Yongkang Ran(冉勇康),Xiwei Xu, Wenshan Chen,L ichun Chen,Shaopen Dongand Hu Wang

1)National Centre of Active Fault Studies,Institute of Geology,CEA,Beijing 100029 2)Department of Geosciences,Taiw an University,Taipei 106

The activity of the Longmenshan thrust belt has concentrated mainly on its Wenchuan-Maoxian Fault,Yingxiu-Beichuan Fault,and Guanxian-Jiangyou fault since the Quaternary time.Results of field investigation show that the length of surface ruptures of the Wenchuan earthquake along the Yingxiu-Beichuan fault and Guanxian-Jiangyou fault are more than 250 km and 72 km respectively.During the historical records(beginning at about 2300 aBP),no earthquake of magnitude comparable with that of the Wenchuan MW7.9 earthquake has ever been reported in Chengdu and Longmenshan region.

We excavated a trench at the north of Yingxiu Tow n on the second terrace(T2)beside the Minjiang River.The trench log revealed as:the medium-fine sand on the upper part of the T2,was cumulatively vertical offset of 7.4±0.4 m and about three times larger compared to the coseismic displacement(2.4 m)on the T1.Among the sedimentation unit before scarp,a chaotic sedimentation unit,developed after the sand unit and w as offset,but the fault tip was covered by slope wash and man-made accumulation,so we infer that the scarp on the T2 was generated from three events.Two detrital charcoal samples collected from the chaotic sedimentation unit and the sand unit yielded ages of 3360～3160 aBP and 5 920～5730 aBP respectively. We conclude that one event occurred before the historic record and after 3160 aBP,and another earlier seismic event occurred after 5730 aBP.

The coseismic vertical displacement of scarps accompanying the Wenchuan earthquake is about 2.1 m on the T1 at Leigu Tow n,while a cumulative vertical displacement is about 4 m on the T2.The trench log on the T2 revealed as:a gravel unit and an above orange sand-clay unit were vertical offset of about 4 m.On the foot wall there are younger strata.Presently,sample from OSL dating yielded ages of 3.3±0.3 kaBP;two detrital charcoal samp les collected from the younger strata dating yielded ages of 1112±27 aBP,we initially conclude that the former event occurred between 3.3±0.3 kaBP and 1112aBP.

We find a grey-black sand clay unit thatmay be a barrier lake depositon in the trench excavated on the T2 at Guixi Tow n.Above the sand clay unit,the upper strata deform with a vertical displacement of 2.2 m,however the vertical deformation is 3.5 m on the lower strata,which show s that two seismic eventsoccurred after deposition of a barrier lake.If formation of a barrier lake supposed to be generated from another earlier seismic event,there would be three events revealed in the trench.The trunk in the barrier lake unit was yielded ages of 8726～8738 aBP,three seismic events occurred after this age.

We conclude that there are three events revealed in trenches along the Yingxiu-Beichuan Fault,the earliest event occurred after 5730 aBP,and the one before the Wenchuan earthquake occurred at about 2600 aBP.The average recurrence interval of events is 2800±200 a.