Abstract:

The Longling-Lancang seismotectonic belt in southwestern Yunnan is critical for accurately defining the boundaries of active blocks and evaluating seismic risks. Using pre- and post-earthquake high-spatial-resolution satellite imagery to study strong earthquakes retrospectively proves to be a practical method in such study. Strong earthquakes frequently cause secondary effects such as coseismic landslides, collapses, and debris flows, which lead to considerable loss of life and property. These secondary effects, often as the most dramatic manifestations of an earthquake, show geologic signatures providing evidence of historic or prehistoric seismic activities. The use of satellite imagery captured shortly after historic earthquakes to interpret these secondary effects is particularly beneficial in determining the intensity and influence radius of earthquakes, thereby helping study on seismogenic faults of earthquakes.

On May 29, 1976, two strong earthquakes with M_S7.3 and M_S7.4 occurred in Longling county, southwest China, followed by intense aftershocks. The seismogenic structure of these earthquakes still remains undetermined to present. These earthquakes triggered numerous coseismic landslides in the regolith of the granitic rock mass. The seismic zone, located in subtropical regions, is characterized by high precipitation and dense vegetation. Apart from the ancient landslides in the northwest and southeast, no records of landslides and debris flows persisted in the epicenter zone for a century, making the occurrence of substantial landslides post the main earthquakes unexpected. Currently, these landslides have undergone reshaping by land surface processes and re-vegetation, which makes them indistinguishable in recent remote sensing images. Using Keyhole satellite images with a resolution of 0.6~1.2m offers a useful means to identify the coseismic landslides of the Longling mainshocks. In this study, we employ these images for a comprehensive visual interpretation of the coseismic landslides. To ensure the accuracy and reliability of the results, we used images captured in 1981(the most recent following the earthquakes)to extract coseismic landslides and substantiated them with images from 1974, field investigation photos from 1976, and relevant records. Finally, we have compiled an exhaustive database of coseismic landslides triggered by the 1976 Longling cases.

Our results are summarized as follows: 1)A total of 14 448 landslides were interpreted, encompassing an overall area of 17.2km². The area of individual landslides primarily ranged from several hundreds to one thousand m², and most were superficial slides in the surface regolith with short sliding distances. The regional stratigraphy is complex, with 70.9% of the landslides occurring in the regolith of granitic rock mass, 15.3%in sandstones or siltstones, and a mere 13.8%in other areas such as limestones. Consequently, these landslides were relatively small compared to those in other regions like the Loess Plateau in north China, where the surface sediment is extremely loose. 2)A strong correlation exists between the intense area of coseismic landslides and the earthquake sequence, which tends to migrate from south to north. Notable aftershocks(e.g., M_S6.2 on June 9 and M_S6.6 on July 21)particularly exhibited the general NNW distribution direction of the earthquake sequence and triggered scattered landslides outside the epicenter zone. Through synthesizing field surveys, combining other records and the findings of this study, we believed that the two main earthquakes triggered numerous coseismic landslides, and the continuous strong aftershocks led to the destabilized regolith of the granitic rock mass creeping successively, resulting in subsequent landslides. 3)The concentration areas of coseismic landslides do not match the high-earthquake-intensity areas, instead, they are all located on one side of active faults, which suggests that the seismogenic fault is neither the Longling-Ruili Fault nor the Wanding Fault. The spatial distribution of the landslides suggests that the scope of the surface rupture zone is about 30km. The conjugated strong earthquake ruptures in southwestern Yunnan may limit the spatial scale of single strong earthquakes, so it is crucial to pay more attention to the intersection zone of NE and NW trending active faults when assessing regional strong earthquake risk in the future.

Key words: southwestern Yunnan, 1976 Longling M_S7.3, M_S7.4 earthquakes, earthquake-triggered landslides, Keyhole satellite imagery, remote sensing interpretation, seismic hazard risk

摘要：

深入研究中国滇西南地区发育的龙陵-澜沧新生地震构造带对于合理划分活动地块边界及未来开展强震危险性评价工作具有重要意义。使用高分辨率卫星影像对已发生的双强震震例开展回溯研究是一种可行的技术手段。文中对震前、 震后高分辨率Keyhole卫星影像进行遥感解译, 结合野外验证和前人研究结果, 获得了1976年 M_S7.3、 M_S7.4 龙陵双强震触发滑坡较为完整的数据库。结果显示: 1)共解译滑坡14 448个, 总面积为17.2km², 70.9%的滑坡分布在花岗岩质的岩体风化层中, 单体滑坡面积集中在数百至1 000m²区间, 多为表层风化层内的浅层滑坡, 滑动距离较小; 2)同震滑坡密集区与高烈度区不匹配, 其空间分布显示破裂区规模约为30km, 且均位于活动断裂的一侧, 表明其发震构造并非为龙陵-瑞丽断裂或畹町断裂。滇西南地区共轭强震破裂可能局限了单次强震的空间规模, 因此在确认未来强震危险区时应重点关注NE向与NW向构造的交会部位。

关键词: 滇西南地区, 1976年龙陵M_S7.3、, M_S7.4双强震, 地震滑坡, Keyhole卫星影像, 遥感解译, 地震危险性