利用海床底質回聲影像探討臺灣西南海域沉積物液化對海床穩定性影響

Abstract

臺灣西南海域因海底重力流極端事件導致一系列的海底電纜斷裂，並造成嚴重的經濟損失，本研究結合地球物理與地球化學方法，探討導致海床不穩定的可能原因，並針對臺灣西南海域陸棚、陸坡區，評估易發生海底重力流之敏感區分布。本論文以2006年12月26日屏東地震於枋寮峽谷引發重力流之事件為例，利用高解析水深、海床底質回聲剖面以及反射震測資料，藉由比對事件前後地形與地層特徵之變化，先確認該事件所引發之塊體運動發生區域，再探討為何地震後會在該處誘發海床崩塌。研究結果顯示，崩塌區域海床下之地層並未有明顯的斷層、褶皺或貫入體構造，進而可直接引發海床崩塌；反而在海床底質回聲剖面資料中發現崩塌區域淺部地層中，廣泛存在一層聲波反白透明層，再搭配高解析三維海床底質回聲影像的分析，發現該透明層與沉積物發生崩塌有所關連。藉由該透明層之地物特徵以及箱型岩心分析結果，透明層之發生應與地層液化有關，而地層液化則是導致海床不穩定之關鍵因素。因此本研究提出一套由淺部沉積層中液化地層引發海床崩塌發生的機制：海床下的某特定沉積層在流體的充填下，使得該層的孔隙水壓逐漸上升，同時有效應力下降，當上覆沉積物給予的荷重較少時，此沉積層容易形成不穩定的狀態，最後在一定規模地震的擾動下，使其有效應力歸零，造成沉積層發生液化。而由於液化地層可藉由底質回聲剖面中的透明層辨識，所以檢視整個高屏棚坡區域回聲剖面中的透明層分布，顯示發現顯示除了枋寮峽谷頭部東側區域外，高屏峽谷與枋寮峽谷之間的陸棚區域也有不少潛在發生液化的海床不穩定區域，其中又以枋寮峽谷頭部西北側分布最為密集。本研究認為這些富含流體的沉積層，在有適當的誘發機制情況下，例如2006年的屏東地震事件，容易使得這些含有流體且處於不穩定狀態的沉積層受到擾動而發生液化，導致海床崩塌而引發重力流。

Large scale submarine cable breakages often occur along the submarine canyons offshore southwest Taiwan after some extreme gravity flow events caused significant economic losses. This study analyzes both marine geophysical and geological/geochemical data to investigate the possibly causes of seafloor instability, and identify the regions prone to submarine landslides that trigger submarine gravity flows in the area off southwest Taiwan. Taking Pingtung Earthquake as an example, this earthquake occurred on 26 December 2006, which triggered gravity flow in the Fangliao Submarine Canyon. By analyzing high-resolution bathymetry, chirp sonar, and reflection seismic data to search for the locations of seafloor failures that caused an extreme gravity flow event, and to investigate why this event occurred in that region. Our study shows that there are not obvious active faults, folds, and diapers to trigger the seafloor failures in our study region. However, from chirp sonar profiles, an acoustic transparent layer is widely distributed in the shallow substrata in the study region. We also construct a 3D chirp sonar image to reveal the distribution of this layer, which we think is closely linked to the seafloor failures. Based on the analyses results of box core data and geophysical characteristics of the transparent layer, we suggest that sediment liquefaction caused the transparent layer, and liquefaction is the major factor of seafloor instability in our study area. We establish a model to show how the liquefied layer affects the shallow substrata and causes seafloor failures: when a layer is supplied with large amount of fluid in the shallow substrata, pore water pressure increases and effective stress decreases in the sediments. With inadequate loading pressure from the sediments above, the fluid saturated layer is easy to become unstable. When a large earthquake hits the region with unstable seafloor strata, the effective stress in the fluid over-saturated substrata will decrease to zero, and cause the liquefaction. Because the liquefied strata can be identified on chirp sonar profiles as a transparent layer, we examine all the chirp sonar profiles in the Gaoping shelf and slope region to map this layer. Our investigation results show that there are many potential liquefaction regions between the Gaoping and Fangliao Submarine Canyons in the shelf, and the area near the northwestern side of the Fangliao Submarine Canyon is the most unstable region. In conclusion, we suggest that the layer with fluid could be easily liquefied when an appropriate events happen, such as 2006 Pingtung Earthquake.