comprehensive analysis shows that during the Lushan earthquake the Jianganlin surface fracture experienced thrust motion firstly, and then sinistral strike-slip. The focal mechanism solution shows that the earthquake is a thrust event with a NE-striking fault plane. The WNW-trending Jianganlin surface fracture is possibly a secondary one of the event, and it is worth studying whether there exist seismogenic structures striking NW-WNW. The find of Jianganlin surface fracture associated with the Lushan M_S7.0 earthquake provides important information for studying the seismogenic structures and earthquake damage.