青藏高原东南缘基于背景噪声的Rayleigh面波方位各向异性研究

Rayleigh wave azimuthal anisotropy on the southeastern front of the Tibetan Plateau from seismic ambient noise

青藏高原东南缘地区是现今地壳形变和地震活动最强烈的地区之一,也是研究青藏高原现今变形机制和构造演化规律的重要区域.本研究使用云南区域地震台网的55个宽频带地震台站连续地震背景噪声数据,采用双台站互相关方法获得Rayleigh（瑞利）面波经验格林函数,提取相速度频散曲线,反演得到云南地区周期5~34s范围内方位各向异性分布图像.反演结果揭示：短周期（5~12s）Rayleigh面波快波优势方向与区域断裂走向有很好的一致性,快波方向随着断裂走向的变化而变化.周期16~26s快波优势方向与反映上地壳特性的5~12s图像总体图像相似,但细节略有不同.其中,滇中块体内易门断裂和滇中块体内东侧的普渡河断裂附近,各向异性快波方向从NS向NW方向旋转;易门断裂以西呈NW向.这反映了青藏高原物质东流和川滇块体受到青藏块体的南东向挤压作用.周期30~34s范围的各向异性,滇缅泰块体和印支块体,快波优势方向为NS和NNW向;而在滇中块体内部,各向异性快波方向呈顺时针旋转变化,可能与青藏高原物质向东逃逸有关.本文还开展了与体波各向异性的对比分析,通过与近震S波分裂、Pms转换波分裂和远震SKS、PKS和SKKS（以后简称为XKS）分裂的对比研究,发现随着周期的增大,得到的快波优势方向与XKS剪切波快波偏振方向趋向一致,与地壳快剪切波偏振方向呈一定夹角.本研究认为,青藏高原东南缘地区壳幔各向异性具有不同的特征和形成机制.

The southeastern front of the Tibetan Plateau is one of the strongest regions in present crustal deformation and seismic activity,and also plays an important role on studying deformation mechanism and tectonic evolution of the plateau.We make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from regional Yunnan Seismic Network.Ambient noise tomography is a rapidly emerging field of seismological research.The processing procedure divides into four principal phases.The first step is single station data preparation.Continuous data are decimated to five samples per second,the mean and trend are removed,periods between 5and 50 sare band-pass filtered,and time domain normalization is applied with an absolute mean normalization method.After the preparatory phase,the next step is cross-correlation between all possible station pairs.To obtain reliable measurements,we stack the daily cross-correlated signals over a 1-year time window for each station pair.The third step is phase velocity dispersion curves determination,and we thus obtained 3072 dispersion curves from 5to 34 s.Finally,using the dispersion curves,we invert to image Rayleigh wave azimuthal anisotropy with periods between 5and 34 s.The result shows as follows：the predominant direction of fast wave at 5~12speriod is consistent and varying with the strike of the regional fractures.In the north of YunnanMyanmar-Thailand block,the fast wave direction trends NS,and in the south of YunnanMyanmar-Thailand block,the direction trends NE-SW.In the Indosinian block,the fast wave direction varied with the strike of Honghe fault and Lancangjiang fault.In the Middle-Yunnan block,the fast wave direction is relatively scattered,while in the eastern part of Yunnan block,the direction is NNW and NW.At period of 16~26s,the image of the fast wave direction is generally similar to the image of 5~12speriod,which reflects the upper crustal feature,besides some differences in the details.In nearby Yimen fault and Puduhe fault in middle Yunnan block,the fast wave direction rotates from NS to NW,and in the west of Yimen fault,the direction trends NW,which suggest that plateau materials flow east,and meanwhile,Sichuan-Yunnan block is squeezed by the Tibetan plateau.At 30~40speriod,in Yunnan-Burma block and Indosinian block,the fast wave direction trends NS and NNW.And in middle Yunnan block,there is clockwise rotation about fast wave direction,which is possibly related to eastward escape of materials under the Tibetan Plateau.According to comparison with body wave anisotropy such as S-wave splitting,Pms splitting and SKS,PKS and SKKS（hereafter referred to as XKS for short）shear wave splitting,we find that along with the increase of the period,the fast wave direction is similar to the XKS shear wave splitting,and sites at a certain degree angle to the direction of fast wave polarization in the crust.All in all,crust-mantle formation mechanism may be different in the southeast of Tibetan Plateau.