臺灣半動態基準地表變形模式精進研究──同震格點位移模型新增時機之探討

Coseismic Grid Displacement Model of the Semi-dynamic Datum in Taiwan

臺灣位處歐亞板塊及菲律賓海板塊之交界。由於板塊的相對運動，國家基本控制點間之網形會因塊體相對位移而扭曲變形，造成臺灣現行之靜態坐標系統TWD97[2010]逐漸失去原有的精度。而在原有靜態坐標基準中加入地表變形模型之半動態坐標基準，則是維護臺灣坐標基準精度之有效方法。其中，「同震格點位移模型」是地表變形模型中的其中一個重要元素，但是該如何決定新增同震格點位移模型的時機？為了解決這個問題，本研究首先分析GNSS連續站坐標時間序列，確認臺灣地區GNSS之平面坐標精度約為18mm。換言之，當地震同震位移量大於18mm，就可以被現行之GNSS觀測網記錄到。此外，由於中央氣象局在地震發生後所提供的即時地震資訊為地震位置、深度與區域地震規模（ML），因此本研究透過臺灣及其鄰近地區之背景地震分布，搭配斷層正演模型，搜尋會對臺灣陸地造成最大地表同震位移量可能超過18mm之地震的震源位置、深度與規模。根據模型結果顯示，要在臺灣陸域產生大於18 mm之同震水平位移量，其地震規模至少要超過ML 6.1；越遠離臺灣陸域，則所需的最小地震規模就越大。此外，隨著震源深度的增加，所需的最小地震規模亦由ML 6.1增加至約ML 6.4。最後，本研究蒐集自1999年集集地震後，臺灣地區已發表之地表同震位移場文獻，驗證本研究所建議之時機的正確性。 Taiwan is located at the boundary between the Eurasian and Philippine Sea plates. The accuracy of static geodetic datum TWD97[2010] in Taiwan is therefore gradually decreasing because the network geometry of the control points is distorted due to the relative motions among plates. The semi-dynamic geodetic datum which contains the surface deformation model is proposed to maintain the accuracy of geodetic datum. The coseismic grid displacement model is one component in the surface deformation model. However, it is unclear that whether the earthquake size is large enough and its location is proper to build a coseismic grid displacement model when an earthquake occurs. To answer this question, the resultant precision of approximately 18 mm according to the analysis of continuous GNSS time series was first estimated in horizontal coordinate at Taiwan in this study. Because only three parameters, including the epicenter, focal depth and ML, related to the coseismic surface displacement are announced in the rapid report by Central Weather Bureau, the background seismicity in Taiwan area and the forward fault modeling based on aforementioned three parameters were adopted to estimate the locations and sizes of earthquakes surrounding Taiwan which may generate the inland displacement larger than 18 mm. Based on the modeling results, the size of earthquake should exceed ML 6.1. The farther from the land area of Taiwan, the required magnitude is larger. As the focal depth increases, the required minimum magnitude also increases from ML 6.1 to ML 6.4. Finally, the published coseismic displacement fields since 1999 Chi-Chi earthquakes were collected to confirm the correctness of the forward modeling results in this study.