Chirp Sonar Images on Liquefaction and Their Implications on Seafloor Stability Offshore SW Taiwan
Date Issued
2016
Date
2016
Author(s)
Lu, Yi-Wei
Abstract
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.
Subjects
seafloor instability
mass movement
liquefied strata
chirp sonar profile
3D chirp sonar image
SDGs
Type
thesis
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