新丰江库区二维P波速度结构——英德一河源一陆河深地震测深剖面探测结果

2-D p-wave velocity structure in the Xinfengjiang reservoir area——Results of Yingde—Heyuan—Luhe deep seismic sounding profile

以新丰江水库为中心完成了一条深地震宽角反射/折射剖面.采用动力学射线追踪方法获得了沿剖面的壳幔二维速度结构图像.结果表明:在新丰江水库东南的大坝峡谷区至河源一带,上地壳基底界面至C_1界面(上地壳上部的底面)之间速度结构存在较明显的纵横向非均匀性,速度以高、低速相间分布为特征.结合近期新丰江库区地方震精确定位结果分析发现,地震分布与这一结构异常区有较好的相关性.推测剖面120 km桩号附近可能存在至少切至上地壳上部的深部断裂.该断裂主要以水平错动为主,且在近地表并不明显,这与新丰江大坝峡谷区分布的北东东向断裂特征具有一致性.研究区基底埋深约1.2～2.5 km,新丰江水库下方基底相对两侧呈现上隆.上地壳下部层速度较为均匀,层厚度无明显变化,上、下地壳分界面埋深20.5～21.5 km.下地壳存在一弱低速层体,莫霍界面由南东向北西呈缓慢加深的趋势,起伏变化不大.埋深约31.0～32.5 km.沿剖面地壳平均速度6.21～6.25 km/s.

A deep seismic wide-angle reflection/refraction profile has been completed in the Xinfengjiang reservoir area. The two-dimensional crustal velocity structure image along the profile is obtained by using dynamic ray tracing method. The results show that there is a obvious velocity anomalies characterized by alternating high and low velocity between Xinfengjiang Reservoir dam canyon and Heyuan area from the basement interface to C1 discontinuity（bottom of the upper part of the upper crust）. Combining with the recent local earthquakes precise location results of Xinfengjiang reservoir area, we find that these earthquakes is related to the velocity anomalies. We infer that there is a deep fault at least extending to the top surface of the C1 discontinuity in the upper crust near pos＊ 120 km of the profile. The fault mainly behaves as horizontal fracture dislocation, and is not distinguished easily near surface. Which consisls with the characteristics of the NEE-trending faults existing in the Xinfengjiang darn canyon regin. The depth of the basement is about 1. 2 - 2. 5 km. The basement rises slightly under the Xinfengjiang reservoir. The variation of the velocity and the thickness is small within the lower part of the upper crust. The boundary between the upper and lower crust is 20. 5-21.5 km deep. There is a weak low velocity body within tile lower crust. The Moho discontinuity deepens from southeast to northwest slowly and it is about 31.0-32.5 km deep. The average crustal velocity along the profile is 6.21-6.25 km/s.