利用人工地震测深剖面研究银川盆地及两侧区域的S波速度结构

Study on S-wave velocity structure difference of Yinchuan basin and blocks on both sides using artificial seismic sounding profiles

本文利用人工地震探测剖面获得的E-W向S波数据,参考P波资料,利用正演算法对该条剖面获得的Sg、Sc1、Sc2、SmS及Sn波波组信息进行了相关计算与处理,获得了沿剖面S波的地壳二维速度结构,结果显示:银川盆地及其两侧区域的地壳速度结构和壳内界面分层都显示出较大的差异.银川盆地及其以西区段呈现四层地壳结构,鄂尔多斯块体内部由西向东C2界面逐渐缺失,显示三层地壳结构特征.且壳内界面及速度等值线变化相对平缓,显示出稳定的地壳结构特征.剖面中段的银川盆地沉积盖层埋深最深达到7.6 km,盆地下方Moho界面的隆起与基底的凹陷呈镜像关系,隆起最浅处深度为38.5 km左右.银川盆地西缘至阿拉善块体边缘中下地壳内出现局部低速异常,横向影响范围达到120 km左右,速度值明显比周围介质低0.15~0.28 km/s个量级,壳内界面起伏变化剧烈,与两侧块体接触及其耦合部位速度结构异常紊乱,显示出断陷活动、地壳不稳定的特征.阿拉善块体壳内速度等值线变化较为平缓,Moho界面相对其他块体显示出较深的深度.沿剖面从获得的S波二维速度结构特征可以看出各块体地壳速度结构特征、深部构造环境、以及深部介质物性差异,揭示了银川盆地在周围块体相互运动、碰撞、挤压背景下的应力变形特征.

We take advantage of S-wave data in EW obtained from artificial seismic sounding profiles deployed in the Project,supported by the State Key Program of National Natural Science of China,＂Study on the Lithospheric Fine Structure and Shallow Tectonic Response in the Western Part of North China Craton by Comprehensive Detection Methods of Deep Seismic Reflection and Refraction＂,calculate and process wave-group information Sg,Sc1,Sc2,Sm S and Sn of the profile by using positive calculus method and referring to P-wave data,and then obtained 2 D S-wave crustal velocity structure along the profile. Results show that： the crustal velocity structure and intracrustal layered interface in Yinchuan Basin and the region on both sides show great differences. Yinchuan Basin and its western section present four-layer crustal structure.The C2 interface within the Ordos block presents the gradual loss from the west to the east,showing the three-layer crustal structure characteristics,with smoothing overburden and its average thickness being 4. 5 km. The depth of Moho varies at about 43 km,and S wave velocity shows positive gradient increase from the surface to the Moho interface and its intracrustal interface and velocity contour changes gently which shows stable crustal structure characteristics.In the middle section,the depth of sedimentary cover can reach to7. 6 km at the deepest part,the Moho uplift and basement sag beneath the basin is a mirror image relationship with depth of the shallowest in the uplift being 39 km,and there is local low-velocity anomaly and violent interface undulation in the middle-lower crust of the western margin in Yinchuan Basin, and velocity structure disorders in contact sites and coupling parts on both sides,which show features of rift activities and crustal instability. The overburden of Helan orogenic belt is the thinnest with the thickness is only about 0. 8 km,and the basement uplifts while intracrustal interface is depressed in different degrees. Moho interface concave is obvious and its depth is about 45 km below the Helan Mountain uplifting area. Between the concave below the Helan mountain and the uplift of Yinchuan basin,the Moho interface develops a mutation belt,and there is a low-velocity zone in the middle-lower crust,with transverse influence scope being about 120 km and the speed being obviously 0. 15 - 0. 28 km s-1 order of magnitude lower than the surrounding medium. In the northern section,the overburden of Alashan block changes smoothly and its thickness is 2. 5 km. The intracrustal interface gradually deepens from the east to the west,and the depth of Moho interface is about 47 km,the buried depth being much deeper relative to each layer interface of other blocks.From the 2 D S-wave velocity structure along the profile,it can be seen that the crustal velocity structure features, deep tectonic environment and physical properties characteristics difference of deep medium is significant among blocks（ basin）.