龙门山大川-双石断裂在都江堰隐伏段的综合探测研究

Comprehensive Survey and Study on the Buried Segment of the Dachuan-Shuangshi Fault in the Dujiangyan Area

摘要:

大川-双石断裂位于龙门山前山断裂南段，因其在都江堰地区隐伏于岷江河流阶地之下，在一定程度上影响了对龙门山断裂带南段的地震危险性评价和发震能力评估。因此，掌握大川-双石断裂的地下分布特征和活动性，对都江堰城区的避灾规划和重大工程选址意义重大。本项研究在野外地质调查基础上，垂直断裂走向布设浅层地震反射剖面、高密度电阻率剖面和钻孔联合剖面。通过多种方法共同揭示了大川-双石断裂在都江堰地区的精确空间展布位置、近地表构造形态、上断点埋深及断裂活动性。大川-双石断裂在都江堰地区的岷江西岸阶地处隐伏通过，为逆断层，倾向NW，倾角在地表浅层为60°~70°，近地面为20°~30°；且破碎带向地面逐渐变浅变薄，上断点向上延伸至第四系灰褐色黏土层和棕黄色粗砂夹砾石层，埋深约3.5 m。上述研究表明大川-双石断裂在都江堰市区附近具有全新世活动特征。

Abstract:

Dachuan-Shuangshi fault is located in the southern segment of the front-range of the Longmenshan fault, which is hidden under the terrace of Minjiang River in Dujiangyan area of Chengdu. To a certain extent, it affects the evaluation of both seismic risk and seismic capacity along the southern segment of the Longmenshan fault zone. Therefore, it is important to study the underground distribution characteristics and activity of Dachuan-Shuangshi fault in preparing disaster avoidance strategy and recovery planning. The effective disaster management planning helps in selecting sites for major projects in Dujiangyan City. Based on the field geological survey, shallow seismic reflection profile, high-density resistivity profile, and borehole joint profile are arranged along the vertical fault direction. Various methods are used to reveal the precise spatial distribution location, near-surface structural morphology, buried depth of upper breakpoint, and the activity of Dachuan-Shuangshi fault. This fault passes through the west bank terrace of Minjiang River in Dujiangyan area. It is a reverse fault inclined to NW with a dip angle of 60°-70° in the shallow layer of the surface and 20°-30° near the surface. The upper breakpoint extends upward to the Quaternary gray-brown clay and brown-yellow coarse sand-gravel beds with a burial depth of about 3.5 m. The study shows that the Dachuan-Shuangshi fault near the urban area of Dujiangyan has the Holocene activity characteristics.