龙门山中央断裂南段盐井—五龙断裂的浅层地球物理方法探测

在2008年5月12日汶川MS8.0地震和2013年4月20日芦山MS7.0地震中,龙门山中央断裂南段的盐井—五龙断裂均未发现地表破裂现象,加之该断裂浅层地球物理资料极度匮乏,在一定程度上限制了对龙门山断裂带南段地震危险性的评价和发震能力的评估。针对龙门山中央断裂南段的盐井-五龙断裂经过区段的主要乡(镇)所在地多为宽度不大于300m的山间峡谷地区,且探测场区存在交通条件不便、场地工作面狭窄等问题,在浅层地震反射波法探测工作中采用小道间距、小偏移距、多道短排列接收和共反射点多次覆盖观测的地震数据采集方式,并经数据处理后获得地震反射剖面图像。浅层地震探测定位结果结合高密度电阻率成像断面、探槽开挖和钻孔联合剖面资料,共同揭示了NE向的盐井—五龙断裂在宝兴县五龙乡东风村一带精确的空间展布位置、产状规模和近地表构造形态。探测结果表明盐井—五龙断裂于五龙乡北东风村西河两岸的T1阶地处隐伏通过,性质为倾向NW的逆冲断裂,近地表倾角50°~60°,上断点埋深19m。该断裂断错宝兴西河T2、T3阶地,西河右岸T1阶地断裂通过处两侧基岩的断距6~8 m,其破碎带及其影响带宽度约30m。本文浅层地球物理探测成果可对判定盐井—五龙断裂的近地表构造活动提供可靠的地震学证据,也为地震重灾区(宝兴县城)的灾后工程选址重建、地震危险性评价和制定抗震防灾规划提供了科学的基础资料。

Detection of the Yanjing-Wulong Fault in the Southern Segment of the Longmenshan Central Fault Zone by Using Shallow Geophysical Method

During the Wenchuan M_S8.0 earthquake on May 12, 2008 and the Lushan M_S7.0 earthquake on April 20, 2013, surface rupture phenomena did not occur on the Yanjing-Wulong fault in the southern segment of the Longmenshan central fault zone. Moreover, shallow geophysical data for the Yanjing-Wulong fault are extremely scarce. The seismic hazard and the capacity for potential earthquakes were evaluated in the southern section of the Longmenshan fault zone to a certain extent. However, the major counties (towns) crossed by the Yanjing-Wulong fault are mostly located in valleys with widths less than 300 m. As a result, such detection areas are subject to inconvenient traffic conditions and narrow working spaces. Therefore, detection through shallow seismic reflection must be applied with small trail spacing, small displacement distance, reception of multiple short arrangements, multiple coverage observations of common reflection points, and data processing. In the present study, shallow seismic reflection is combined with high-density resistivity tomography, trench excavation, and drilling to reveal the size, spatial distribution, and near-surface structure of the NE-trending Yanjing-Wulong fault in the area of Dongfeng village, Wulong, Baoxing County. These detection results not only provide reliable seismological evidence for determining the near-surface activity of the Yanjing-Wulong fault but also offer scientific fundamental data for selecting sites for post-disaster reconstruction, earthquake risk assessment, and planning of earthquake resistance and hazardous prevention in the Baoxing County.