中国大陆中上地壳剪切波速结构

MIDDLE AND UPPER CRUST SHEAR-WAVE VELOCITY STRUCTURE OF THE CHINESE MAINLAND

为使已获得的中国大陆中上地壳结构更为可靠,本文搜集了很多对浅部结构分辨率较好的短周期面波资料.与传统面波层析成像反演方法不同,本文在第二步由面波频散得到剪切波速的过程中不再对每个结点进行一维波速模型分别反演,而是直接将所有结点上的区域化频散转换成三维波速结构的线性化约束,实现了直接的三维反演.检测板测试结果显示该方法可以得到理想的反演结果.本文得到的波速模型显示,中国大陆中上地壳的速度分布存在明显的横向变化和分区特征.较低的波速异常很好地勾勒出我国主要的沉积盆地,波速异常在不同深度上的变化在一定程度上反映了各盆地结晶基底的深度.以东经95°为界,特提斯构造域西部具有明显的低速异常,而东部基本没有低速异常.基于油气资源多存在于沉积层中,而沉积层表现低速异常,我们推测特提斯构造域西部油气前景比东部好.另外,由于特提斯构造域西部低速非常明显,这可能也说明了其地壳温度较高.兴安造山带的低速异常可能也说明了其地壳温度较高.20km深度上鄂尔多斯盆地西侧的弱低速带,很好地勾勒出中国大陆近似沿105°经线的强震带的走势.

In order to give a more reliable shallow crust model for the Chinese mainland, the present study collected many short-period surface wave data which are better sensitive to shallow earth structures. Different from traditional two-step surface wave tomography, we developed a new linearized surface wave dispersion inversion method to directly get a 3D S-wave velocity model in the second step instead of inverting for 1D S-velocity profile cell by cell. We convert all the regionalized dispersions into linear constraints for a 3D Svelocity model. Checkerboard tests show that The distribution of the middle- and upper-crust this method can give reasonable results. shear-wave velocity of the Chinese main- land in our model is strongly heterogeneous and related to different geotectonic terrains. Low-velocity anomalies delineated very well most of the major sedimentary basins of China. And the variation of velocities at different depths gives an indication of basement depth of the basins. The western Tethyan tectonic domain （on the west of the 95°E longitude） is characterized by low velocity, while the eastern Tethyan domain does not show obvious low velocity. Since petroleum resources often distribute in sedimentary basins where lowvelocity anomaly appears, the low velocity anomalies in the western Tethyan domain may indicate a better petroleum prospect than in its eastern counterpart. Besides, low velocity anomaly in the western Tethyan domain and around the Xing＇an orogenic belt may be partly caused by high crustal temperature. The weak low-velocity belt along -105°E longitude at 20 km depth with 20 km depth corresponds to the N-S strong seismic belt of central China.