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

摘要： 齐心庄探槽位于1679年三河-平谷8级地震夏垫地表地震破裂带东段。探槽内出露的断错地层、崩积楔、堰塞塘堆积、张裂楔及微细地层层理的揉皱现象,显示夏垫断裂全新世以来曾发生4次强震事件。这4次强震分别发生在距今(10.85～9.71)ka、(7.39～6.68)ka、(5.416～2.233)ka及1679年(即三河-平谷8级地震)。这4次强震的时间间隔分别为(3.245±0.336)ka、(3.211±0.815)ka及(3.553±0.796)ka,平均强震间隔为(3.336±0.396)ka。4次强震的平均同震垂直位移为(1.4±0.5)m。在齐心庄实施的探槽工程,除邻近断面开挖的探槽外,还在探槽南北400m范围内开挖了4个探坑,对认识断层下降盘地层的展布形态及确定断面堰塞塘的分布提供了帮助。在这些野外工作基础上,对齐心庄探槽研究结果与前人沿夏垫断裂其它地点的探槽及钻孔资料进行了对比,在古地震事件认识上大部分是一致的。

关键词: 北京平原, 夏垫断裂, 古地震, 地震破裂带