利用衛星測地資料分析花東縱谷北段之地殼變形

Analysis of Crustal Deformation Along the Northern Longitudinal Valley from Satellite Geodesy

臺灣東部為歐亞板塊與菲律賓海板塊交界處，地震發生頻繁，但近20年來花東縱谷北段僅有少數規模大於五的地震，因此此段的縱谷斷層被視為震間鎖定狀態，和南段的淺部潛移深部發震行為不同。透過觀測縱谷北段的地表變形有助於瞭解兩板塊間現今的碰撞及運動方式，並可提供地震潛勢分析的依據。本研究結合全球衛星定位系統（GPS）及永久散射體差分干涉（PSInSAR）兩種衛星測地資料，分析2004年至2010年間縱谷北段的三維地表變形。結果顯示此區的水平速度大致以20-25mm/yr朝向西北，並呈現由南往北順時針變小的情形。垂直運動上，海岸山脈在23.4ºN以北呈現最大10mm/yr的下沈，推測此運動可能和菲律賓海板塊在此處外海向北隱沒至歐亞板塊或是東部外海一西傾斷層的震間滑移有關。此外，應變率分析與地震觀測指出：（1）中央山脈東側發生的正斷層地震應與此處接近東西向伸張的地表變形有關；（2）縱谷北段有快速的應變能累積，加上近20年僅發生過兩個地震矩規模大於五的的地震，顯示此區可能有較高的地震潛勢。 While frequent earthquakes have occurred in eastern Taiwan due to the interactions between the Philippine Sea and Eurasian plates, the northern portion of the Longitudinal Valley (LV) has experienced few M>5 earthquakes in the past 20 years and is considered locked interseismically, different from the southern portion where near-surface fault creep and deep seismicity have been observed. Therefore surface deformation near the northern LV fault is worth investigating for evaluating earthquake potential of eastern Taiwan. Consedering the data advantages of three-component GPS (Global Positioning System) and high-spatial-sampling PSInSAR (Persistent Scatterer InSAR), we apply a velocity interpolation method to intergrate both geodetic observations for analyzing the interseismic ground deformation of the northern LV area. Our results reveal a decrease of horizontal ground motion to the north from 25 mm/yr to <10 mm/yr, and a clockwise rotation near the northern end of LV. The vertical velocities show subsidence in most of the study area with rates up to 10 mm/yr, which may relate to a postulation that the northern Costal Range is subducting to the north under the Eurasian plate. In addition, geodetic strain-rate field and earthquake focal mechanism point out that (1) the occurrence of normal-faulting earthquakes in the eastern Central Range is consistent with the observed extensional strain field; (2) while the northern LV has large magnitude of strain-rate, no observation of fault creep and relative low rate of large (M>5) earthquakes in the past 20 years may imply high earthquake potential in the area.

While frequent earthquakes have occurred in eastern Taiwan due to the interactions between the Philippine Sea and Eurasian plates, the northern portion of the Longitudinal Valley (LV) has experienced few M>5 earthquakes in the past 20 years and is considered locked interseismically, different from the southern portion where near-surface fault creep and deep seismicity have been observed. Therefore surface deformation near the northern LV fault is worth investigating for evaluating earthquake potential of eastern Taiwan. Consedering the data advantages of three-component GPS (Global Positioning System) and high-spatial-sampling PSInSAR (Persistent Scatterer InSAR), we apply a velocity interpolation method to intergrate both geodetic observations for analyzing the interseismic ground deformation of the northern LV area. Our results reveal a decrease of horizontal ground motion to the north from 25 mm/yr to <10 mm/yr, and a clockwise rotation near the northern end of LV. The vertical velocities show subsidence in most of the study area with rates up to 10 mm/yr, which may relate to a postulation that the northern Costal Range is subducting to the north under the Eurasian plate. In addition, geodetic strain-rate field and earthquake focal mechanism point out that (1) the occurrence of normal-faulting earthquakes in the eastern Central Range is consistent with the observed extensional strain field; (2) while the northern LV has large magnitude of strain-rate, no observation of fault creep and relative low rate of large (M>5) earthquakes in the past 20 years may imply high earthquake potential in the area.