临汾盆地及周边地区双差层析成像研究

Double-Difference Tomography in Linfen Basin and Its Surrounding Areas

使用固定台网与喜马拉雅三期台阵数据,采用双差层析成像方法对临汾盆地及周边地区的地震位置和速度结构进行联合反演,最终获得研究区地震精定位和精细速度结构。地震重定位结果表明,临汾盆地地震具有2个优势地震地层,分别为5~9 km和18~25 km,且下层地震多于上层;地震具有明显的丛集性,发震层下界清晰;洪洞凹陷和辛置凸起下方地震成簇,震源深度最深的小震全部集中在该区域。速度结构研究表明,研究区浅层速度梯度与出露地质构造高度吻合,说明反演结果具有可靠性,12 km以下速度结构与浅部构造特征不同,说明该区域地壳总体呈现深浅构造不一致的特征;灵石隆起先期存在连通临汾、太原盆地的古河道,后期发生断块式穹状隆起,本研究可为其提供地球物理佐证,并推测古河道位于灵石隆起西侧;地震条带处于灵石隆起西侧的原因为西侧较东侧具有低速构造,在鄂尔多斯块体逆时针转动作用力下西侧易于破碎;临汾盆地下方至18 km处整体呈现低速低密度特征,这与构造含水有关;临汾1695年M7地震发生在速度转换带上,洪洞1303年M8地震发生在高速构造区,这说明两次地震的发震构造及机理有所不同。

We use the fixed station network and the Himalayan Phase III array data,and adopt the double-difference tomography method to jointly invert the seismic position and velocity structure of Linfen basin and its surrounding areas,and finally obtain the precise location and velocity structure of earthquakes in the study area.The seismic relocation results show that the Linfen basin earthquakes have two dominant seismic strata(5-9 km and 18-25 km),and there are more earthquakes in the lower layer than the upper layer.The earthquakes have obvious clustering,and the lower boundary of seismogenic layer is clear.The earthquakes below Hongdong depression and Xinzhi uplift are clustered,and the small earthquakes with the deepest focal depth are all concentrated in this area.The velocity structure results show that the shallow velocity gradient in the study area is highly consistent with the exposed geological structure,which confirms the reliability of inversion results.The velocity structure below 12 km is different from the shallow structure,indicating that the regional crust generally presents the inconsistency characteristics between deep and shallow structures.In the early stage of Lingshi uplift,the paleochannels connected Linfen basin and Taiyuan basin,and occurred fault-block dome uplift in the later stage.This study can provide geophysical evidence for that,and speculates that the paleochannels located on western Lingshi uplift.The reason why the seismic belt located on western Lingshi uplift is that the west side has low-velocity structure compared with the east side,and is easy to break under the anticlockwise rotation force of Ordos block.From the surface of Linfen basin to 18 km,the overall low speed and low density structure is presented,which is related to the structural water content.The Linfen 1695 M73/4 earthquake occurred in the velocity conversion zone,and the Hongtong 1303 M8 earthquake occurred in the high-speed tectonic zone,which shows that the seismogenic structures and mechanisms of the two earthquakes are different.