川西地区台阵环境噪声瑞利波相速度层析成像

2006年中国地震局地质研究所地震动力学国家重点实验室在川西地区(26°N~32°N,100°E~105°E)布设了由297台宽频带数字地震仪组成的流动观测台阵.利用该密集台阵29°N以北156个台站2007年1~12月份的地震环境噪声记录和互相关技术,我们得到了所有台站对的面波经验格林函数和瑞利波相速度频散曲线,并进一步反演得到了观测台阵下方2~35 s周期的瑞利波相速度分布图像．本文结果表明,观测台阵覆盖的川滇地块、松潘-甘孜地块和四川盆地的地壳速度结构存在显著差异,具体表现为：(1)短周期（2~8 s）相速度分布与地表构造特征相吻合,作为川滇地块、松潘-甘孜地块和四川盆地之间的边界断裂,龙门山断裂带和鲜水河断裂带对上述三个地块上地壳的速度结构具有明显的控制作用,四川盆地前陆低速特征表明相应区域存在较厚的（约10 km）沉积盖层;(2)中周期（12~18 s）相速度分布表明,川滇地块和松潘-甘孜地块中上地壳速度结构存在明显的不均匀横向变化,并形成了尺度不同且高、低速相间的分块结构,而四川盆地中地壳整体上已经表现出相对高速;(3)长周期（25~35 s）相速度分布表明,松潘-甘孜地块,特别是川滇地块中下地壳表现为广泛的明显低速异常,意味着它们的中下地壳相对软弱,而四川盆地的中下地壳呈现整体性的相对高速,意味着四川盆地具有相对坚硬的中下地壳,并且以汶川地震的震中为界,龙门山断裂带的地壳结构显示了北段为高速异常,南段为低速异常的分段特征．

Phase velocity array tomography of Rayleigh waves in western Sichuan from ambient seismic noise

A transportable array with 297 broadband seismic stations was deployed in the western Sichuan (26°N~32°N,100°E~105°E) in 2006 by the State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration．From the ambient noise data recorded at 156 stations of this array to the north of 29°N from January to December of 2007, we have obtained the surface wave empirical Green′s functions (EGF) using cross-correlation technique and measured the Rayleigh-wave phase velocity dispersion curves for all possible station pairs．These dispersion measurements were then used to invert for phase velocity maps of Rayleigh waves between periods 2~35 s．Our results manifest the significant discrepancies between the crustal structures of the Chuandian block,Songpan-Garze block and Sichuan basin, which can be summarized as follows．(1) The phase velocity maps at the short periods (2~8 s) are well-correlated with the surface geological feature, and the upper crust structures of the Chuandian block,Songpan-Garze block and Sichuan basin are well confined by the Longmen Shan faults and Xianshuihe faults, especially in the foreland of the Sichuan basin, where the significant low-velocity structure suggests that a thick sediment does exist;(2) The phase velocity maps at the intermediate periods (12~18 s) manifest that the middle and lower crust velocity structure of the Chuandian and Songpan-Garze block has apparent lateral heterogeneities, including low-velocity zones of different sizes, and the upper and middle crust beneath the Sichuan basin shows high velocity structure;(3) The phase velocity maps at the long periods (25~35 s) manifest that the Songpan-Garze block and Chuandian block have an evident low-velocity anomaly in the middle-lower crust,suggesting that the middle-lower crust of these two blocks is relatively weak, and the middle-lower crust of Sichuan basin has high-velocity anomaly, suggesting a mechanically strong middle-lower crust in the basin．In particular, the crustal velocity structure along the Longmen Shan faults has a high-velocity anomaly to the north and a low-velocity anomaly to the south of the hypocenter of the Wenchuan earthquake．