伊豆-小笠原與馬里亞納隱沒板塊下方之非均向性

Abstract

地表觀測提供了板塊運動的數據，在板塊之下的地函作用模式卻無法用儀器直接觀測，利用剪力波分離量測隱沒帶附近之非均向性，則能為隱沒帶的動力隱沒機制提供直接的證據，將有助於我們了解地函在板塊學說中所扮演的角色。本研究使用發生在馬里亞納與伊豆-小笠原隱沒帶的地震，移除了測站下方之非均向性，使用遠震的S波量測隱沒板塊下方的非均向性，量測結果顯示兩區域呈現不同的快方向分布模式，馬里亞納地區的快方向主要平行於海溝的方向，伊豆-小笠原地區的快方向則不與海溝平行或垂直，與板塊運動方向亦無關聯，兩區域的延遲時間平均落在2 s左右，最大可達3 s。進一步分析量測結果，發現伊豆-小笠原的剪力波分離參數隨著S波之初動排序呈現以90˚為週期的消長，且根據震源深度及波線穿越的區域，大多數的非均向性來自於轉換帶與下部地函，此結果暗示了在上部地函之下可能存在兩層可造成非均向性的物質，我們結合了過去研究的成果，即給定地函溫壓條件下應力造成之晶格優選排列，推論出伊豆-小笠原隱沒板塊下方之非均向性可能貢獻於存在轉換帶的wadsleyite與下部地函的bridgmanite。

Understanding the mechanism of plate subduction helps us put together a whole picture of how the mantle works under plate tectonics. The sub-slab seismic anisotropy serves as a direct tool for illuminating subduction dynamics, implying the flow direction and deformation patterns of subducting slabs. We measured source-side shear wave splitting with receiver-side correction for the Izu-Bonin and Mariana subduction zones, and find different fast direction (ϕ) patterns of these two regions. Trench parallel fast splitting directions dominate beneath the Mariana slab except for shallow events, while the fast directions of Izu-Bonin seem to be random with respect to the slab contour. The delay times (δt) of the two regions are ~ 2 seconds in average, maximum value may up to ~ 3 seconds. The variations of splitting parameters with S wave initial polarizations in Izu-Bonin region implies the sub-slab anisotropy materials more complex than a single anisotropic layer. A forward approach shows that a two-layer structure better explains the splitting observations than a single anisotropic layer. In Izu-Bonin, these splitting parameters are mainly at transition zone depths, suggest the presence of anisotropy at transition zone and lower mantle. Combined with the deformation-induced crystallographic preferred orientation (CPO) evidences, the wadsleyite and bridgmanite may cause the two-layer anisotropy at midmantle beneath Izu-Bonin.