鄂尔多斯西南缘及邻区三维速度结构全波形反演

Three-dimensional full waveform inversion of velocity in southwestern margin of Ordos and adjacent areas

鄂尔多斯地块西南缘作为青藏高原东北缘扩张的最前端,获取其高精度三维速度结构对揭示地块间物质转移和相互作用机制有着重要的研究意义.本文收集并整理了677个流动台站上记录到的8个4.7级以上地震事件的数据,采用基于伴随谱元的全波形反演方法,获得了研究区内0~60 km的三维速度结构.模型成像结果表明,在鄂尔多斯地块低速沉积层下方的地壳相较于同一深度的地壳速度结构呈明显高速,上地幔速度结构连续.华南地块内部速度结构表现为古老克拉通稳定的岩石结构,但北部地壳并不具备相似特征.相较鄂尔多斯地块和华南地块,阿拉善地块内部壳幔速度结构要复杂得多.在北纬38°以北,鄂尔多斯地块内部部分地壳速度结构呈高速,上地幔呈低速,而地块南部地壳速度结构则具有相反的特征.地块内部的20 km处存在不连续分布的低速体表明,鄂尔多斯地块稳定克拉通的岩石结构发生了一定改变.此外,研究区内断裂附近对应有显著速度结构异常,这也为研究区深入开展深浅构造耦合研究提供有力的地震学成像约束.

The southwest margin of the Ordos block is located at the foreland of the northeast margin of the Tibetan Plateau.The three-dimensional velocity structure is of great significance to reveal the mechanism of material transfer and interaction between blocks.We collected seismic records of 677 seismographic stations,where 8 earthquakes of which magnitude are greater than 4.7 are selected.Using the full waveform inversion method based on the adjoint spectral element method(SEM),the 3-D velocity structure from 0 to 60 km beneath the southwest margin of the Ordos and its adjacent area is obtained.The results show that compared with the velocity structure of the crust at the same depth,the velocity of the crust under the sedimentary layer with slow velocity,beneath the Ordos block,memorably increases sharply and the velocity feature of the upper mantle is continuously.The velocity structure under the interior of the South China block appears characteristics of the stable rock structure of ancient craton,whereas velocity under the northern crust of the South China shows different characteristics.Compared with Ordos block and South China block,the velocity structure beneath the inner Alxa block is much more complex.In the north of 38°N,the crustal velocity beneath the Ordos block is high and the upper mantle is slow,whereas the shear velocity,in the southern region,shows opposite characteristics.Beneath the Ordos block,slow shear velocity zones that distribute discontinuously in the crust at the depth of 20 km indicated that the old stable craton has been changed to some extent.In addition,prominent velocity anomalies are observed near faults in the study area.This study also provides seismological imaging constraints for further research on the coupling of shallow and deep structures in the study area.