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

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