以海原断裂甘肃老虎山段为例浅析 走滑断裂古地震记录的完整性——兼论古地震研究中的若干问题

On the completeness of paleoseismic records of strike-slip faults:an example from the Laohushan segment of the Haiyuan fault in Gansu,China,with a discussion of several problems in the paleoearthquake study

在简单介绍海原断裂老虎山段松山探槽点古地震工作的基础上,分析了制约走滑断裂上古地震记录完整性的因素。松山探槽点跨断裂开挖的2个探槽揭示颜色和粒度不同的地层互层的沉积序列呈近水平面状展布。在断裂带内地层发生错断、旋转和拖曳等变形现象。多个层位上的这种变形表明该点揭示了6次古地震事件。炭屑的^(14)C年龄限定了6次事件均发生在距今3500～3900年以来,其中最新的4次事件从老到新分别为0～410aAD、890～1000年之后不久、1440～1640 aAD和1990年景泰-天祝Mw5.8级历史地震。该序列及其他例子表明连续的、高沉积速率的微构造环境是走滑断裂上探槽选取的重要原则。

In this paper, taking for example the paleoseismic investigation of the Laohushan segment of the Haiyuan fault, the authors analyze the constraints on the completeness of paleoseismic records. Two fault-perpendicular trenches at the Songshan trench site have revealed that nearly horizontally-bedded sedimentary layers with alternating colors and grain sizes are offset, rotated and sheared. Such deformations in multiple stratigraphic horizons demonstrate the occurrence of six paleoseismic events. ^14C dating of charcoal from the trenches determines that all the six events occurred during the past 3500-3900 years, of which the youngest four events took place, from the oldest to youngest, during 0-410a A.D., shortly after 890-100Ga A.D., during 1440-1640a A.D. and in 1990 when the Jingtai-Tianzhu Mw=5.8 earthquake took place. This record and other examples suggest that the structural environment with few or no depositional hiatuses and high rates of sedimentation are the best sites for trenching in a strike-slip fault.