甘肃北山旧井地区活动断裂的形成及其地震危险性评价

The active Alytn fault zone is a world-famous great sinistral wrench structure. It extends from Gozhaco in the northern part of Tibet eastward to Kuantanshan Mt in the western part of Gansu Province in a total length of 1600km. The fault zone consists of a series of faults and stretches as a straight-line structure in NEE direction. It represents a natural boundary of northwestern part of Qinghai-Tibet Plateau, strikingly cuts Kunlun Mts and separates Tarim basin from Qaidam Basin, and bisects the Qilian Mts. The fault zone occupies an important position in the tectonic evolution of north China. Thus, research of the active Alytn fault zone is significant for understanding its formation mechanism, genesis of earthquakes, intraplate kinematics and resent regional geodynamics and hence it attracted great attention of the world geoscientists..Altyn fault zone composes of Alytn fault, Jianggasaiyi-Hongliugou fault, Sanweishan fault and Che’erchen fault from south to north. According to historical records, many times over seven strong earthquakes has occurred along the Altyn fault zone. Paleoseismic studies suggest that many times 7.5-8.0 magnitude earthquake events has occurred Along the Altyn fault. Che'erchen fault is a most northern fault in ALytn fault zone, is also a buried fault. Sanweishan fault is located in the north 60km, and parallel to the Altyn fault. In the vicinity of Dunhuang City – Guazhou, Sanweishan fault is linear distribution along the north Piedmont of Sanweishan Mts. .By the structure and geomorphological interpretation of high-precision satellite remote sensing data, aerial remote sensing data, we can determine that the northern tip of Sanweishan fault cut through the Hexi Corridor, and into the long-term stability block of Beishan. At the clear positions of active fault geomorphological performance, horizontal displacement and vertical displacement can be measured with differential GPS. Across the active fault scarps, we dig some trenches for revealing the directing evidence of active fault. Some dating samples(OSL、14C ) will be collected from some stratum profiles, and determinate the deposition age of samples. Analysis to determine the formation times for related terraces, alluvial fan, we can determine formation era of Jiujing fault system and its related basin. Then, we can calculate the rates at different parts of Sanweishan fault, and determine the spatial variation of slip rate. Using the fault dislocation model, we written calculation program in Visual Fortran platform, calculate the surface deformation field, and confirm the relationship between Sanweishan fault with Jiujing fault system.

通过对高精度的卫星遥感数据、航空遥感数据的活动断裂和构造地貌解释，确定三危山活动断裂（阿尔金断裂带中的分支断裂）东北端切穿河西走廊，进入长期稳定的甘肃北山地块。选择活动断裂构造地貌表现清晰的位置，用差分GPS实测断错地貌标志的水平位移和垂直位移。横跨活动断层地貌陡坎，揭露出断层活动的直接证据和新鲜的地层剖面。在地层剖面上采集释光（OSL）、碳十四（14C）测年样品，实验室测定样品的沉积年代。分析确定相关阶地面、洪积扇面的形成时代，确定旧井等断层等活动参数和古地震序列,以及相关盆地的形成时代。计算三危山断裂及其东北端分支断裂上的活动速率，分析断层活动速率的空间变化和变形的转化。采用断裂位错模型编写位错计算程序计算地表变形场,分析三危山活动断裂与旧井活动断裂系的关系。确定阿尔金断裂带向北、向东方向的扩展影响范围和活动强度，评价北山旧井地区活动断层的地震危险性。

通过对高精度的卫星遥感数据、航空遥感数据的活动断裂和地貌解释，获得了三危山断裂、旧井断裂的几何结构和剖面分布。在活动断裂构造地貌表现清晰的位置，用差分GPS、无人机移动测量实测断错地貌标志的水平位移和垂直位移。在开挖的新鲜地层剖面上采集释光（OSL）、碳十四（14C）、采集宇宙成因核素（10Be）测年样品，实验室测定样品的沉积年代和核素含量，获得约32个沉积物的沉积年代数据和多个地貌面的剥露年龄。在旧井盆地完成了3个揭穿盆地沉积地层钻孔（合计钻探进尺96m）、1条长约3km 的地质雷达探测剖面，认为旧井盆地为一个半地堑性的掀斜盆地。根据埋藏宇宙成因核素（10Be）样品年代，认为旧井盆地开始接受沉积的年代为距今约5.5Ma左右。通过断错地貌的年龄和位移量，计算得到三危山断裂、旧井断裂的左旋滑动速率分别为0.06-1.25mm/a和~0.03mm/a。通过探槽分析和沉积物测年，在三危山断裂上晚更新世以来的3次古地震事件，在旧井断裂上获得4次古地震事件。.通过对北山地块东南缘北河湾断裂的构造活动性和深部构造特征研究，获得了断裂的第四纪运动学特征和深部电性结构。北河湾断裂是一个全新世左旋走滑断裂，局部有逆冲或正断分量。断裂晚更新世左旋走滑和垂直逆冲速率分别为~2.69mm/a和~0.35mm/a。跨北河湾断裂的大地电磁剖面揭示断裂深部为近垂直的低阻带，向下延伸到下地壳。综合区域地质和地球物理资料，认为北河湾断裂与南部的阿尔金断裂系和祁连山逆冲体系构造上不相连，青藏高原向北扩展的前缘位于金塔盆地南缘。.旧井断裂几何学和运动学特征及其与东西向走滑断裂带的构造关系表明，旧井断裂是正在发展的走滑转换双重构造的一部分，这些北东向的断层在阶区内吸收走滑位移和正断分量。红旗山断裂、金庙沟断裂和旧井断裂系在北山南部形成了独立的变形带，没有表面地质证据表明它们与南部三危山或河西走廊其他变形带，或北侧的马鬃山变形带有运动学上的关联。北山稳定地块活化机制以激活现存断裂构造为主，局部发育新生断层为辅。