南极上地幔结构异常对长周期自由振荡的影响

The effect of upper mantle structure on long period Earth′s free oscillation at the South Polar

2012年发生的苏门答腊大地震激起了强烈的环形地球自由振荡,本文利用南极26个地震台站记录的此次大地震激发的0T10简正模,分析南极上地幔结构异常对长周期自由振荡的影响,我们将震后20h的自由振荡观测数据与利用PREM地球模型模拟的结果进行比较,结果表明:在震后较短时间内区域性(区域的范围远远小于简正模的波长)上地幔结构异常能显著影响长周期自由振荡质点的偏振,引起简正模强烈的偏振异常.以往研究主要关注地球自转和全球范围的上地幔结构异常对长周期自由振荡简正模的影响,局部范围的上地幔结构异常对长周期自由振荡的影响并未被重视.对南极地区0T10垂向偏振异常的进一步分析表明:南极大陆上地幔存在方位各向异性,上地幔各向异性主要分布在横贯南极山脉下方,深度范围约为70~660km.利用震后较短时间内的长周期地球自由振荡观测资料可以对局部区域上地幔的各向异性及其深度范围提供约束,可作为剪切波分裂和面波层析成像技术的补充.

This paper analyses the effect of upper mantle structure on0 T10 normal mode of free oscillation of Earth caused by 2012/04/11 MW8.6 Sumatra earthquake.We compare the observations with simulation calculated by the PREM,which shows a fact that is often ignored in previous studies of long period normal modes：lateral variation of upper mantle structure,whose wavelength is much smaller than the wavelengths of normal modes at low frequencies,can cause significant polarization anomalies for the normal modes,while usually only the effect of Earth′s rotation and global upper mantle lateral variation are taken into account.The further analysis of the0 T10 vertical polarization suggests that at South Polar the anisotropic area is mainly distributed beneath the Transantarctic Mountains with a depth range 70~660 km.Using the polarization of long period normal modes in a short time after the earthquake we can constrain the scope and depth ofthe lateral variation of local mantle structure.