東北歐亞大陸地幔底部之極低速帶的探討

Abstract

極低速帶（ULVZ）指的是發生在下地幔底部核幔邊界（CMB）上方厚度約數十公里，P波速（Vp）和S波速（Vs）驟減的構造，通常被發現零星地散布在全球。本研究利用ScP經過此一構造衍生出的反射和轉換波相來限制ULVZ的性質。我們分析西藏地區INDEPTHⅢ（1998－1999年）陣列，以及Hi-CLIMB（2002－2005年）陣列的資料。在諸多遠震波形中只找到一個在庫頁群島附近的地震，其ScP後方約9秒處出現明顯的多餘波相，在前方13秒也出現微弱的擾動，藉合成波的模擬得知ScP的後擾波為ScsP，前擾波為SdP。模擬ScsP-ScP時間差和ScsP/ScP振幅比得到ULVZ最適模型為Vp和Vs分別減少10%和30%，密度略增10%，而厚度約為60公里。此一模型也能解釋SdP的振幅和時間。此一ULVZ的速度變化和其它ULVZ模型十分類似，但厚度卻是最厚的。本論文的ULVZ落在俄羅斯的海參威附近，因為其他地震在鄰近反彈的ScP沒有伴隨任何明顯的異常波相，ULVZ的水平分布可能小於200公里。

Mapping global distribution of ultra-low velocity zone (ULVZ) at the core-mantle boundary is crucial to understanding of mantle dynamics and core-mantle interaction. We analyzed records of two Tibetan arrays, INDEPTHⅢ and HiCLIMB, to search for evidence for the presence of ULVZ. The array-stacked ScP phase from one event in Kurile region exhibits significant energy about 9 s after ScP and a small perturbation about 13 s before ScP. Synthetic waveform experiments show that the post- and pre-cursors to ScP can be modeled as ScsP and SdP with a ULVZ of the following properties: δVP = -10 to -12%,δVS = -30%, δρ= 10%, and thickness D = 60-65 km. The thickness is so far the greatest among all the ULVZs previously identified over the globe. This ULVZ is located at a spot beneath NE Eurasia where the lowermost mantle has never been explored before. The distribution of normal ScP bouncing in the neighborhood constrains the lateral dimension of this ULVZ to be about 200 km.