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研究生:陳勁吾
研究生(外文):Chen, Chin-Wu
論文名稱:下地幔底部D''''層的剪波速梯度構造
指導教授:郭本垣喬凌雲喬凌雲引用關係
指導教授(外文):Kuo, Ban-YuanChiao, Ling-Yun
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:海洋研究所
學門:自然科學學門
學類:海洋科學學類
論文種類:學術論文
論文出版年:2000
畢業學年度:88
語文別:中文
論文頁數:38
中文關鍵詞:D'''' 層繞射剪波
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核幔邊界上方厚約200~300公里的D" 層,是下地幔中物理性質最不均勻的區域。位於地幔對流系統的底層,D" 包含了瞭解地幔動力機制的關鍵,而地震波的速度變化是重要的性質之一。近年來,全球三維地幔速度模型的研究已顯示出D" 大尺度側向變化的特徵。然而,這些全球模型往往無法有效分辨波速在深度方向的變化,因此,針對速度梯度的區域性研究便顯得格外重要。本研究利用Sdiff /SKS振幅比和Sdiff-SKS差別走時同時限制D" 的區域平均速度構造。Sdiff在D" 中長距離的傳播使得Sdiff /SKS振幅比能敏感反映出D" 的速度梯度。Sdiff-SKS走時則反映了D" 的平均速度,對D" 的速度構造提供相輔的約束條件。
本論文分析了通過西太平洋、北太平洋及中大西洋下方D" 的震波資料,將量得的Sdiff /SKS振幅和一維地球參考模型PREM相比,得到振幅殘餘及其隨距離變化的曲線。雖然各區域的振幅曲線變化不一,但值皆為正,代表實際的Sdiff振幅比PREM預測的來的大,即實際的速度構造較能讓Sdiff在繞射時保存其能量。這個觀察表示D" 可能普遍存在波速隨深度遞減的負梯度構造。另一方面,走時殘餘的分布則顯示,西太平洋和中大西洋南半部的D" 平均速度較慢,而北太平洋和中大西洋北半部的D" 是快異常。
在各區域中,我們以簡單的一維模型來模擬觀察到的兩種殘餘。假設地幔中的聚焦作用及D" 中的衰減等因素只能平移振幅殘餘曲線而不能改變其變化趨勢,我們只模擬振幅曲線的斜率。我們發現,西太平洋和中大西洋的D" 速度梯度達每百公里-0.05 km/s,遠負於任何全球參考模型所制定的值。如果全歸因於熱效應,每百公里-0.05 km/s的梯度代表在300公里的深度內溫度變化達1000 K的數量級。本論文最後也討論了不同的地幔熱力學性質對此一估計的影響。
謝誌……………………………………………………………………….i
摘要………………………………………………………………………ii
目錄……………………………………………………………………...iii
圖目……………………………………………………………………...iv
第一章 緒論……………………………………………………………1
1.1 不均勻的D" 層……………………………………………...1
1.2 D" 的波速梯度研究………………………………………..6
1.3 研究方向……………………………………………………...7
第二章 Sdiff和SKS的振幅與走時……………………………………..9
2.1 Sdiff /SKS振幅殘餘……………………………………………..9
2.2 Sdiff -SKS走時殘餘…………………………………………...11
第三章 資料分析……………………………………………………...14
3.1 資料的選取………………………………………………….14
3.2 各區域分析結果…………………………………………….15
3.3 與其他區域的研究比較…………………………………….19
第四章 一維速度模型………………………………………………..22
4.1 基線偏差…………………………………………………….22
4.2 一維模型模擬……………………………………………… 22
第五章 討論…………………………………………………………..28
5.1 梯度與熱效應……………………………………………….28
5.2 熱力學參數………………………………………………….29
第六章 結論…………………………………………………………..32
參考文獻………………………………………………………………..33
附錄一、本論文使用的地震資料……………………………………..36
附錄二、本論文使用的測站資料……………………………………..38
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