Abstract: An M_S 7.0 earthquake attacked southern segment of the Longmenshan Fault zone on 20 April 2013, in Lushan County, Sichuan Province, southwest of China. The seismic intensity of the meizoseismal area of the Lushan event is Ⅰ Ⅹ(the Chinese Seismic Intensity Scale). The meizoseismal area strikes NE, and is approximately 24km long and 11km wide. In the post-earthquake emergency scientific survey, some members found a series of co-seismic surface rupture signs at Longmen Township, one of the most damaged areas by this quake, north of Lushan County. The reported typical surface rupture signs include intensive shear-fissures along the channel at Zhanghuo Group, rotation of bricks near a white tower at Wanghuo Group, and lots of extensive cracks. On the basis of analyzing such surface rupture signs, Han et al., (2013)deduced that there might be a blind fault along Lushan County and Longmen Township(named the Lushan-Longmen presumed blind fault), and this fault probably was the seismogenic fault. Therefore, to confirm whether there is a potential seismogenic fault along Lushan and Longmen is very important not only to research of seismogenic fault for this earthquake but also to the reconstruction in the disaster-hit areas. Through surface ruptures surveying and trench excavating, we conclude that: there are no co-seismic surface ruptures at Longmen. Meanwhile, artificial seismic prospecting outcome, which shows nonexistence of the Lushan-Longmen presumed blind fault at least 800m below the ground surface, also supports our idea. Consequently, the reported shear-fissures and extensive cracks are not produced by the seismogenic fault, but most likely by ground shaking during the earthquake.

Key words: the Lushan M_S 7.0 earthquake, Longmen Township, ground fissures, seismogenic structure

摘要： 4.20芦山地震后，有学者在芦山县龙门乡发现一系列的线性裂缝和砖块的旋转变形等“地震地表破裂迹象”，由此推测芦山—龙门一线存在隐伏逆断裂，并认为该断裂有可能是此次地震的发震断裂。因此，进一步探讨芦山—龙门一线是否存在潜在的发震断裂，无论是对研究芦山7.0级地震的发震断裂，还是对灾区的重建指导都十分重要。在龙门乡开展了地质灾害调查、跨谷地的地质剖面实测，槽探和人工地震勘探等工作。结果显示：至少在800m深度范围内，不存在芦山-龙门隐伏断裂。此带上的地裂缝等现象不是由断层位错引起，而更可能是地震动在阶地陡坎附近造成的地基或边坡效应所致。

关键词: 芦山7.0级地震, 龙门乡, 地裂缝, 发震构造