臺灣位於菲律賓海板塊與歐亞板塊的碰撞邊界，而臺灣東部縱谷地區被認為是兩板塊的縫合帶。在縱谷的東邊是以火山島弧組成的海岸山脈地質為主，西邊則是以變質岩組成的中央山脈地質為主。菲律賓海板塊向西北方向隱沒到歐亞板塊下方，造成縱谷北段有著複雜的板塊相互作用。2018年2月發生了規模6.4的花蓮地震，以致多處道路斷裂以及大樓倒塌等嚴重的災情發生。而後郭陳等人(Kuo-Chen et al., 2019)在此佈設了密集的臨時地震陣列以追蹤後續餘震序列，因此對於縱谷北段地區有了更進一步的探討與了解。在本研究中延伸郭陳等人(Kuo-Chen et al., 2019)的餘震序列研究資料並結合2017年架設的密集地震網資料，進一步利用三維速度模型來探討東臺灣縱谷北段的地下構造。 本研究利用聯合體波以及表面波資料的反演方法(Fang et al., 2016)，希望藉由結合體波和表面波各自的優點來獲得解析度較高的Vp與Vp/Vs 模型。在逆推過程中使用2,892個地震以及短週期表面波資料。結果顯示重新定位相較於只用一維速度模型定位後的地震還要來得聚集，並且地震深度大約落在5到20公里處。在2017年以及2018年的震源機制解分布比較，可以看到在2017年靠近米崙地區震源機制解是與2018年的餘震類型相同，皆以正斷層機制為主，並對比地表變形的結果顯示與震源機制解分佈是相吻合。從震波速度模型來看，其構造分佈大致吻合縱谷北段的地表地質特徵，在縱谷西邊為中央山脈其岩性為變質岩所組成，縱谷本身與東邊分別以沖積層與火成岩碎屑岩塊為主的海岸山脈，故呈現縱谷西邊速度比起東邊高的現象。然而靠近縱谷下方有一個明顯高低速的介面產生，根據震源機制解以及地震分布，在中央山脈下方有一逆斷層構造可延伸至海岸山脈，而此構造可能與板塊邊界有關，並從南至北的速度剖面中顯示在靠近23.75˚N中央山脈下方有一高速體往上延伸，至花蓮地區中央山脈下高速體並不明顯，推測與菲律賓海板塊逐漸往西北隱沒至中央山脈東翼下方現象有關。 ;The northern part of eastern Taiwan is situated on the plate boundary between the Philippine Sea plate and the Eurasian plate. The Philippine Sea plate subducts northwestward beneath the Eurasian plate with a very complicated plate interaction in this area. At the surface, the Central Range belongs to the Eurasian plate composed with metamorphic rocks, whereas the Coastal Range belongs to the Philippine Sea plate composed of sedimentary and volcanic rocks. These two geological units are separated by the Longitudinal Valley (LV) as a suture zone. Seismic tomography is one of powerful tools to investigate the subsurface structures. In the past, a few of high resolution seismic tomography have been performed in this area. Taking the advantage of the data sets of Kuo-Chen et al., (2019) deployed a dense seismic array to capture the aftershock sequence of 2018 Mw6.4 Hualien earthquake and another dense seismic array in 2017, we applied a joint inversion of body and surface wave data to get better Vp and Vp/Vs velocity models. The advantage of this method is the complementary strengths of each data set. A total of 2892 local earthquakes and short period surface wave data were used for inversion. As a result, the earthquakes located with our new velocity model are more clustered than those by using the 1-D model, and the depth range of these events are between 5 and 20 km. Based on the focal mechanisms, the focal mechanisms are dominated by normal faults with few strike-slip faults in Hualien City in our 2017 and 2018 data set. The patterns of our velocity models roughly match the geological units at the surface but provide more information at deeper depths. The velocities in the east of LV are relatively lower than those in the west. Also, an interface between the relatively high and low Vp/Vs ratio zones beneath LV. According to the focal mechanism solution and seismic distribution, the interface could show the geometry of the plate boundary at deeper depths. Another pattern can be found that a high-velocity block extends upward from 20km to 5km in the east flank of LV, which could relate to serpentinized rocks in this region.
