利用多种地震数据联合反演青藏高原地壳上地幔剪切波速度结构

郑现,赵翠萍,郑斯华,周连庆

利用青藏高原东南缘170个固定台站记录的连续波形、远震面波、远震体波数据,提取了各台站对间6—120 s瑞雷波相速度频散、6—50 s群速度频散及各台站下方接收函数,反演得到研究区地壳上地幔剪切波速度结构、地壳厚度及泊松比(图1-2)。成像结果显示10 km深的剪切波速度分布能够勾画出四川盆地的沉积层分布范围;30 km深的剪切波速度图像表明研究区中下地壳广泛分布低速层;40-60 km深及100 km深度速度分布则分别揭示地壳厚度及岩石圈厚度分布变化。根据对研究区地壳低速层分布研究,川滇地块、松潘-甘孜地块北部及扬子克拉通西部20—40 km广泛分布着地壳低速层,但是低速层的深度、厚度及速度值存在强烈的不均匀性,并且受到研究区主要断裂带及构造块体的影响,尤其是丽江-小金河断裂以南,低速层厚度及速度值急剧衰减,近乎消失。研究区泊松比变化范围很大,揭示了研究区地壳物质组成存在很强的横向不均匀性。川滇块体西北部及四川盆地以南区域同时存在高泊松比(σ >0.3)及强低速层(Vs < 3.4 km s–1),表明这些区域可能存在中下地壳部分熔融。在右江地块及腾冲火山与红河断裂带之间区域观测到上地幔低速层,可能是由于岩石圈减薄,低剪切波速反映了软流圈介质性质。在松潘-甘孜褶皱带东部、香格里拉次级块体及雅江次级块体,地幔低速层与地壳低速层及地表高海拔区域存在一定程度的一致性对应关系,表明地壳及上地幔存在密度低、介质软弱区域,与布格重力异常结果一致。

Summary

Based on continuous waveform, surface wave, and body wave data from teleseismic events recorded by broadband seismic network stations in the southeastern margin of the Tibetan Plateau, we retrieved 6–120 s Rayleigh wave phase velocity and 6–50 s group velocity dispersion curves for station pairs, as well as receiver functions beneath all of the stations. The Rayleigh wave phase velocity dispersions at overlapping periods based on ambient noise empirical Greens functions were found to be congruent with those based on teleseismic surface wave data. We then performed inversion to obtain a reliable three-dimensional shear wave velocity structure by using both the surface wave dispersion measurements and receiver functions. The resulting shear wave velocity distribution revealed that low-velocity zones (LVZs) are ubiquitous in the mid-lower crust beneath the Chuandian block, the Songpan-Ganze fold belt, and some parts of the western Yangtze platform. This result differs from those of previous studies, which indicated that LVZs are mainly concentrated in the lower crust beneath the Chuandian block. We also found that the LVZs vary substantially in terms of occurrence depth, thickness, and anomaly strength. They are also influenced by the major fault zones, especially the Lijiang-Xiaojinhe fault, and the tectonic blocks in the study area. Extremely high Poisson’s ratios (σ > 0.3) and low shear wave velocities (Vs < 3.4 km/s) coincide in the northwestern margin of the Chuandian block and the region to the south of the Sichuan basin, indicating the existence of partial melt in the mid-lower crust in that area. The mantle LVZs exhibit spatial consistency with crustal LVZs to some degree in the eastern Songpan-Ganze fold belt, Shangrila sub-block, and Yajiang sub-block, implying the existence of a weak and low-density layer in the upper-most mantle.


Figure 1. Images showing the distributions of (a) Moho depth and (b) Poisson’s ratio (b).

图3 沿不同垂直剖面的剪切波速度分布

其中每组剖面顶部黑色线及灰色区域表示沿该剖面的地形地貌,白色虚线表示由H–κ叠加搜索法计算的地壳厚度,白色实心圆表示汶川地震及芦山地震的余震序列在剖面上的投影(Ms ≥ 3.0). 

Figure 3. Shear-wave velocity distributions along the vertical cross-sections shown in Fig. 1b. (a) AAʹ, (b) BBʹ, (c) CCʹ, (d) DDʹ, (e) EEʹ, (f) FFʹ, (g) GGʹ, and (h) HHʹ. The black line at the top of each cross-section delineates the topography along the profile, the white lines indicate the crustal thickness calculated by using the H–κ stacking algorithm, and the solid white circles mark the earthquake sequences of the Wenchuan and Lushan earthquakes (Ms ≥ 3.0)


上述研究成果发表在国际地球物理学期刊Geopysical Journal International上 (Xian Zheng, Cuiping Zhao, Sihua Zheng, Lianqing Zhou. 2019. Crustal and Upper Mantle Structure Beneath the SE Tibetan Plateau from Joint Inversion of Multiple Types of Seismic Data. Geopysical Journal International, doi: https://doi.org/10.1093/gji/ggz027).

原文链接:https://academic.oup.com/gji/article/217/1/331/5304619