利用背景噪声成像技术反演陕西及邻区地壳剪切波速度结构

利用地震背景噪声层析成像技术处理陕西及邻区所布设的257个宽频带台站的连续背景噪声数据,采用基于射线追踪的面波频散直接反演方法获得陕西及邻区地壳(6～39 km)高分辨率剪切波速度结构。成像结果显示:(1)渭河盆地顶部形成于新生代,厚的沉积层造成其浅部显著的低速异常,盆地中、上地壳为低速结构。渭河盆地与南北两侧地质构造单元交界区域的下方存在高速与低速结合带,以及在块体间相互运动的作用下,在块体内部,特别是界带深部可能存在着物质与能量的强烈交换,为渭河盆地及邻区的地震孕育发生提供深部环境。(2)南鄂尔多斯块体并不是一个均匀的整体,块体地壳浅层东薄西厚的低速异常结构,可能与鄂尔多斯自显生宙以来的整体掀斜,以及晚白垩纪以来差异性整体抬升和受强烈而不均匀的剥蚀有关。块体中地壳速度比上地壳和下地壳较高。壳内不存在显著的低速体,说明壳内低速体并没有贯穿整个鄂尔多斯地块。我们推测南鄂尔多斯块体仍保留着稳定克拉通的属性,其地壳结构可能反映了克拉通早期形成时的结构特征,至今还未遭受明显改造。(3)秦岭造山带东,西深部结构存在显著差异,具有分段分区的特征。造山带中地壳速度较高,可能因在板块碰撞和造山过程中,下地壳物质被抬升进入中地壳,从而造成中地壳速度偏高。

Crustal Shear Wave Velocity Structure beneath Shaanxi Province and Its Adjacent Region Inverted from Ambient Noise Tomography

In this study, we applied seismic ambient noise tomography technology to process the continuous ambient noise data of 257 broadband stations in Shaanxi province and its adjacent regions, and used the surface-wave-dispersion direct inversion method based on ray tracing to obtain the high-resolution shear wave velocity structure of the crust (6-39 km) beneath Shaanxi province and its adjacent regions. The results demonstrate the following:(1) The thick sedimentary layer covering the top of the Weihe Basin is the cause of the low-velocity anomaly in its shallow crust, and the middle and upper crusts of the basin feature low-velocity structures. There is a high-speed and low-speed combination belt beneath the boundary area between the Weihe Basin and geological tectonic units on both the northern and southern sides and under the interaction of blocks. Moreover, there may be a strong exchange of material and energy within the block, especially in the deep part of the boundary zone, which provides a deep environment for the preparation and generation of the earthquakes in the Weihe Basin and its adjacent areas. (2) The southern Ordos block is not a uniform whole; the low-velocity structure of the shallow crust in the block is thin in the east and thick in the west, which may be related to the overall tilting of the Ordos block in the Phanerozoic eon and the strong differential denudation of the eastern uplift and western fall since the Late Cretaceous. The velocity of the middle crust beneath the Ordos block is higher than those of the upper and lower crusts. There is no significant low-velocity structure in the crust of the block, which shows that the low-velocity structure in the crust does not penetrate the whole Ordos block. We speculate that the southern Ordos block still maintains a stable craton property, and its crustal structure may reflect the structural characteristics of the early formation of the craton, but so far, the craton has not been significantly reformed. (3) The deep structure beneath the eastern and western Qinling orogenic belt is different and is characterized by segmentation and the presence of a subarea. The reason for the higher velocity of the middle crust beneath the orogenic belt may be that in the processes of plate collision and orogeny, the material of the lower crust rises into the middle crust, leading to the higher velocity of middle crust.