利用GPS觀測資料探討宜蘭平原之現今地殼變形

Abstract

宜蘭平原位於沖繩海槽的西南端，一般認為此處以橫移伸張為主的地殼變形作用，主要是受到呂宋島弧斜向碰撞歐亞板塊、菲律賓海板塊隱沒及沖繩海槽張裂構造活動影響。近期的地震活動顯示，在平原的南半部，主要的地震機制走向橫移斷層並帶有少許伸張分量。根據中國石油公司之震測資料和地面地質之對比與解釋，宜蘭平原地下構造相當複雜，被數條斷層切過。雖然此地區之活動構造有各種不同的討論，但由於平原地區覆蓋大量的沉積物且斷層未出露地表，因此宜蘭平原之活動構造的確切位置及活動特性仍不清楚。因此，本研究藉由分析中央地質調查所2002年至2006年35個非連續式記錄之GPS觀測站，以及12個中央氣象局和2個宜蘭大學之連續式記錄之GPS觀測站的資料，來闡明宜蘭地區主要活動斷層之空間分布，並探討其斷層發育與沖繩海槽西延之拉張作用間的關係。由於2005年3月5日發生宜蘭雙主震，對宜蘭地區的測站觀測造成一定的影響，本研究欲了解此次地震活動的影響，藉由連續追蹤站之時間序列及半彈性空間斷層錯位模型的分析，修正2005年宜蘭雙主震所造成的同震位移，再利用修正後結果估算各測站相對於穩定大陸邊緣（澎湖白沙站，S01R）之速度場。GPS水平速度場之分析結果顯示，宜蘭地區向東南方移動，速度值約為0.9至37.3 mm/yr，速度方位角介於3°- 174°之間；而最大值位於蘇澳附近。整體速度場有兩個趨勢：一為由北向南遞增，另一為由西向東遞增。由南北向速度剖面來看，蘭陽溪南側之速度分量（速度量值為3.1- 37.3 mm/yr，速度方位角為20°- 162°）遠大於該溪北側之速度分量（速度量值為0.9- 9.5 mm/yr，速度方位角為3°- 174°），故推論沖繩海槽為不對稱之弧後張裂，且南側之張裂速度較北側快。此外，沿著宜蘭平原南緣分布（牛鬥至蘇澳）之速度場，由西往東的變化為順時針旋轉，整體的變化由26°至148°，共旋轉了122°。宜蘭平原之主軸應變環境大致以西北-東南方向伸張變形為主，其最大主軸應變率為0.15-2.32 μstrain/yr，最小主軸應變率為-2.24-0.65 μstrain/yr；其中，值得注意的是平原內部伸張應變方向，由西向東的變化從西北-東南向往東逐漸轉成接近南北走向，故其伸張方向由西向東順時針一共旋轉了43°。速度場及伸張應變旋轉之現象，可能是由於宜蘭平原西側同時受到菲律賓板塊的擠壓應力及琉球海溝隱沒後退所產生之伸張應力，形成了西北-東南之伸張方向之橫移壓縮環境；而平原東側則受到沖繩海槽張裂作用向南延伸之影響大於板塊碰撞之擠壓應力，因而轉為南北方向之伸張應變。此外，藉由速度場速度向量的不連續，推測宜蘭平原上有四條構造，由北而南分別為牛鬥斷層、坑四斷層、濁水斷層及三星斷層等活動構造。綜合速度剖面及剪應變之結果，蘭陽溪北側平原內為一系列正斷層系統，而南側為具左移分量之正斷層系統。近期台灣東北地區受到沖繩海槽張裂之影響，造成早期造山時呈東北-西南走向之山脈，在受力彎曲時發生層間的左移滑動，此一構造行為即沿著地質弱帶發生，使得觀測到宜蘭平原南邊之正斷層帶有左移的分量。

The northeast Taiwan is undergoing the transtensional deformation in a transition zone due to the collision and subduction between the Philippine Sea Plate and the Eurasian Plate. The Ilan Plain is a triangular-shape flat alluvial plain located on the southwest end of the Okinawa Though which behind the Ryukyu trench-arc system is an active marginal back-arc basin. It is generally considered that the active crustal deformation is deeply influenced by the westward opening of the Okinawa Trough. Moreover, focal mechanisms of a doublet (ML 5.9 and 6.0) occurred on 5 March 2005 showed that the strike-slip faulting with minor normal component are dominated in the southern part of plain. According to seismic reflection profile, the fault systems in the Ilan Plain were preliminary proposed. However, how these faults accommodate the deformation is a crucial topic for the characteristics of crustal deformation at the transition zone between subduction and collision. Therefore, we use the recent GPS measurements to better understand the current deformation pattern in study area. We analyzed the GPS data from 33 stations observed by the Central Geological Survey, 12 continuous GPS stations of the Central Weather Bureau and 2 continuous GPS stations of the National Ilan University to understand the crustal deformation and the spatial distribution of active structures in the Ilan Plain of NE Taiwan. The crustal deformation of the Ilan area is affected by the 2005 Ilan earthquake doublet, therefore, we analyze time series of continuous GPS stations and evaluate co-seismic displacement of the Ilan earthquake doublet by using dislocation model. Thus the secular velocity field related to a stable continental margin station, S01R is recalculated. The horizontal station velocities relative to the Chinese continental margin show that the Ilan area is moving southeastward（the station velocities between 0.9 mm/yr and 37.3 mm/yr in azimuths between 15° and 340°）and the fastest station is near the Suao. The tendency of the Ilan area is that the stations velocities increase from north to south and from west to east. The station velocities at the northern flank of the Ilan Plain are insignificant（the station velocities between 0.9 mm/yr and 9.5 mm/yr）. On the contrary, the magnitudes of station velocities at the southern flank（the station velocities between 3.1 mm/yr and 37.3 mm/yr）are larger than those of the north part. Thus, the opening of Okinawa trough is asymmetric, where the extension rate in the southern part of the trough is faster than the northern one. In addition, the station velocities rotate clockwise along the direction from 26° to 148° in the southernmost margin of the Ilan Plain. The principal extension strain rate axes in the Ilan plain mostly trend NW-SE directions. The maximum principle strain rate is 0.15-2.32 μstrain/yr and the minimum principle strain rate is -2.24-0.65 μstrain/yr. The extension directions rotate clockwise in 43° form western part to eastern part of Ilan Plain. The western plain was affected by both the compressional force by plate’s collision and the tensile force by Ryukyu trench retreat. Thus, the transtensional regime is predicted by the strain rate field. Based on the discontinuous velocity vectors, we supposed there are four faults in the Ilan plain, from north to south, the Niutou fault, the Kengssu fault, the Choushui fault and the Sanhsing fault, respectively. Due to the results of the velocity profiles and shear strain rate field, the faults in northern Ilan plain are dominant with normal fault systems and sinistral fault systems in southern plain. The southward opening of the Okinawa trough is a main driving force in the northeastern Taiwan and results in bending the north Taiwan mountain belt. The left-lateral fault system appears at southern plain was caused by the bending of the mountain belt.