关键词: 汶川地震, 龙门山, 活动断裂

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