以SmKS波到時探討地球外核頂部的速度結構

Abstract

地球的固態內核之主要成份為鐵和鎳，另外還有少部分的氧、硫和矽等之比重相對較輕的物質。動力學研究認爲這些較輕的物質會因浮力作用上浮而聚集在地球液態外核的頂部，但目前零星的地震學觀測結果尚未能夠對此說法給出明確的證據。從地震波觀測的角度來看，地球外核頂部若聚集有此較輕的物質，將會使核幔邊界(CMB)以下一部分深度區域内的密度和P波（即外核内K波）速度低於它們在該深度上的全球平均模型PREM的速度值。因此，通過在全球不同地區對在外核頂部傳播的K波之系統性觀測，並且與不同外核頂部速度模型所得到的理論模擬結果進行比較，就可以判斷外核頂部是否有低速區域存在。 本研究中，我們首先收集震央距為120°-140°範圍内的測站之寬頻地震波形記錄，並測量SKKS波與SmKS組合波（包括S3KS、S4KS和S5KS，但主要是S3KS）的到時，將SKKS波與希爾伯特轉換之S3KS波透過交互相關函數得到兩者之間觀測的相對到時差。然後我們使用Direct Solution Method (DSM)來模擬全球平均模型PREM中在相同測站的理論波形,並從理論地震圖上得到模型預測的相對到時差。我們還可以進一步降低CMB以下一部分深度範圍内的P波速度,再計算其DSM理論地震圖，得到外核頂部有低速層存在的模型預測之相對到時差。由於S3KS波在外核頂部的K波部分射綫長度大於SKKS波，外核頂部速度對S3KS波的影響會比對SKKS影響大。因此外核頂部低速層的存在會使S3KS-SKKS相對到時差增大。我們從全球地震數據中心IRIS搜集全球分布規模大於6.0且深度大於400公里之地震，並系統性地處理時間從1990年至今之78個地震，測量得到606筆精確的S3KS-SKKS相對到時差。由我們測量之結果顯示CMB底下400公里的部分有相對於PREM模型的低速層存在。最後透過Bayesian逆推方法來尋找統計上的期望值模型。逆推結果也顯示，在CMB底下550公里深度範圍内地球外核的實際速度低於PREM模型將近0.11%，所以我們的研究結果為外核頂部有輕物質的存在提供了強有力的證據。

The solid inner core of the Earth consists of heavy minerals Fe and Ni with a fraction of light elements such as O, S and Si. These lighter elements are expelled from the inner core during its formation, rise up through the outer core as the result of buoyancy, and are trapped below the core-mantle boundary (CMB). Seismological evidence has been presented both for and against the existence of light materials at the top of the outer core. In this study, we use waveforms of recorded and modeled SmKS waves to investigate the effect of velocity perturbation under the CMB on the differential traveltimes between SKKS and S3KS waves. Due to the long propagation distance and interference with neighboring phases, the arrival times of SKKS and S3KS waves are difficult to define accurately in the records. Therefore, in our analysis we measure both the observed and model-predicted traveltimes by cross-correlating the waveform of the Hilbert-transformed S3KS with that of SKKS. We obtained 606 high-quality S3KS-SKKS differential traveltimes from 78 deep earthquakes (depth >= 400 km). We use synthetic seismograms calculated by the direct-solution method (DSM) in a suite of one-dimensional models with different structural profiles under the CMB to examine the existence of a zone of lowered velocity at the top of the outer core. Then we conduct a Bayesian inversion of the observed differential traveltimes for the velocity structure at the top of the outer core. The Metropolis-Hastings Monte Carlo algorithm is adopted for an efficient sampling of the model space. Inversion result indicates that the seismic velocity in the 550-km layer under the CMB is on average 0.11% lower than that in PREM. The clear depth-dependent velocity profile strongly favors the existence of light elements and chemical stratification at the top of the Earth's outer core.