青藏高原东部基于噪声的面波群速度分布特征

通过收集青海、甘肃、四川三省的76个地震台记录的2008年1—12月三分量的连续噪声数据,利用噪声面波层析成像的方法获得了青藏高原东部的面波群速度分布特征。首先采用多重滤波方法提取了1 000多条台站对5～50 s的三分量面波群速度频散曲线,然后将研究区域划分为0.2°×0.2°的网格,利用O ccam方法反演了瑞利波(R-R)和勒夫波(T-T)的群速度分布。反演得到的群速度分布特征与地表地质和构造特征表现出较好的相关性,清晰地揭示了地壳内部的横向速度变化。层析成像的结果显示在短周期(8～20 s)内,拥有较厚的沉积层的四川盆地表现为明显的低速特征,而青藏高原东部则表现为较高的群速度分布特征;随着周期的增加(>20 s),群速度的分布特征呈现出与短周期相反的特性,青藏高原东部下方的速度远远低于四川盆地,这可能与青藏高原东部中、下地壳低速层相关联,同时也意味着研究区域的地壳结构具有明显的横向不均匀性。在群速度分布图上,龙门山不仅是四川盆地与青藏高原的地形和构造分界带,同时也对应着高群速度与低群速度的过渡带。

Surface wave group velocity distribution based on ambient noise analysis in eastern Tibet

We obtained surface wave group velocity distribution in eastern Tibet from ambient noise analysis with the broadband seismic data recorded from January to December of 2008 at 76 stations from the Qinghai,Gansu and Sichuan Earthquake Administration digital seismic network.First,we used the multiple-filter analysis method to extract more than one thousand three-component surface wave group velocity dispersion curves from inter-station paths at periods from 5 to 50 s.Then the study area was reticulated into a 0.2°×0.2° grid,and using O'ccam inversion method to obtain the Rayleigh(R-R)and Love(T-T) wave group velocity distribution.The results of group velocity distribution maps generally demonstrate good correlations with surface geological and tectonic features;they also clearly revealed the lateral velocity variation in the crust.The results of the group velocity tomography show that at the short periods(820 s),Sichuan Basin is clearly resolved with low group velocity due to its thick sedimentary layer,and the eastern part of Tibet shows relative higher group velocity distribution.With the increase of period(>20 s),the group velocity distribution appears the contrary characteristics compared to that of the short periods,and the group velocity beneath the eastern part of Tibetan Plateau is much lower than Sichuan Basin,which possibly correlates with the low-velocity zone exhibited in the middle crust beneath the eastern part of the Tibetan Plateau,and also means that the crust in our study area has conspicuous lateral heterogeneity.Our group velocity distribution maps clearly show that the Longmenshan fault is not only the boundary of topography and tectonic zone,but also the transition zone of high and low group velocity.