Abstract:

Historical records with time information are useful for determining the time of earthquake events, while the investigation of historical damage phenomena such as earthquake-triggered landslides can help determine the magnitude of historical earthquakes by analyzing the correlation among historical earthquake-caused landslides, historical earthquakes and related active faults. A series of small basins were developed along the southern segment of the Xiaojiang Fault(XJF), with relatively flat and open topography and concentrated human activities. In most of the southern segment of the XJF, the terrain is relative flat, but some landslide accumulations are still clear, which are obviously different from the surrounding settings and are easy to be identified. Based on remote sensing interpretation and field investigations, landslides with different scales have developed in more than 10 locations along the southern segment of the XJF. Some of them are large with a volume of more than 1 million m³, and some are small with a volume of less than 100 000m³. They are the ancient landslides with a stable state. These landslides are mainly distributed in basins and their border areas with gentle terrain slopes. They are likely to be earthquake landslides rather than rainfall induced. The main scarp angles of these landslides are relatively concentrated, most of which are between 29~31 degrees, indicating that these landslides are caused by one geological event. We use light detection and ranging(LiDAR) measurement technology to obtain the digital elevation model(DEM)data of the landslide development section. The generated three-dimensional topographic shadow map presented in this paper suggests that there is a close relationship between these landslides and the latest surface ruptures of the southern segment of the XJF, indicating that these landslides should be triggered by the latest seismic event along the southern segment of XJF. The fault section was faulted in the latest earthquake events on the surface, triggering clusters of landslides. Based on the age test results of samples from the trench on the landslide body and historical literature data, the co-seismic landslides were triggered in 1606AD. According to the latest research results of the earthquake surface rupture zone in the southern segment of the XJF and empirical formula, combined with the comparative analysis on the intensity of geological disasters and the number of casualties of different earthquake cases, the authors re-assess the magnitude of the 1606 Jianshui earthquake and find that the magnitude of this historical earthquake could not be less than 7½(≥7.5). It means that the southern segment of the XJF, as a part of Xianshuihe-Xiaojiang fault(XSH-XJF) system, shows strong activity and has the ability to generate large earthquakes. GPS observations have verified that the crustal material on the southeastern margin of the Tibetan plateau rotates clockwise around the Eastern Himalaya Syntaxis(EHS), which requires a continuous left-lateral strike-slip fault system as the eastern boundary. The results presented in this paper are useful for deeper study of such an eastern boundary.

Key words: ancient landslides, southern segment of Xiaojiang Fault, 1606 Jianshui earthquake, seismogenic structure

摘要：

历史地震资料在研究地震活动性和评价地震危险性方面具有重要作用, 沿活动断裂丛集状发育的地震滑坡可为识别和复核历史地震提供重要的线索。通过遥感影像和野外地质调查发现, 沿小江断裂南段至少有10个地点发育滑坡, 规模大小不一, 既有体积>100万立方米的大型滑坡, 也有体积<10万立方米小型滑坡, 但均为目前处于稳定状态的历史古滑坡。这些滑坡主要分布在地形坡度较缓的盆地及其边界地区, 很可能为地震滑坡, 而非由降雨诱发。滑坡后缘陡坎的角度基本集中在29°~31°之间, 表明其应该为1次地震事件的结果。基于LiDAR测量建立滑坡发育段落的数字高程模型(DEM), 在生成的三维地形阴影图上清晰地揭示了滑坡体与小江断裂南段最新地表破裂带的密切关系, 该断裂段最晚一次断错地表的地震事件触发了丛集状分布的滑坡。综合经验估算结果、 滑坡体探槽中年代样品¹⁴C的测试结果和历史文献资料, 地震滑坡的发生时间可以推断为公元1606年。对这些历史地震滑坡的认识一方面为将小江断裂南段确定为该地震的发震构造提供有力的证据; 另一方面也为重新评价地震震级提供新的切入点。根据小江断裂南段地震地表破裂带的最新研究成果及经验关系式, 同时结合不同震例的地质灾害强度以及人员伤亡数的对比分析, 对1606年建水地震震级的复核结果表明: 其震级很可能≥7½级(即≥7.5级)。鲜水河-小江断裂系的强烈活动和发生大地震的能力至少可一直延续到小江断裂南段。目前由GPS观测资料所证实的青藏高原东南缘地壳物质绕东喜马拉雅构造结(EHS)的顺时针转动需要一个连续的左旋走滑断裂系统作为东部边界, 文中的工作和认识可促进对该东部边界更深入的研究。

关键词: 历史古滑坡, 小江断裂南段, 1606年建水地震, 发震构造