| 摘要: | 台灣東部花東縱谷是歐亞板塊與菲律賓海板塊之縫合帶，而縱谷北部結束端是台灣地震活動最頻繁之地區之一，而米崙斷層是此區域最活躍之活動斷層，其活動特性以左移為主，逆衝為輔。此區在1951年發生過規模7以上的大地震，震央在米崙斷層往北向海延伸之海域。由於過去對於縱谷北端海域的地質構造研究甚少，本研究透過多音束水深、側掃聲納影像、高解析底質剖面及多頻道反射震測調查縱谷北端海域。結果顯示，花蓮海脊之構造可以分成三種型態，表層沉積物受侵蝕產生之線型，淺部構造則是因地層滑移所產生的正斷層，而較深部構造則呈現一正開花突起構造(pop-up structure)，且在西側之峽谷的地層有擠壓之特徵。透過震源機制解得知，花蓮海脊深部所受之應力主要為西北-東南向擠壓。因此，縱谷北端海域可能為菲律賓海板塊向下碰撞並擠壓歐亞板塊而隆起之突起構造。淺部之構造為一系列因為深部地層擠壓，而在淺部產生拉張應力所形成的正斷層以及地塹。為了瞭解與陸上構造之關係，將研究結果與陸上的前人研究作討論。從前人研究結果可以得知米崙台地上之地表變形為同震期抬升，間震期沉降之狀態，且帶有順時針旋轉之應力。此區域之地層滑移產生之正斷層的走向由北偏東30度轉為北偏東45度，與米崙台地上為順時針旋轉之應力狀態相吻合。此區域地震來自深部應力擠壓，主要沿突起構造兩側分布，而地層在深部擠壓使得間震期在淺部和地表產生拉張之應力。從底質剖面、震測剖面以及陸上淺層震測剖面結合以上討論，推測米崙斷層確實沿花蓮海脊最西側地形隆起處延伸，至於是否延伸至新城海脊上則需要更多資料來證明。從地震分布以及底質剖面和震測剖面之特徵來看，花蓮海脊南段比北段活躍。在縱谷北端海域有兩種不同型態之斷層，其一為花蓮海脊西側峽谷，也就是米崙斷層延伸之位置，屬於深部應力擠壓之逆衝斷層，若是較大規模的地震會沿此處發生。其次是花蓮海脊上之淺部構造因地層滑移所產生的正斷層，這一系列的正斷層長度與深度均不大，對於地震災害之影響應該較小。;The Longitudinal Valley (LV) is considered as the collisional suture between the Philippine Sea and the Eurasian plates. The northern end of the LV is close to the western end of the Ryukyu Trench. Earthquakes are very frequent in such a complex regime. The Milun Fault is located in the northernmost portion of the LV. However, the offshore portion of the Milun Fault is still enigmatic. Particularly, because of the lack of data the offshore extension of the Milun Fault was rarely addressed. For that, in this study we use new marine seismic reflection, sub-bottom chirp sonar profile, and multi-beam bathymetic data to analyze the seafloor features and tectonic structures of the offshore portion of the Milun Fault. The Milun Fault is a well-known active structure associated with the rupture of the Milun tableland by a M7.3 earthquake in October 1951. From south to north, the geological units include Hualien ridge, the top of Hualien Canyon, and Hsincheng Ridge. Based on the SBP profile and bathymetry, we can observe a creeping linear structure on the sediment and seafloor, and three structure directions and two types structure on the Hualien Ridge. Firstly, we find normal fault for sliding strata in shallow depths. The strike of those normal faults is N30˚E in southern Hualien Ridge and N45˚E in northern Hualien Ridge. Secondly, the Hualen ridge is cut by the strike-slip fault zone in N120˚E. So, the SBP results indicate that the Hualien Ridge is under a clockwise rotation stress. Base on the MCS profile, we could observe the area in the Milun Tableland and Hualien Ridge is a pop-up structure. According to the earthquake focal mechanisms, we could know the region is subject to compressive stress and compression direction is NW-SE. Moreover, the SBP profile and MCS profile indicate that the Milun Fault could extend northeartward to the west side of the Hualien Ridge. However, we need more data to understand whether Milun Fault extends to the Shingcheng Ridge. In summary, the Milun Terraces and Hualien Ridge is a pop-up structure due to the NW-SE regional compressive stress, but we could observe the normal faults and sliding strata due to the extensional stress on the shallow depths. The northern Longitudinal Valley is under a clockwise rotation. The Hualien Ridge is therefore cut by a strike-slip fault zone and is divided into southern Hualien Ridge and northern Hualien Ridge. |
