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The Relics,Ages and Significance of Prehistoric Large Earthquakes in the Angang Graben in South Tibet

Wu Zhonghai;Ye Peisheng;Wang Chengmin;Zhang Keqi;Zhao Hua;Zheng Yonggang;Yin Jinhui;Li Huhou;Institute of Geomechanics,Chinese Academy of Geological Sciences;The Institute of Hydrogeology and Environmental Geology,CAGS;Institute of Geology,China Earthquake Administration;College of Resource Environment and Tourism,Capital Normal University;  
The Angang basin is a typical half-graben of the Nyemo graben group of the central Yadong-Gulu rift in south Tibet.Multiple-generation fault scarps,earthquake-induced large landslides,and colluvial wedges of fault scarps along the west margin boundary normal fault of the graben contain rich information of prehistoric earthquake events and are effective marks to identify prehistoric large earthquakes(M ≥7.0).Based on the measured vertical displacements of multiple-generation fault scarps and OSL and 14 Cdating results of 36 samples from river terraces and fault scarp profiles,it can be estimated that the vertical displacement rate of western main boundary fault zone of Angang graben is 0.8-1.3mm/a and at least 6large earth-quakes have occurred in the Angang graben since about 23-26 ka.According to the length and the maximum vertical displacement of earthquake fault,the magnitude of such large earthquakes should be in a range of Mw7.0-7.2.The newly study result on paleoearthquake show that the ages of the last two large earthquakes should be about 5.8±1.0ka and 2.4±0.2ka before presents respectively,and the average recurrence interval between two large earthquakes would roughly be 3.3-3.6ka in Angang,but the recurrence interval of large earthquakes in the whole Nyemo graben group should be short and inferred to be 1.0-1.2ka.Prominent normal faulting and frequent large earthquakes indicate that the large earthquake activity is consistent with clusterd-slip model and are in the seismic active period since about 23-26 ka in the Angang graben and Nyemo graben group.
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