- 學位論文
臺灣花蓮北部板塊交接點上部岩石圈週期碰撞與鬆弛模式
Cyclic Upper Lithospheric Collision and Relaxation at the Junction of Collision to Subduction in Northern Hualien, Taiwan
摘要
本研究利用連續GPS測站、井下應變儀與地震資料，探討花蓮北部之歐亞大陸板塊與菲律賓海板塊交接點之地殼變形機制，我們發現兩板塊間震期於此區聚合作用常伴隨許多背景地震，地殼週期性彈性回跳之反向運動易引發較大規模地震以及更多地震。花蓮北部連續GPS時間序列中普遍存在著異常大幅東西向年週期位移變化，其中波峰波谷差在原始資料中最大可達30 mm，15個連續測站平均亦有20 mm變化，多數連續GPS站於每年11月至4月維持間震期向西移動方向，5月至10月則反向往東移動。許多研究指出GPS易受季節性環境因子之影響而產生年週期或半年週期之訊號，對於垂直分量影響較大，通常為水平分量變化幅度三至十倍之間。我們利用花蓮北部最新之大地測量與地震觀測資料檢驗過去普遍認為由季節性變化所主導週期性訊號之概念，多重比較結果顯示此區域並不適用此一概念解釋，各種主要季節變化因子(如：大氣壓力、海水面位差、溫度、地下水、降雨等)所導致之地殼變形，以及與GPS時間序列之相關性皆不顯著。另外，由連續GPS時間序列評估之半年反向運動平均速度場，我們得到近乎菲律賓海板塊斜向碰撞方向結果；GPS計算之時間域平面應變與井下應變儀亦呈現相同之東西向年週期壓縮與鬆弛，且能與區域性規模大於5之地震有良好對應。所有證據皆明顯指出：板塊碰撞過渡至隱沒之交接點之花蓮北部，於大地測量所觀察到水平向年週期性大幅度變化，並非是地表季節性因子改變所造成，而是源自更深部之地體構造動力。因此，本研究以新模式對此區域地殼變形做出不同解釋，我們認為上部岩石圈於此板塊交接點具有年週期碰撞與鬆弛之機制，其規律地以彈性回跳方式釋放累積應變，如此深部大尺度運動便易反應於地表GPS與井下應變儀所觀測之強烈年週期訊號。
並列摘要
Continuous GPS (CGPS) and seismic data from northern Hualien in Taiwan reveal that the convergence between the Eurasian plate and the Philippine Sea plate produces abundant background seismicity and cyclically leading to more seismicity containing larger events with opposite direction of the plate convergence. We discover unusually large horizontal displacement variations of annual east-westward cycle with peak-to-peak more than 30 mm from 15 CGPS time series in northern Hualien. Most stations move eastward during May to October, and move westward during November to April every year. Several studies have shown that CGPS vertical component is typically influenced by seasonal changes of environment factors, and CGPS horizontal components are one-third to one-tenth those of the vertical displacement. We test the perception with newly acquired geodetic and seismological data and found it inadequate for the case of surface displacements in northern Hualien. The evidence for contribution of environment factors in the ground displacements is very weak in this region. We thus calculated the average velocity of northwestern and reversal southeastern motion; the observed displacements are nearly parallel to the direction of subduction of the Philippine Sea plate. The CGPS areal strain also implies high correlation between occurrences of local earthquakes Mw > 5.0 and surface displacements. All observations clearly demonstrate a cycle of the converging and relaxation behaviors of the Eurasian plate and the Philippine Sea plate within the junction of collision to subduction in northern Hualien. As an alternative explanation, the cyclic movement linking to deeper crustal deformation is proposed for the junction area of subduction and collision, instead of the common perception for influence of environmental factors.