利用雷利波探討菲律賓海盆之速度構造及非均向性

Abstract

菲律賓海板塊位於太平洋西側，周圍為聚合性板塊邊界，太平洋板塊沿著伊豆-小笠原海溝及馬里亞納海溝向西隱沒至菲律賓海板塊下，在西側的板塊邊界，菲律賓海板塊沿著琉球海溝向西北隱沒至歐亞大陸邊緣下，而在台灣地區形成弧陸碰撞帶，南方則由加洛林板塊沿著雅浦海溝隱沒至菲律賓海板塊南部。菲律賓海板塊內部可劃分成三個主要的海盆:位於西部的西菲律賓海盆，大約從45個百萬年前開始擴張至35個百萬年前；位於東部的四國海盆及帕里西维拉海盆則由30個百萬年前發展至15個百萬年前。而馬里亞納海槽從7個百萬年開始至今持續以弧後擴張機制發展中。而前人研究中探討此區域震波速度非均向性的結果不盡相同，說明此區域的演化仍具有爭議。 在本研究中，我們分析雷利波相速度並建立菲律賓海區域的速度結構以及非均向性模型。首先，我們選用1998年至2014年，規模(Mw)大於5.0的天然地震，共7,914筆，利用這些地震紀錄計算出沿著397條波傳路徑上的相速度頻散曲線，接著，將測量到的頻散曲線配合逆推理論解算在菲律賓海區域子空間中之速度異常值以及速度的非均向性情形。逆推過程中，選用格點間距400公里，並給予適當的參數使得模型正規化以及平滑化，最後獲得週期20至160秒雷利波相速度異常及非均向性模型。 速度異常結果反應出海盆的地質特徵及演化過程，在西菲律賓海盆顯示正速度異常值，而在四國海盆及帕里西维拉海盆顯示負速度異常值。沿著伊豆-小笠原海溝及馬里亞納海溝北段，速度異常呈現正值，此結果反應出太平洋板塊隱沒至菲律賓海板塊下的板塊之幾何形狀。另外，在週期60至100秒，非均向性的快軸方向在菲律賓海板塊的北部呈現東北-西南走向，此方向與海張裂方向平行。非均向性情形在菲律賓海板塊的南部相對較弱，在週期80至140秒，快軸方向與菲律賓海板塊的絕對移動方向相同，並且在南方的非均向性快軸方向也與加洛林板塊的絕對移動方向相同。由此，我們合理的推測，在南部的非均向性反應了地幔流場方向，而西南側的碰撞帶形成天然的屏障使得地幔物質沿著西北側的聚合邊界延伸，非均向性快軸方向受海溝走向影響呈現東北東-西南西走向。相較於前人研究，本研究提供大規模及大尺度下的地幔流場與板塊運動之間的關係，同時也顯示不同的隱沒帶對應不同的地幔流場。

The Philippine Sea plate (PSP) is surrounded by convergent boundaries. On the eastern side, the Pacific plate subducts beneath the PSP along the Izu-Bonin trench and Mariana trench. On the western side, the PSP subducts to the Eurasian continental margin along the Ryukyu trench, driving the arc-continent collision at Taiwan. Further south, the Caroline plate subducts under the southern part of the PSP along Yap trench. There are three major basins in the Philippine Sea: the West Philippine basin developed in 35-45 Ma at west, and the Shikoku and Parece Vela basins opened in 15-30 Ma at east. The back-arc spreading Mariana trough is active since 7 Ma, next to the Mariana trench. Previous studies show a significant discrepancy in seismic anisotropy structures for the PSP region, which perform different scenarios of tectonic evolution for this area. In this study, we conduct both isotropic and anisotropic Rayleigh-wave phase velocity structure of the PSP. The earthquakes larger than Mw 5.0 are screened out from the global seismic databases. Totally, 7,914 events are screened out in the period of 1998-2014 which can form the measurements of Rayleigh-wave dispersion curves along 397 station-pairs over the PSP. The measured dispersion curves are then inverted into the isotropic and azimuthally anisotropic (2Ψ) velocity maps at different periods with the proper damping and lateral smoothing in the LSQR inversion. The inversion is framed by the knots’ spacing in 400 km. The consequent velocity anomalies are referenced to the averaged values corresponding to the periods between 60 and 160 seconds. The derived velocity anomalies are consistent with the basins’ ages. The positive velocity anomalies are seen in the West Philippine basin, whereas the negative anomalies are found in the Shikoku and Parece Vela basins. A positive anomaly is also seen in the eastern PSP along the Izu-Bonin trench and north part of Mariana trench. It may correspond to the subducting Pacific slab beneath the PSP. On the other hand, the fast direction of the Rayleigh-wave anisotropy is found in NE-SW in the northern PSP at 60-100 s, which is parallel to the direction of extension. The anisotropy in the southern PSP is relatively minor. It is found to follow the direction of absolute plate motion (APM) of the PSP and the neighboring Caroline plate at 80-140 s. Such the APM-parallel pattern may reflect the mantle materials flowing as the direction of APM in the south. The plate collision around Taiwan forms a natural barrier to mantle flow resulting an ENE-WSW flow dominated by the NW subducting Ryukyu slab. Comparing to the other studies for the PSP, our result provides large scope of the mantle flows at the level of lithosphere and asthenosphere, reflecting the plate motions joint around the PSP. The variation in kinematic behaviors of the mantle flow at different subduction zones is also revealed.