南海地区地震背景噪声成像与壳幔深部结构

利用南海地区28个陆地地震台站和2个布设于太平岛和东沙岛的新增海岛地震台站2011—2016年间的连续地震背景噪声波形数据,使用互相关方法计算得到了台站间的互相关函数,并提取出Rayleigh面波群速度和相速度频散曲线.采用快速行进和子空间方法反演获得了南海及周边地区12～40s周期的Rayleigh面波群速度和相速度图像,并联合反演得到了研究区深至60km的三维S波速度结构.考虑到南海数千米厚海水层对于面波频散反演的严重影响,本文在反演模型中加入了水层,显著提高了反演结果的可靠性.成像结果表明:南海及周边地区地壳上地幔顶部S波速度结构存在显著的横向不均匀性,并与这一区域的主要构造单元具有较好的空间对应关系.在5～10km深度,莺歌海—宋红盆地区的低速异常特征可能与盆地较厚的沉积层有关.在5～15km深度,海域高速异常区与海盆空间位置具有高度一致性,推测与海盆区地壳厚度相对陆缘区明显偏薄有关.当深度从20km增加至30km,海盆区的高速特征扩展至了陆缘地区,反映了地壳厚度从海盆至陆缘逐渐增厚的趋势,与OBS(海底地震仪)深地震剖面给出的地壳精细结构结果一致.至35～60km深度,海盆的高速异常特征依然明显,且速度值随深度增加整体呈现上升的趋势,推测南海海盆区的岩石圈厚度应该大于60km.

Ambient noise tomography and deep structure in the crust and mantle of the South China Sea

Using the continuous seismic ambient noise data recorded by 28 land seismic stations in the area surrounding the South China Sea and 2 island stations on Dongsha Island and Taiping Island during January 2011 and December 2016, we calculated the cross-correlation functions between station-pairs utilizing the cross-correlation method, and extracted the Rayleigh surface wave group velocity and phase velocity dispersion curves based on it. The group velocity and phase velocity images of South China Sea over period range of 12~40 s were inverted by using Fast Marching and Subspace method. Then, the 3D S-wave velocity structure up to 60 km in depth was inverted by joint inversion. During the inversion process, we added a water layer into the inversion model, considering that several kilometers seawater layer could strongly change the surface wave dispersion behavior. In fact, it is proved that an additional water layer can significantly improve the reliability of the inversion results. From the inverted result, there exists strong lateral variations in the crust and upper mantle structures within the study areas. These variations spatially correlate with the main structural units in this area. At depth of 5~10 km, the Yinggehai-Song Hong Basin is of low velocity, which may be related with the thick sediments layer under the sea. At depth of 5~15 km, the high velocity anomaly in our model correlates well with location of the South China Sea Basin, which may infer that the crust thickness of the sea basin area is much thinner than that of the continental margin area. At depth of 20~30 km, the high-velocity feature of the sea basin extended further to the continental margin area. This reflects the gradual thickening of the crust thickness from the sea basin to the continental margin, which is consistent with the deep seismic profile result obtained using OBS data. In the depth range over 35~60 km, the high-velocity feature of the South China Sea Basin is still obvious and the velocity increases with the depth in general, by which we surmise that the lithosphere thickness of the sea basin should be greater than 60 km.