Abstract: With the occurrence of the 12May 2008 Wenchuan,Sichuan,China earthquake,two high-relief surface ruptures were formed on the Yingxiu-Shikan segment of the range-front fault and the Bailu-Hanwang segment of central fault along the Longmenshan Fault zone,and around the faults almost all buildings were razed.Furthermore,surface ruptures on the central fault broke through the accustomed activity segmentation boundary and extended to the northern segment by about 60km.Aftershocks also have a trend of migrating to the northern segment from the middle segment of Longmenshan Fault zone.It would be interesting to know as what kinds of effects this giant earthquake has on the earth's surface along the northern segment of Longmenshan Fault zone,and whether there have been earthquakes occurring since late Quaternary? In order to accumulate more reliable and detailed data and have a primary understanding to solve these questions and for post-quake reconstruction,we made a detailed geological and geomorphologic survey of the co-seismic earth's surface transformation and the late Quaternary Fault activity along the northern segment of the range-front fault on the basis of researches by predecessors.And then we excavated trenches at two relatively heavily-damaged and suspicious sites with legible linear shadows on image to investigate whether there exists active fault or fold.Our observation suggests that the northern segment is quite distinct in geology and geomorphology,and the coseismic earth's surface transformation is different from that in the middle segment of front-range fault and there is no clear trace to indicate that it is a late Quaternary active fault.So the late Quaternary active segment of range-front fault may terminate nearby the south of the Yong'an town.Our observation also suggests that the so-called active fault scarp as argued by some researchers near the Yong'an village is in fact an eroded river bank.

Key words: Wenchuan earthquake, Longmenshan, active fault

摘要： 5月12日汶川8.0级地震沿龙门山断裂带中央断裂映秀—石坎段、前山断裂白鹿—汉旺段形成了典型的逆断层-褶皱地震地表形变带,两侧构筑物遭受了毁灭性的破坏。中央断裂地震地表形变带突破了以往所认识的断裂活动分段边界,向北扩展了约60km,余震亦具有从中段向北段迁移的趋势。龙门山断裂带北段在此次地震中地表有什么影响或破坏?该段晚第四纪是否有过地震活动?在前人工作的基础上,我们对前山断裂北段的地震地表特征和晚第四纪活动性进行了详细的地质地貌调查,并重点选择2个影像线性特征清晰、震害较强烈的疑似地点进行了探槽揭露,以期为解决这些问题以及灾后重建积累翔实可靠的基础资料及获得相应的初步认识。主要结论是:前山断裂北段地质地貌、构造、5月12日汶川8.0级地震的地表表现等与其南侧的灌县-安县断裂(中段)均存在显著差异,晚第四纪活动迹象不明显,前山断裂晚第四纪活动段可能终止在永安镇往南一带;永安镇一带前人认为的"活动断裂陡坎"应为侵蚀河岸。

关键词: 汶川地震, 龙门山, 活动断裂