利用表面波探討地下構造

The lithospheric structures: insights from studying surface waves

葉玉蓮(Yu-Lien Yeh)

指導教授 ： 陳朝輝

國立中正大學/理學院/地球與環境科學系地震研究所/博士(2013年)

摘要

本研究重點分為兩部分研究，第一部分乃利用IRIS(1984-2007)及OHP(1997-2007)測站所記錄的長週期及寬頻雷利波的頻散資料反演，詳細探討菲律賓海板塊岩石圈的速度構造及非均向特性，並藉由非均向性隨週期變化的情形，進一步了解菲律賓板塊岩石圈的演化與變形。所得結果顯示在週期30秒以下，菲律賓海板塊內的海盆內部呈現群速正異常及非均質的特性。在菲律賓板塊周圍的聚合帶，均呈現速度負異常。特別在琉球海溝，在較深部的地區存在速度在南段及北段的海溝有明顯差異的情形。另外結果野顯示菲律賓海板塊的非均向性隨深度不同而有不同;在短週期，Western Philippine Basin和Shiukou Basin的快波方向為東北-西南方向，和最後階段的海盆擴張方向一致。但在Parece Vela Basin的快波方向較複雜，可是在較長週期大致呈現東北-西南方向。在岩石圈和軟流圈的交界處 (週期約100秒)，快波方向和板塊絕對運動方向一致；在板塊聚合帶及弧後地區，本研究結果顯示其快波方向垂直海溝走向，所得結果符合 A-type 橄欖石(olivine)，在板塊拉力下，礦物排列方向平行最大應力方向；但在深部的快波方向應和板塊隱沒所引起的區域地函流動 (mantle flow)有關。
第二項研究重點為利用短週期的表面波探討台灣西南部地區淺層剪力波速度構造。台灣西部麓山帶為菲律賓海板塊和歐亞板塊碰撞的變形前緣，因此造成複雜的褶曲和斷層系統，而觸口斷層位於西南部麓山帶和西部海岸平原之間。觸口斷層帶附近的地殼構造，特別是觸口斷層以東的山區部分，因地形關係，缺少鑽井資料及進行其他地球物理方法的探測，例如人工震測方法。因此本研究除利用長週期的表面波資料來研究較大區域的岩石圈構造，對於小區域的地下構造是否可以用短週期的表面波來探求進行探討。近幾年所發展出來的噪响比方法(cross-correlation of seismic ambient noise)，此方法對於研究觸口斷層帶淺部地殼特性非常適合，在2004年於觸口斷層東部及西部共設置十一個寬頻地震站，共記錄約一千個微震資料，因地震大部分發生的深度在10公里的位置，雖可藉遊走時資料反演速度構造，但在0-7km的解析度較差，因此本研究企圖利用噪响比方法來探求淺層速度構造，結果顯示所的S波速度構造較之前所得結果較慢，在1-2 km存在一低速層，此低速層應和因造山運動所造成的高度的破碎地殼有關。

關鍵字

地震噪嗓交對比 ； 非均向性 ； 雷利波群速構造 ； 表面波層西成像 ； 菲律賓海板塊 ； 格林函數 ； 觸口斷層

並列摘要

This research is to investigate the lithospheric structures of the Philippine Sea Plate and the collision front of the Philippine Sea plate, using surface wave. Long period and broadband waveform data from 1984 to 2007 extracted mainly from the IRIS Data Center with additional data from 1997 to 2007 extracted from the Ocean Hemisphere Project is used to perform a detailed mapping of Rayleigh wave group velocity and anisotropic structures of the Philippine Sea Plate (PSP). Our results show prominent high Rayleigh wave group velocity anomalies and heterogeneity in three marginal basins of the PSP at period up to 30 s. The convergent boundaries around the PSP are associated with low-velocity anomalies. Heterogeneity of velocity distribution at deep depths along the Ryukyu trench is also observed. Our results indicate that the azimuthal anisotropy in the PSP is depth dependent. The fast direction in the West Philippine Basin and the Shikoku Basin is in NE–SW at shorter periods, which is correlated with fossilized fabric produced by final phase of deformation, whereas the fast direction in the Parece Vela Basin is complex at shorter periods and becomes uniformly NE–SW-oriented at longer periods. At the depths near the lithosphere-asthenosphere boundary, the fast directions become parallel to the absolute plate motion for various parts of the PSP. The fast directions along subduction zones around the PSP are predominately trench-normal. The relatively simple patterns of azimuthal anisotropy may imply that the observed anisotropy originates from the A-type LPO of olivine fabric at shallow depths in the back-arc region and with slab-entrained mantle flow at deep depths.
The Chia-Nan (Chiayi-Tainan) area is in the southwestern Taiwan, and is located at the active deformation front of the collision of the Eurasian continental plate and the Philippine Sea plate, which causes complex folds as well as thrust fault systems in the area. The Chukuo fault zone is a boundary between the Western Foothill and the Western Coastal Plain in the Chia-Nan area. The nature of the crustal structure beneath the fault zone, especially the eastern part of the fault zone with mountain topography, has not been well known in detailed due to lack of drilling data as well as its limitation in using other geophysical methods, such as active source survey. In this study, we deployed an array with 11 broadband seismic stations to monitor the seismicity of the Chukuo fault zone. The array has recorded more than 1000 microearthquakes around this area. It provides an opportunity to use P- and S-wave travel time data to investigate the both the crustal P- and S-velocity in the fault zone, however due to the nature of the earthquake distribution, the ray density is relatively low at depth between 0 and 7 km. In addition, the uncertainty of S-wave reading for small earthquake also a limit in building precise S-velocity profile, Thus, we take the advantages of using cross-correlation of seismic ambient noise to investigate crustal S-velocity profile in the Chukuo Fault area, especially in the mountain area where crustal faulting is a dominated phenomenon. The results indicate that S-wave velocity in the uppermost crust in the Chukuo Fault zone is shown to be slower than previous studies. A low velocity layer exists at depth between 1 and 2 km in the east of the Chukuo Fault. The low S-velocity is related to a highly fractured upper crust due to intensive deformation caused by the orogenic process.
Keywords: Philippine Sea Plate, Surface wave tomography, Rayleigh wave group velocity structures, Azimuthal anisotropy, Trench-normal, P-wave travel time, Seismic ambient noise, Green’s function, Rayleigh wave, Shear-wave velocity structure, Chukuo fault

並列關鍵字

Azimuthal anisotropy ； Rayleigh wave group velocity structures ； Surface wave tomography ； Philippine Sea Plate ； Seismic ambient noise ； Chukuo Fault

參考文獻

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