鲜水河断裂带乾宁段晚第四纪走滑速率及区域强震危险性研究

Late Quaternary slip rate and earthquake hazard along the Qianning segment,Xianshuihe fault

活动断裂几何学特征及滑动速率是研究断裂运动学、动力学机制及其评估区域强震危险性的重要依据。青藏高原东缘左行走滑的鲜水河断裂带是控制高原物质向南东挤出的重要边界,是中国陆内活动性最强的断裂之一。本文以鲜水河断裂带北西段为研究对象,通过高精度遥感影像解译、野外考察、OSL(光释光)和^(14)C测年方法以及LiDAR(激光雷达)扫描获得乾宁段龙灯乡冲积阶地的位错量和废弃年龄。T4和T3′水平位错量分别为106±5 m和77±2 m,T4阶地垂直位错量为9.6±0.5 m。T4和T3′阶地的废弃年龄分别为11±1 ka和7±1 ka。结合对应的年龄和位错量,得到乾宁段晚第四纪走滑速率左行走滑速率为10.5±1 mm/a,垂直滑动速率为0.9±0.1 mm/a,断层倾向北东,具有正断运动学特征。通过重新计算断裂两侧GPS矢量沿断裂方向分量,得到鲜水河断裂带炉霍段、炉霍—康定段、磨西段现今左行走滑速率分别约为8.1 mm/a、8.2 mm/a、9.4 mm/a,整体表现为自北西向南东递增。综合乾宁段晚第四纪走滑速率和最新强震活动的离逝时间估算,认为鲜水河断裂带乾宁段目前应变累积达到了发生一次M_(W) 6.8(M_(S) 7.2)大地震的潜能,在区域防震减灾工作中应对此加以重视。

Determining the geometric characteristic and slip rate alongactive faults is essential to understand fault dynamic mechanism and evaluate regional earthquake hazard.The left-lateral strike-slip Xianshuihe fault in eastern Tibet is one of the most active intra-continental faults in China,acting as a significant boundary fault controlling eastward material extrusion from Tibet.Here,we determined the offsets and abandonment ages of Longdeng terraces on the Qianning segment,NW Xianshuihe fault,using tectonic-geomorphology approaches(high-resolution remote sensing images and field investigation,LiDAR(Light detection and ranging) with OSL(Optically stimulated luminescence) and ^(14)C dating.Yielded the abandonment ages of 11±1 and 7±1 ka for T4 and T3′,respectively.Horizontal offsets measured on the high-resolution DEM(Digital elevation model) obtained from our LiDAR surveys are 106±5 and 77±2 m for T4/T3′ and T3′/T1,respectively,and the vertical offset of T4 is 9.6±0.5 m.Combining the terrace ages with their offsets,we obtain an average late Quaternary horizontal slip-rate of 10.5±1 mm/a,and a throw rate of 0.9±0.1 mm/a.In addition,by recalculating the fault parallel component of GPS vectors on both sides of the fault,we derive a horizontal rate along the Luhuo,Luhuo-Kangding,and Moxi segments of 8.1,8.2,and 9.4 mm/a,respectively,suggesting that the slip rate along the Xianshuihe fault currently increases from NW to SE.Combining the slip-rate we determined along the Qianning segment with empirical equations of maximum offsets and earthquake magnitude,we suggest a seismic risk of a M_(W) 6.8(M_(S) 7.2) earthquake in the near future along that segment of the Xianshuihe fault,which should be taken seriously in the earthquake disaster mitigation.