以絕對重力研究台灣造山運動: 觀測與模式比較(II)

Abstract

本計畫是用重力及GPS 方法來研究台灣造山運動，內政部提供研究所需要之儀器設備，交通大學、中央大學、工研院及法國Frederic Masson 教授所帶領的團隊共同合作。由於台灣地處於歐亞大陸板塊及菲律賓海板塊交界，而且以每年7 公分的速度向西北方移動。板塊運動所造成之垂直位移和質量變化可以用GPS 及重力來監測。本計畫主要的三個目標為：(1)利用絕對重力儀FG5、超導重力儀SG、相對重力儀Graviton-EG 及GPS來驗證地殼活動跟台灣的造山運動的關係(2)增進對台灣造山運動之瞭解及(3)研究在1946 年新化斷層發生6.3 級的地震，其北側屬沉降之前陸盆地至今未發生隆起的原因。由大量降雨、潮汐和地震的影響，造成很明顯與時間相依的重力變化，在研究造山運動之前這些雜訊都要被去除。我們將建立影響重力變化之固體潮、海潮覆載、地下水文及3-d 大氣模式改正。建立優化的數學模式來處理有關絕對重力及相對重力之數據。我們也將建議在這實驗區內擺設超導重力儀。超導重力將提供連續且長週期之重力數據，以獲得時間相依的重力變化及造山運動相關之重力訊號。在新化斷層附近的GPS 顯示板塊向西隱沒形成島弧的同時，也造成歐亞板塊邊緣張裂，而形成嘉南平原。本計畫的研究範圍涵蓋新化斷層，用GPS 及重力觀測資料將提供新化斷層相關現象之有力證據及新的資訊。

In collaboration with a French team led by Frederic Masson, this project aims to study Taiwanese Orogeny using gravimetry and GPS, taking advantage of the gravimetry facility provided by the Ministry of the Interior in southern Taiwan and Hsinchu. Taiwan is located at the converging zone of the Eurasia and the Phillippine Sea Plate and moves northwestwards at a rate of 7 cm/year. Such a plate motion creates vertical displacements and mass changes that can be detected by repeat gravimeter and GPS measurements. Three major objectives of this project are (1) use measurements of a FG5 absolute gravimeter, Superconducting gravimeter, Graviton-EG relative gravimeter and GPS to validate the tectonic motion model associated with Taiwanese orogeny, (2) improve our understanding of the Taiwanese orogeny and (3) investigate the pattern of deformation on both sides the frontal thrust fault corresponding to the 1946 Hsinshua earthquake. Due to large rainfall and oceanic and seismic effects, temporal gravity variations in Taiwan are significant, and aliased into the gravity signal of orogeny and must be removed. We will develop models accounting for such gravity variations as solid earth tide, ocean loading, hydrological and atmospheric effects. Optimal methods of processing absolute and relative gravity data will be developed. We will also propose to install a superconducting gravity site in the study area. A superconducting gravimeter will provide continuous and long-term gravity measurements that can be used to model temporal gravity changes and provide gravity sginatures originating from orogeny. Current GPS results near the Hsinshua fault show that in this region the shortening is accompanied by a large subsidence of the SW coastal plain. GPS and gravimetric measurements will be collected in this region to verify this shortening and provide further information.