Structure of Crust and Upper Mantle in Tianshui-Wudu Strong Earthquake Region of North-South Tectonic Belt
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摘要: 利用两条相互垂直的高分辨地震折射/宽角反射剖面和相应的非纵观测的多个扇形剖面取得的人工地震资料, 研究天水和武都8级大震区的地壳和上地幔顶部结构和构造.二维剖面结果显示, 地壳沿垂向可分为上地壳和下地壳两大层.上地壳中部存在低速层, 层内介质速度比背景值低0.3~0.5km/s.莫霍面深度大约为46~48km.NE向的天水-武都剖面下地壳速度在横向上变化剧烈, NW向的成县-武山剖面, 在礼县以西, Moho面和C界面有被上涌物质改造过的迹象.三维速度成像显示, 在105°E附近, 从7至11km的深度范围内, 存在一条近NS向的断裂带, 在该带的两侧速度结构有明显的差异, 西侧为低速异常, 而东侧为高速异常, 这一近NS向的断裂带与二维剖面的下地壳深断裂在位置上很接近.该地区的几个8级大震均发生在105°E附近, 并且呈一近NS条带.Abstract: The structure of crust and upper mantle in Tianshui-Wudu strong earthquake region is studied by using DSS data from two cross high resolution refraction /wide angle reflection profiles and corresponding fan profiles.The 2-D result shows that the crust can be vertically divided into two layers: upper crust and lower crust.There exists a low velocity layer in the upper crust with velocity difference of 0.3-0.5 km/s.The depth of Moho is about 46-48 km.There is a strong lateral variation in the lower crust along the NE Tianshui-Wudu profile.And along the NW Chengxian-Wushan profile, there are some evidences showing that the Moho and C discontinuities are changed by substances from the deep layer.The 3-D result shows that there is a fault with near NS strike at the depth of 7-11 km near 105°E.The velocities at the two sides of this fault are quite different: low velocity at the west side and high velocity at the east side.This NS fault is very close to the deep fault of 2-D profile.All the large earthquakes occurred near 105°E and formed a NS belt.
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图 1 研究区地形和剖面位置及地震分布
Fig. 1. Topography of study area, positions of profiles and earthquake distribution
图 2 成县-武山人工地震剖面地震记录截面(SP9)
Fig. 2. Record section of Chengxian-Wushan DSS profile from SP9
图 3 天水-武都剖面(PF-1)SP6炮点射线追踪和走时对比
Fig. 3. Ray tracing and travel time comparison of Tianshui-Wudu DSS profile (PF-1) from SP6
图 4 天水-武都剖面(Pf-1)二维地壳速度结构
Fig. 4. 2-D velocity structure of Tianshui-Wudu DSS pro-file (Pf-1)
图 5 成县-武山剖面(PF-2)二维地壳速度结构
Fig. 5. 2-D velocity structure of Chengxian-Wushan DSS profile (PF-2)
图 6 炮点SP8 (天水-武都测线接收) 扇形地震记录截面
Fig. 6. Fan record section of SP8 observed along Tianshui-Wudu DSS profile
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