Seabed gas emissions and submarine landslides off SW Taiwan

Abstract

Methane emissions out of the seabed could seriously affect Earth's climate and are usually associated with the dissociation of gas hydrates stored in marine sediments on the continental margins. Spatially, gas emissions out of the seafloor are not evenly distributed in continental margins. Gas emissions out of the seabed generally occur through submarine mud volcanoes and gas seeps. To understand the seabed gas emissions off SW Taiwan, we investigate the distributions of active submarine mud volcanoes, gas seeps, and gas plumes off SW Taiwan. We examine all of the available sub-bottom profiler and EK echo sounder data. We identified 19 submarine mud volcanoes, 220 gas seeps, and 295 gas plumes. The gas emissions are generally distributed at the crests of mud diapiric ridges. Most of the active mud volcanoes and gas seeps cluster at the KASMVG (Kaoping submarine mud volcanoes group) area. We speculate that the intensive mud volcanism and gas seepage at the KASMVG area are ascribed to submarine channel erosion along the continental slope base. The erosion causes a deep V-shaped channel and a steep BSR (Bottom-Simulating Reflector) slope curve across the continental margin. The upward migration rate of free gas beneath the BSR is thus increased and intensifies mud volcanism and gas seepage at the KASMVG area. The gas seeps can reduce the slope stability and generate small-scale slides. The development of mud volcanoes in an area could effectively disturb the seabed morphology so that large-scale submarine landslides cannot easily happen.

Fangliao Slide — a large slope failure in the upper Kaoping Slope off southwest Taiwan

“…The high amplitude reflections display an acoustically reversed polarity relative to the seafloor, suggesting that the onlap structures have trapped free gas (Audemard and Serrano 2001;Liu et al 2006). The gas hydrate equilibrium curve in the offshore area of SW Taiwan indicates that the water depth of ~600 m could be a minimum pressure to enable the gas hydrate formation (Hsu et al 2018). Because the water depth ~600 m is also the boundary between the upper and lower domains of the Fangliao Slide (Figs.…”

“…Gas seeps and gas plumes are widely distributed on tops of the mud diapirs off SW Taiwan (Fig. 3) (Hsu et al 2018). Some gas seeps and gas plumes do exist near the headwall of the Fangliao Slide and in the terrace III (Fig.…”

“…The active continental margin off SW Taiwan is characterized by accretionary wedges, numerous submarine canyons across the continental slope (Liu et al 1997(Liu et al , 2004), a few kilometers thick sedimentary deposits Hsu et al 2013a), numerous active mud diapirs and mud volcanoes, and gas seeps (Sun and Liu 1993;Liu et al 1997;Chiu et al 2006;Chen et al 2010Chen et al , 2014Hsu et al 2013bHsu et al , 2018. A disastrous tsunami hitting the coastal area of southwest Taiwan in 1781 was reported (Chen 1830;Hsu 1983).…”

Introduction to the special issue on submarine geohazard records and potential seafloor instability

“…1). In this special issue, Hsu et al (2018b) have examined the gas emissions distribution and further evaluated the seafloor stability between the Kaoping and Fangliao canyons area. Chen et al (2018a) have discovered and delineated a large submarine landslide (named Fangliao Slide) to the west of the Fangliao submarine canyon; they have also discussed the possible mechanisms of the slope failure.…”

“…They are briefly described in the following. Hsu et al (2018b) first systematically summarize the all of the sub-bottom profiler and EK echo sounder data to map the gas emissions distribution off SW Taiwan and obtain the sites of 19 mud volcanoes, 220 gas seeps and 295 gas plumes off SW Taiwan. Their results show that most of gas emissions are located between the Kaoping and Fangliao submarine canyons.…”