Vol. 17 Supp. (32~42) ACTA SEISMOLOGICA SINICA Nov., 2004 Article ID: 1000-9116(2004)Supp.-0032-11 2-D P-wave velocity structure in the mideast segment of Zhangjiakou-Bohai tectonic zone: Anxin-Xianghe-Kuancheng DSS profile* WANG Fu-yun 1) (_T__~L~_) ZHANG Xian-kang l) (~f~) CHEN Yong 2) (~ LI Li 2) ( ~ ~) CHEN Qi-fu 2) (~,~) ZHAO Jin-renl)(~:'~--) ZHANG Jian-shi l) (~:~) LIU Bao-feng 1) (~)J ~) 1) Geophysical Exploration Center, China Earthquake Administration, Zhengzhou 450002, China 2) Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China Abstract In order to get the 3-D fine velocity structure in the Capital-circle area of China, 6 explosions, ranging from 1800 to 2500 kg, were conducted and recorded by an array of 240 seismographs. A reflection/refraction survey was car- fled out along the profile extending from Anxin county, Hebei Province northeastward to Yanshan Mountains, crossing the Zhangjiakou-Bohai tectonic zone. The 2-D velocity structure of P wave was imaging along the profile. The results show that abnormality exists in the deep structure of the Zhangiiakou-Bohai tectonic zone: The base- ment is significantly depressed, the interfaces and Moho are uplifted, and a strong velocity gradient layer is existed above the Moho that may be dislocated by deep fault. The crust of Huabei basin is thin and low velocity body ex- ists in the crust. The Yanshan Mountains' crust is thick, the layers in the crust are quite clear and the velocity in the layer is homogeneous. Huabei basin differs from Yanshan Mountains in structure. Key words: 2-D P-wave velocity structure; artificial explosion; Capital-circle area of China; Zhangjia- kou-Bohai tectonic zone; crust-mantle transition zone CLC number: P3 t 3.2 Document code: A Introduction The Capital-circle area is the center of politics, economy and culture of China, and many strong earthquakes occurred here in history. The Zhangjiakou-Bohai tectonic zone located in the area is an important active zone. In the period of 500 years between AD 1481~1981, 18 earth- quakes with magnitude more than 6 occurred in the area and 11 of them lay in this active tectonic zone. During 1992-2001, continuous GPS observation found sinistral relative movement between Yanshan Mountains and Huabei basin (YANG, et al, 2002). More and more studiers are interested in the deep structure of Zhangjiakou-Bohai tectonic zone, in which both Cenozoic tectonic motion and seismicity are very intensive and frequent (SUN, et al, 1988; ZHANG, et al, 1996, 1998; ZHANG, et al, 2002a; ZHAO, et al, 1999; HE, et al, 1998; LIU, 2001). * Received date: 2003-05-22; revised date: 2004-02-18; accepted date: 2004-04-12. Foundation item: Preliminary study of State Key Basic Research Project (2001CCA02100) and National Natural Science Foundation of China (40234038). Contribution No.RCEG20031 l, Geophysical Exploration Center, China Earthquake Administration.