Discussion on appearance of surface fractures along the rear-range of Longmenshan mountain during 2008 MS8.0 Wenchuan earthquake

The information of surface fractures at eleven investigated sites along the Wenchuan-Maoxian fault zone in the rear-range of Longmengshan mountain are presented in this paper. The investigation sites include, from south to north, Yusidong village of Wolong town in Wenchuan county, Niuping of Gengda town, Jinbo village，Zhangpai village and Zuwan village of Caopo town, Gaodongshan village of Miansi town, Linggang village of Yulong town, Qipangou village of Weizhou town, Qiaoqiaozhai village of Maoxian county, to Malianggou gully of Fengyi town, with a total length of 100 km. Although most surface fractures are located at the top or on the slope of the mountain, with the same strike as the mountain trend, there are also a few surface fractures stretching on terraces of rivers, with the fracture trends perpendicular to or obliquely intersecting the rivers. The northwestern side of most fracrures among the 11 sites are uplifted. Vertical displacements are mostly 20mdash;40 centimeters. Among them, the strike of surface scarp on the top of Zoumaling range is perpendicular to the trend of Qipangou gully in the Weizhou town of Wenchuan county, with the scarp on north-western hanging wall being just opposite to the Minjiang river. On the wall of this fault scarp straight smooth frictional mirror surface and striations with 57deg;plunge-angle can be seen, indicating a displacement with dextral strike-slip component. Here we would query whether these fracture surfaces were formed solely by gravitational slide or probably caused by co-seismic displacement along Wenchuan-Maoxian fault during the Wenchuan earthquake. Note that, among 11 investigated sites, there are 6 sites with land scarps, among which two surface fractures are located in fault trough on mountain tops. This shows that the surface fractures on mountain tops during Wenchuan earthquake could have been formed by tectonic activity. Besides, surface dislocation profiles on 5 sites are described in the paper. The three main fault belts in Longmenshan mountain have distinct dislocated geomorphology during Late Quaternary. We think it shows that the three fault belts did not move following the frontward style of Mesozoic tectonic motion, but were synchronal activity during the same events of Late Quaternary with different active intensity. The distribution of small-displacement of surface fractures along the Longmengshan rear-range formed by Wenchuan earthquake is probably the reflection of such synchronal activity. Investigation in this paper would be helpful in calling for more attention to surface fracture survey along the rear-range of Longmenshan mountain during Wenchuan earthquake, facilitating an overall assessment of the Longmengshan tectonic zone activity during the Wenchuan earthquake.