龙门山断裂带映秀-北川断裂的古地震及其复发间隔初步研究
- 出版年：2010
- 作者：Yongkang Ran(冉勇康)；Xiwei Xu；Wenshan Chen；Lichun Chen；Shaopen Dong；Hu Wang
- 单位1：National Centre of Active Fault Studies
- 语种：中文
- 起始页：28
- 总页数：2
- 刊名：国际地震动态
- 是否内版：否
- 刊频：月刊
- 创刊时间：1971
- 主管单位：中国科学技术协会
- 主办单位：中国地震学会；中国地震局地球物理研究所
- 主编：许绍燮
- 地址：北京市海淀区民族大学南路5号
- 邮编：100081
- 电子信箱：rdws@cea-igp.ac.cn
- 网址：http://www.gjdzdt.org.cn
- 期：6
- 期刊索取号：P263.06 627-3
- 数据库收录：中国核心期刊（遴选）数据库收录期刊；中国学术期刊综合评价数据库统计源期刊；中国期刊全文数据库（CJFD）全文收录期刊；中文科技期刊数据库原文收录期刊
- 核心期刊：中国核心期刊（遴选）数据库收录期刊
摘要
The activity of the Longmenshan thrust belt has concentrated mainly on itsWenchuan-Maoxian Fault, Yingxiu-Beichuan Fault, and Guanxian-Jiangyou fault since theQuaternary time. Results of field investigation show that the length of surface ruptures ofthe Wenchuan earthquake along the Yingxiu-Beichuan fault and Guanxian-Jiangyou fault aremore than 250 km and 72 km respectively. During the historical records (beginning at about2 300 aBP) , no earthquake of magnitude comparable with that of the Wenchuan Mw7. 9earthquake has ever been reported in Chengdu and Longmenshan region.
We excavated a trench at the north of Yingxiu Town on the second terrace (T2) besidethe Minjiang River. The trench log revealed as: the medium-fine sand on the upper part ofthe T2, was cumulatively vertical offset of 7. 4±0. 4 m and about three times larger com-pared to the coseismic displacement (2. 4 m) on the T1. Among the sedimentation unit be-fore scarp, a chaotic sedimentation unit, developed after the sand unit and was offset, butthe fault tip was covered by slope wash and man-made accumulation, so we infer that thescarp on the T2 was generated from three events. Two detrital charcoal samples collectedfrom the chaotic sedimentation unit and the sand unit yielded ages of 3 360 3 160 aBP and 5920～5730 aBP respectively. We conclude that one event occurred before the historic recordand after 3 160 aBP, and another earlier seismic event occurred after 5 730 aBP.
The coseismic vertical displacement of scarps accompanying the Wenchuan earthquake isabout 2. 1 m on the Tl at Leigu Town, while a cumulative vertical displacement is about 4 mon the T2. The trench log on the T2 revealed as: a gravel unit and an above orange sand-clayunit were vertical offset of about 4 m. On the footwall there are younger strata. Presently,sample from OSL dating yielded ages of 3. 3±0. 3 kaBP；two detrital charcoal samples col-lected from the younger strata dating yielded ages of 1112±27 aBP, we initially concludethat the former event occurred between 3. 3 ± 0. 3 kaBP and 1 112aBP.
We find a grey-black sand clay unit that may be a barrier lake depositon in the trench ex-cavated on the T2 at Guixi Town. Above the sand clay unit, the upper strata deform with avertical displacement of 2. 2 m, however the vertical deformation is 3. 5 m on the lower stra-ta, which shows that two seismic events occurred after deposition of a barrier lake. If forma-tion of a barrier lake supposed to be generated from another earlier seismic event, therewould be three events revealed in the trench. The trunk in the barrier lake unit was yieldedages of 8726～8 738 aBP, three seismic events occurred after this age.
We conclude that there are three events revealed in trenches along the Yingxiu-BeichuanFault, the earliest event occurred after 5 730 aBP, and the one before the Wenchuan earth-quake occurred at about 2 600 aBP. The average recurrence interval of events is 2 800±200 a.
We excavated a trench at the north of Yingxiu Town on the second terrace (T2) besidethe Minjiang River. The trench log revealed as: the medium-fine sand on the upper part ofthe T2, was cumulatively vertical offset of 7. 4±0. 4 m and about three times larger com-pared to the coseismic displacement (2. 4 m) on the T1. Among the sedimentation unit be-fore scarp, a chaotic sedimentation unit, developed after the sand unit and was offset, butthe fault tip was covered by slope wash and man-made accumulation, so we infer that thescarp on the T2 was generated from three events. Two detrital charcoal samples collectedfrom the chaotic sedimentation unit and the sand unit yielded ages of 3 360 3 160 aBP and 5920～5730 aBP respectively. We conclude that one event occurred before the historic recordand after 3 160 aBP, and another earlier seismic event occurred after 5 730 aBP.
The coseismic vertical displacement of scarps accompanying the Wenchuan earthquake isabout 2. 1 m on the Tl at Leigu Town, while a cumulative vertical displacement is about 4 mon the T2. The trench log on the T2 revealed as: a gravel unit and an above orange sand-clayunit were vertical offset of about 4 m. On the footwall there are younger strata. Presently,sample from OSL dating yielded ages of 3. 3±0. 3 kaBP；two detrital charcoal samples col-lected from the younger strata dating yielded ages of 1112±27 aBP, we initially concludethat the former event occurred between 3. 3 ± 0. 3 kaBP and 1 112aBP.
We find a grey-black sand clay unit that may be a barrier lake depositon in the trench ex-cavated on the T2 at Guixi Town. Above the sand clay unit, the upper strata deform with avertical displacement of 2. 2 m, however the vertical deformation is 3. 5 m on the lower stra-ta, which shows that two seismic events occurred after deposition of a barrier lake. If forma-tion of a barrier lake supposed to be generated from another earlier seismic event, therewould be three events revealed in the trench. The trunk in the barrier lake unit was yieldedages of 8726～8 738 aBP, three seismic events occurred after this age.
We conclude that there are three events revealed in trenches along the Yingxiu-BeichuanFault, the earliest event occurred after 5 730 aBP, and the one before the Wenchuan earth-quake occurred at about 2 600 aBP. The average recurrence interval of events is 2 800±200 a.