海原断裂带老虎山段晚第四纪滑动速率精确厘定与讨论

Late Quaternary slip rate of the Laohushan fault within the Haiyuan fault zone and its tectonic implications

如何准确测定断裂滑动速率是近年来活动构造研究的前沿与热点.随着高精度地形数据获取手段与第四纪测年方法的不断进步,位错量和地貌面年龄的精度均得到大大提高.在进行滑动速率计算时还要考虑地质过程是否合理,蒙特卡洛方法为获取更加符合地质过程的滑动速率提供了重要工具.本文以滑动速率研究程度较低的海原断裂带老虎山段为例,基于LiDAR高精度地形数据,测得T1—T4阶地面年龄分别为1~3ka,9~11ka,15~17ka,40~45ka,陡坎前缘的位错分别为7~14m,28~36m,59~66m,180~190m.综合多地点的左旋走滑位错量及不同时代的地貌面年龄数据,并考虑滑动历史,利用蒙特卡洛模拟方法,将位错-时间两个参数的不确定性定量化,限定老虎山断裂45ka以来平均滑动速率为4.3±0.16mm·a-1,17ka以来的平均滑动速率为4.0±0.15mm·a-1,与前人研究得到的狭义海原断裂滑动速率4.5±1.0mm·a-1基本一致.综合整个海原断裂带滑动速率,本文结果更支持低滑动速率变化趋势,即海原断裂带整体滑动速率趋于稳定,向东至六盘山断裂,滑动速率开始降低,推测海原断裂带的左旋走滑在尾端主要为马东山—六盘山隆起所吸收.结合老虎山断裂历史地震资料和深部锁闭浅部蠕滑的动力学特征,推测老虎山断裂具备与相邻断裂一起触发强震的能力.

How to precisely determine the slip rate is a central topic in active tectonics research.The resolution of offset and dating are greatly improved with the development of high-resolution topographic surveys and Quaternary dating methods.In fault slip rate evaluation,a reliable fault slip rate is derived based on a fault motion under reasonable geological process.Monte Carlo analysis is a method that enables slip rate determination while statistically incorporating displacement and uncertainties.In this study,we target the Laohushan fault within the Haiyuan fault zone.We used 9 dated offsets at three sites along a 25 km segment to derive the average fault slip rate.The ages of T1—T4 terraces were 1~3 ka,9~11 ka,15~17 ka,40~44 ka and the offsets were 7~14 m,28~36 m,59~66 m,180~190 m.The fault slip rate was obtained to be4.3±0.16 mm·a-1 and 4.0±0.15 mm·a-1 during the past 45 ka and 17 ka,respectively,based on high-resolution LiDAR topographic data and Monte Carlo analysis method.The slip rate of Laohushan fault is comparable with that of the Haiyuan fault.By synthesis of previous results,we find that the fault slip rate is uniform along the main WNW-ESE trending Haiyuan fault,which decreases eastward towards the N-S trending Liupanshan fault.Hence,we propose that the Madongshan and Liupanshan is the main uplifts that absorbing the left-lateral motion along the Haiyuan fault at its Eastern termination.Based on the results we present here and previous observation of shallow creep and locking at depth,we propose that the Laohushan fault is capable of participating in strong earthquakes,comparable to the 1920 M8.5 Haiyuan earthquake.Hence,further investigations are urgently needed to study the paleoseismic history,recurrence interval,seismic hazard potential of the Laohushan fault.