台灣花東縱谷中段之震間變形

Abstract

花東縱谷位於歐亞板塊和菲律賓海板塊的交界帶，此地區發生過多次大地震事件。位於花東縱谷的縱谷斷層為現今台灣本島上相當活躍的斷層，為台灣地震頻率最高的斷層；尤其位於縱谷南段的池上斷層，呈現快速潛移，在過去有相當多的研究成果。然而，花東縱谷中段瑞穗和玉里一帶的構造活動性研究則相對較少。前人研究根據野外實地調查或地震資料來判釋花東縱谷中段的主要構造線，但缺乏長期且精確的研究資料進行分析探討。 本研究主要目的是藉由大地測量資料，包含GPS測量資料和精密水準測量資料，來探討中段縱谷斷層和中央山脈斷層的活動特性和震間變形。GPS資料收集分成GPS連續站及移動站資料收集，連續站資料來源為中央研究院及中央氣象局於花東縱谷中段所架設的GPS連續觀測站，而移動站部分，本研究採用和中央研究院地球科學所合作於2003年7月至2012年12月所接收的資料。本研究希望能透過分析花東縱谷中段的大地測量資料，藉以獲得此區域近十年各測站相對位移變化量，建構此區域近期的構造活動特性並推得地下的構造幾何形貌。 本研究以Okada(1985)彈性半無限空間模型進行參數之推估，搜尋斷層參數之可能範圍，並了解參數之間的關係，進而建構震間期縱谷中段之二維彈性半無限空間錯位模型。同震變形部份，則以彈性半無限空間模型，逆推最佳的2013年瑞穗地震斷層幾何參數及計算斷層面上的同震滑移量分佈，建構瑞穗地震同震變形模型。利用以上模型結果，讓我們能夠更加了解縱谷中段的構造幾何型態，並推測未來可能帶來的地震災害潛能。

The Longitudinal Valley in eastern Taiwan is generally considered as the suture between the Philippine Sea plate and the Eurasian plate. Many historical large earthquakes occurred along this suture. The southern part of the Longitudinal Valley has attracted many studies, mostly due to the rapid-creeping Chihshang segment of the Longitudinal Valley fault. However, only a few detailed studies focused on the middle part of the valley. In order to understand the interseismic deformation patterns along the middle part of the Longitudinal Valley, we analyzed GPS (Global Positioning System) measurements and leveling results to obtain the velocity field and strain pattern of the area. We calculated the GPS velocity field from continuous stations installed by the Central Weather Bureau and Academia Sinica that have been measured in the past twelve years, as well as campaign-mode stations measured at least annually from 2003-2012. Our analysis is intended to understand the activities of the Longitudinal Valley fault and the Central Range fault in the Rueisuei and Yuli areas, in the middle part of the valley. We calculated the velocity field and velocity cross-sections across the middle segments of the Longitudinal Valley fault and the Central Range fault, and derived interseismic dislocation model for these two faults using the data from GPS and leveling. The results show that the Rueisuei and Yuli areas are characterized by compressional deformation in northwest-southeast direction. In the northern Rueisuei area, the Central Range fault appears to have higher rates of reverse motion than in southern Rueisuei area. The Longitudinal Valley fault appears to have higher rates of both reverse and sinistral motions in the southern Rueisuei area. Our geodetic analysis provides new observations of interseismic deformation patterns of the middle segments of the Longitudinal Valley fault and the Central Range fault. These new observations will enable us to further understand the earthquake hazard potential of this part of the Longitudinal Valley suture.