以絕對重力研究台灣造山運動---觀測與模式比較

Abstract

台灣活躍的造山運動長年以來已經吸引了科學界相當多的注意，有兩個主要的板塊構造理論:厚皮構造理論與薄皮構造理論，來解釋造山運動的現象。然而到目前為止，兩個理論仍各有其未竟圓滿之處。本研究的目的為試圖引入重力觀測，結合其他相關大地測量方法，以實地觀測數據提供上述板塊理論一個驗證的管道。為此，我們設定一條西起西岸平原，東至海岸山脈，橫貫台灣的剖面線作為研究的對象。針對前述兩種板塊構造理論，我們以模擬計算的方式預估出理論模式所推衍出之地表升降的幾何變化，以及源自地表位移，質量轉移等現象所引發之重力效應。更進一步，規劃中的外業觀測數據蒐集將可提供模式化的板塊構造模型一個邊界值條件，用以驗證模型的合理性甚或改善模型。依據這種構想，我們沿南部橫貫公路為主要作業路線，規畫了一個跨越台灣五大地質區的重力監測網。監測網中的絕對重力點一方面提供重力網可靠的控制基準，另一方面在前後長達五年的研究時段中維繫時間框架的一致性。於此同時，相對重力子網將從主要路線向外延伸，拓展了監測網良好的空間涵蓋程度，並提升了目標剖面額外的資料密度。而為了達成足夠的精度以萃取期望中的地球物理訊號，針對諸如固體潮，海潮重力效應，大氣效應，地下水文效應等周期性或非週期性的背景干擾因素，我們也將發展改正模式，仔細率定模式參數，希望藉由更完善的模式來支持研究的進行。我們預期上述年度反覆監測所得到的數據將可提供板塊構造理論一個堅實的驗證機會，並在可見的將來，發掘更多值得探討的問題。

The active ever orogeny in Taiwan has long attracted the attentions from scientists. Two major consequent theorems, recognized as thick skin tectonic and thin skin tectonic, have been developed to justify the underlying orogenic dynamics, yet neither has been proved to be the solid fact so far. It is the objective of this research to introduce gravimetrical observation, along with other geodetic data, serving as model verification. Along a profile crossing Taiwan from west coast plain to east coastal Range, displacements and gravitational variations inferred from the two tectonic models mentioned above can be predicted over a given time domain through computational simulations. The gravimetrical and GPS observations could then validate the theoretical predictions or even more, take the part as extra boundary conditions for tectonic model refinement. In the wake of this mindset, this research proposes a gravimetrical monitoring network to study the dynamic of mountain ranges along the Southern Crossing highway, which would probe all five primary geological regions of Taiwan. The absolute gravimetry in the network provides reliable datum and keep the temporal consistency from a campaign to next campaign throughout the 5 years monitoring; in the mean time, the relative gravimetry, branched out from the main campaign routine, extend the spatial coverage and enhance the data density for the studied profile. To achieve the accuracy sufficient enough to distinguish the targeted geophysical signal, decent models to segregate periodic or non-periodic background factors, such as earth tide, oceanic gravitational effects, atmospheric effect, and hydrological effects, have to be carefully developed, calibrated, and even customized for this specific study. We expect that the data collected from this series of annually repeating campaigns will offer a solid proof for tectonic theorem and very likely will reveal more subjects to be exploited in the future.