- 學位論文
利用長支距震測資料和重力模擬探討南段琉球弧前地區的地殼構造
Crustal structure of the southernmost Ryukyu forearc region from large offset seismic data and gravity modeling
摘要
在臺灣東部海域琉球弧前地區，過去研究認為此區受到隱沒板塊斜向聚合的影響，整個隱沒帶同時存有垂直海溝與平行海溝的應力分量。同時，受到南沖繩海槽擴張的影響，琉球群島向南遷移，使得上覆板塊的地殼，受到多種大地應力的作用之下，地殼構造變得十分複雜。因此，此區域的地殼構造特徵特別是地殼是否紀錄到這些複雜應力作用所產生的地殼變形，便值得深入探討。本研究重新處理分析2009年臺灣TAIGER計畫中一條在臺灣東部海域，以西北-東南走向橫跨過琉球弧前地區的多頻道反射震測與海底地震儀陣列的測線，MGL0906-22N，獲得反射震測剖面與地殼速度構造模型，並以速度構造模型為基礎進行重力模擬。本研究利用反射震測速度分析所求得的淺層速度構造給予速度模擬初始模型的淺部速度控制，藉以取得較為可信之初始速度構造模型，並利用能夠聯合反演折射與反射走時資訊的tomo2d軟體來建立最終之速度構造模型，以得到反射面的深度資訊。在反射震測剖面觀察到弧前盆地中有水平與傾斜的幾套沉積地層，與過去研究結果相似。速度構造模型中以等速線4.5公里/秒為基盤面；等速線6公里/秒為上部地殼和下部地殼的分界。速度構造模型結果顯示有一個深部反射面其深度大約介在25~30公里之間，本研究認為此反射面很可能為菲律賓海板塊的莫荷面。本研究認為在測線的東南段，上部地殼的厚度比較厚，可能與斜向隱沒有關。重力模擬的結果顯示，理論值與觀測值趨勢相近，在模型的東南方，增積岩體的下方，深度25公里之下，有高密度的物質存在，可能是隱沒的菲律賓海板塊地函。
並列摘要
The southernmost Ryukyu subduction zone, where the Philippine Sea Plate (PSP) subdutcs beneath the Eurasia Plate (EP), is both complex and active, and induces high seismicity offshore eastern Taiwan. Large negative gravity anomaly has been reported in this section of the Ryukyu forearc region, intense crustal deformation was also observed in this forearc region. It is an important issue that how the crustal structures beneath Ryukyu forearc region are deformed due to the subduction-collision process. In this study, we present a re-processed multichannel seismic (MCS) reflection profile, its P-wave velocity-depth structure model and a density model along a large-offset (using a 6-km long streamer) seismic profile MGL0906-22N which runs in NW-SE direction in the forearc region of the southernmost Ryukyu subduction zone. In the western part of the MCS profile, two sedimentary sequences in the Hoping Basin which have been mentioned in previous studies are clearly observed. In the crustal velocity model, the layer boundaries in the upper crust may be defined by two velocity contours of 4.5 km/s and 6.0 km/s, represent the top of the basement and lower crust, respectively. The depth of a deep reflection interface is lying between 25 km to 30 km in our velocity model, we suggest this reflection interface could be the moho of the down-going Philippine Sea Plate. The thicker upper crust is observed in the southeast part of our profile. We suggest that the high subduction obliquity may cause the thicker upper crust. In gravity modeling, the model of gravity modeling profile shows similar variation as the observed gravity anomaly profile. Beneath the accretionary prism, there are some high density material deeper than 25 km.
參考文獻
延伸閱讀
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