关键词: 北京平原, 夏垫断裂, 古地震, 地震破裂带

Abstract: The Qixinzhuang trenches are located at the eastern segment of the Xiadian ground fracture zone caused by the 1679 Sanhe-Pinggu M 8 earthquake. These trenches reveal a lot of deformation such as dislocated strata, colluvial deposits in wedge shape, sag deposits, tensional cracks and corruation textures of microstratification. These phenomena demanstrate that 4 major earthquakes have occurred on the Xiadian fault since Holocene time. They happened on the time of (10.85～9.71)ka B. P., (7.39～6.68) ka B. P., (5.42～2.23)ka B. P., and in 1679, respectively. Their interval is (3.25±0.43)ka, (3.21±0.82)ka, and (3.55±0.80) ka, and the average is (3.34±0.40) ka. The arerage value of coseismic vertical displacement is (1.4±0.4)m. In addition to two trenches near the fault, 4 holes have been dug on the north and south of the trenches. These holes are helpful to observe the stratum shape of the down thrown wall and to define the distribution of sags along the fault. Based on the field investigation, we compare the study results at the Qixinzhuang trenches with previous studies on the other trenches and boreholes along the Xiadian fault. As to the identification of paleoearthquakes, they are mostly in agreement each other.

Key words: Xiadian fault, Paleoearthquakes, Seismic fracture zone