Abstract: On the day Wu-wu of the second month in the second year of Chuyuan period in the reign of Emperor Yuan of the western Han Dynasty, that is, April 17, 47 BC, an earthquake occurred near the county seat of Huandao County, Longxi Prefecture. This earthquake caused serious damages to the city wall, government office buildings and civilian houses in the Huandao county seat, many people died, landslides, ground fracturing and spring gushing, etc. occurred. On the basis of textual research on the historical earthquake data and the field investigation, we affirm that the ancient Huandao county seat locates at the new Wangjia Village of Santai, southeast of Wenfeng Town in Longxi County at present. The ancient Huandao county seat is the most seriously damaged area according to the historical earthquake data, so it should be in the meizoseismal area of this earthquake. The epicenter intensity of this earthquake is about 9~10 degrees, and the magnitude of this earthquake is estimated to be about 7. Considering the intensities of other towns damaged during this earthquake, we draw the isoseismal lines of this earthquake, with its major axis directed NNW. The direction of the major axis of the isoseismal lines and the location of the epicenter are approximately consistent with the strike of the western segment of the Gangu-Wushan secondary fault on the northern margin of western Qinling fault zone. This fault segment has clear evidences of new activity during late Holocene, which are characterized by left-lateral strike-slip faulting with normal components; the fault dips NE and faulted the T₁-T₂ pluvial and alluvial terraces. Up to now, there are some surface deformation traces, such as deeply-incised seismic fault grooves, densely developed structural tensional fractures in the loess stratum, landslides and a series of beaded dolines etc. Combining the results of relocation of small to moderate earthquakes in the research area and comprehensive analysis on their distribution features in plane and profile, we get the results that the causative structure of the 47 BC Longxi earthquake is the western segment of Gangu-Wushan secondary fault on the northern margin of western Qinling fault zone. This fault zone is an important active block boundary fault in the eastern margin of Tibeten block, and has the tectonic condition to generate M ≥ 7 large earthquakes in the past and in the future.

Key words: 47 BC. Longxi earthquake, historical earthquake textual research, Huandao ancient town, causative structure, northern margin of western Qinling fault zone

摘要： 西汉元帝初元二年二月戊午，即公元前47年4月17日，在陇西郡所属的豲道县附近发生了1次强烈地震，造成"坏败豲道县城郭、官寺及民室屋，压杀人众。山崩地裂，水泉涌出"等严重震害。根据历史资料考证和野外实地调查，确认豲道县城在今甘肃省陇西县文峰镇东南的三台王家新庄附近。豲道城是这次地震史料记载中破坏最严重的地方，理应位于极震区内，其烈度达Ⅸ-Ⅹ度，估算其震级达7级左右。结合其他破坏点烈度的综合评定结果，绘制了这次地震的等震线图，其长轴方向为NWW向。这次地震极震区位置和等震线长轴方向与该区的西秦岭北缘断裂带的甘谷-武山断裂段西段相吻合。该段断裂全新世晚期新活动的地质地貌和地质剖面证据明确，以左旋走滑为主兼具向N倾的正断特性，断错了冲沟T₁-T₂级冲洪积阶地，沿断裂至今仍保留有部分深切的地震断层沟槽、黄土中密集发育的张性构造裂缝、滑坡体和串珠状落水洞等地表形变现象遗迹。结合该区现代小震活动精定位结果的综合分析，认为公元前47年陇西地震应为西秦岭北缘断裂带的甘谷-武山断裂段西段新活动的结果。该断裂具备发生7级以上地震的构造条件。

关键词: 公元前47年陇西地震, 历史地震考证, 豲道城, 发震构造, 西秦岭北缘断裂带