臺灣地區重力變化：觀測及分析

Abstract

本論文主要以臺灣地區各式地面重力觀測成果，檢視並分析各地區重力變化情形，配合高程變化等相關資訊對重力變化機制作初步分析。2003年及2007年，共進行了兩次的全臺一等水準點上重力測量，所有重力觀測量皆須經過環境改正後再行應用或做後續計算，其中本文特別針對海潮負載效應改正作較深入的探討及分析。海潮負載效應對地處西太平洋的臺灣而言，重力觀測量的影響極為顯著，本文即以適用臺灣地區的OTL10區域海潮模型，配合SGOTL程式計算求得海潮負載效應，對臺灣各地的重力觀測量作改正，並同時以g7及SPOTL程式計算所得的海潮負載效應對絕對重力觀測量做改正，其中以SGOTL所改正後的絕對重力值，可以得到最佳的精度表現。相對重力觀測量的標準偏差表現，主要來自於觀測時環境干擾的程度，而藉由增加重力的觀測數量，不但可重複檢核觀測量的閉合差，更可有效提升重力成果的精度。就以2003至2007年的重力變化情形來看，臺灣中央山脈北段部分重力值變化量可達-0.085 mgal/year，臺灣西部地區則達到0.061 mgal/year，經比較正高變化情形後，可以明顯研判中央山脈部分地區的造山運動仍持續進行中，而臺灣西部則有地層下陷的情形發生。於2004年至2008年期間，本文利用絕對重力及相對重力方法檢視雲林地層下陷情況，以FG5絕對重力儀於雲林縣同安國小觀測所得重力值變化率達到22.72 μgal/year，同時以相對重力進行六次重力網形觀測，經平差計算後求得雲林地層下陷區的重力變化值，並與該區域各地的正高變化率作比較，可初步驗證重力方法對於地層下陷監測的可行性。

The temporal gravity changes in Taiwan are used to examine the crustal deformation in Taiwan. All gravity observations are corrected by the environmental effects, such as solid earth tide, ocean tide loading, polar motion, atmosphere and underground water. This study will put a special emphasis on ocean tide loading. Ocean tide loading(OTL) effect of gravity is large in Taiwan and around its nearby area. Three different OTL models are tested to improve the accuracy of absolute gravity around the coast of Taiwan and its islets. The model SGOTL with the latest regional ocean tide model of Taiwan is assimilated with local tide gauge records. In Taiwan, OTL10 and the high resolution DEM result in the optimum accuracy in the OTL corrections of absolute gravity measurements. It’s obviously to shown the standard deviations of adjusted gravity have reduced by using mixed method of ladder and star. Two major relative network campaigns around the whole Taiwan are made from 2003 to 2007. The relative gravity observations are adjusted using the weight-constraint least-squares method. With the 4-year gravity changes, we have found that orogeny occurs on the central ridges and land subsidence occurs in southwestern Taiwan with average rates of -0.085 mgal/year and 0.061 mgal/year, respectively. A gravity network in the Yunlin County in western Taiwan is established to determine gravity variations caused by large land subsidence. At station TAES, the gravity change rate from absolute measurements by FG5 is 22.72 μgal/year. The gravity-height admittance factor in Yunlin subsidence area is -5.25 μgal/cm, instead of -1.97 μgal/cm, due to the complicated mechanism of subsidence. This result can be a prototype method of subsidence monitoring by gravimetry.