华北东北部的面波相速度层析成像研究

本文用华北临时地震台阵197个宽频带台站2006年12月～2007年6月记录的面波资料，用双台法测定了435条路径上周期8～75 s的基阶瑞利波相速度频散曲线.通过Ditmar & Yanovskaya 方法反演得到33个周期分辨率为30～50 km的相速度分布图像.分析研究了4个具有代表性周期的相速度分布图像，这些图像揭示了华北东北部地壳上地幔速度结构的横向非均匀性质.10 s周期的相速度分布图像清晰地勾绘出了华北东北部盆地与山区及其内部次级构造单元的边界，平原与山区的相速度存在明显的差异；华北东北部6级以上的强震大都分布在16 s周期高相速度与低相速度急剧变化的过渡带上，规模较大的断裂也大部分位于过渡带的边界附近；在研究区域中部，26 s周期的相速度出现一个明显的呈北东向展布的相速度梯度带，该梯度带与重力梯度带位置相近，说明该重力梯度带两侧的地壳速度结构差异较大.

Surface wave tomography for the phase velocity in the northeastern part of North China

By using the surface waves recorded by 197 broadband stations of the temporary seismic array in North China during Dec. 2006 and June, 2007, the dispersion curves for the period between 8 and 75 s along 435 paths were obtained by the two-station method. The phase velocity images with a resolution of 30~50 km for 33 periods were obtained by using the algorithm developed by Ditmar and Yanovskaya. We analyzed the phase velocity images for four typical periods. These images revealed the heterogeneity of the crustal and upper mantle velocity structure in the northeastern part of North China. The phase velocity image for the period of 10 s clearly sketches the border of the sub-tectonic units between the mountain and basin areas in North China. The phase velocities are very different between the mountain and basin areas. Strong earthquakes (M≥6) mostly located at the transition zones of higher and lower velocity anomalies in the images for the period of 16 s. The large-scale ruptures were also near the edges of the transition zones. In the center of the research area, a remarkable gradient zone of the phase velocity was in the NE orientation in the image for the period of 26 s. The location of the gradient zone of the phase velocity is close to the gradient zone of the gravity, which implies the crust velocity structure along one side of the gravity gradient zone has a lager difference from that along the other side.