基于全球小口径地震台阵的PcP和PKiKP震相研究

Collecting PcP and PKiKP phases pair using global small aperture arrays

由地球核幔边界(CMB)反射的PcP和由内外核边界(ICB)反射的PKiKP震相可以提供关于这两个界面物理性质最直接的信息,如密度和波速的变化,而这些信息对了解地球的核幔动力学过程具有重要的意义.结合PcP和PKiKP来研究CMB性质也比仅利用P和PcP更有优势,因为他们在上地幔的传播路径的相似性可以很好地消除上地幔结构对观测的影响.同时,短周期PcP和PKiKP对小尺度结构具有较高的分辨能力,可以其物性提供更好的约束.然而,由于CMB和ICB的P波反射系数较小,PcP和PKiKP信号与其他主要震相相比很弱,且容易受到其他信号干扰(如小震中距下PcP受P波尾波的影响),通常很难在单个地震记录上同时观测到它们.对于PKiKP,其振幅往往接近于噪声级别,提取该信号常常需要利用到台阵叠加技术.本文利用全球现有的小口径IMS台阵对这两个震相数据进行识取,并对两个震相的全球可观测性进行了比较和分析.在识别弱震相PKiKP的过程中使用相加权技术来增强信号.由于台间距仅有数千米,小口径台阵显著地增加了这两个震相观测的可靠性,同时由于加入多台数据,可以更好估计PKiKP/PcP振幅比等观测的不确定性.本文对前人关于CMB和ICB的研究工作以及该领域最新的研究进展进行了总结,并且对今后利用小口径台阵记录的PcP和PKiKP震相数据研究CMB和ICB结构进行了探讨(如CMB界面的起伏和其上方的超低速层,ICB随时间的变化等).

PcP and PKiKP that reflect from the Earth＇s core-mantle boundary and inner-outer core boundary respectively could provide most direct information about physical properties of the two interface in the Earth＇s deep interior,for example,density and velocity jumps across the boundary. Such information is critical for us to understand the core-mantle dynamics. Combining PcP and PKiKP to study CMB properties also has advantage over the combination of P and PcP,because ray paths of PcP and PKiKP are much more similar in the upper mantle than those of P and PcP,and effects of upper mantle heterogeneity can be well eliminated. However,due to the small reflection coefficients of PcP and PKiKP,they are usually weaker than other main seismic phases,and it＇s often difficult to observe both of them in a single seismograph. Especially for PKiKP,its amplitude is often close to noise level,and stacking techniques are usually required to identify the signal. During recent few decades,the CMB and ICB have been found not simple,and many seismic features related to them has been detected. For example,topography variation and strong heterogeneity near the boundary（ mushy zone near the ICB and ultra-low velocity zone above the CMB）. Short period PcP and PKiKP are very sensitive to those structure,and can provide good resolution for them. In this study,data of the two phases from global small aperture array are collected,and the global observability of the two phases are compared and analyzed. PhaseWeighted-Stacking（ PWS） technique is used to identify the weak PKiKP signal. The small aperture array（ inter-station distance of only few kilometers） significantly improves the reliability of the observation, and due to multi-station data of the array, the uncertainties of observation,for example,PKiKP/PcP amplitude ratio could be better estimated. After summarizing previous works,this study also discusses potential prospect of using this phases pair to study CMB and ICB structure,and proposes several applications（ e. g.,ULVZ,CMB topography and temporary change of ICB）.