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Using echo sounders to detect gas plumes in seawater is common, especially in the context of hydrothermal circulation areas or gas hydrate-bearing cold seeps. To understand the distribution of gas plumes in the southernmost Okinawa Trough, we have conducted 13 cruises with a 38 kHz single-beam echo sounder (EK60). A total of 266 gas plumes of acoustic image, associated with the hydrothermal circulation, are detected. To estimate the near-seafloor bottom current speeds, 201 gas plumes are further used. As a result, the gas plumes around the axial depression of the Okinawa Trough generally tilt to the northeast at rising tides and high tides, suggesting a northeastward flow of the bottom current. However, the gas plumes in the Keelung continental slope tilt to the southwest at ebb tides and low tides, suggesting a southwestward flow of the bottom current. Our results significantly show a good estimation of the near-seafloor bottom currents from EK plume images in the case of lacking real observations. The directions of the bottom currents depend on semidiurnal tides. Assuming a quasi-constant speed of upward gas bubbles out of seabed, we have estimated the bottom current speeds in 6 hydrothermal circulation zones near the rifting center of the southernmost Okinawa Trough. The estimated bottom current speeds in submarine volcanic areas vary largely from 2 to 160 cm s-1 , but bottom current speeds in relatively flat region are between 20 and 50 cm s-1. The large variation of the bottom current speeds in the submarine volcanic areas could be due to the variable emissions of the gases out of the submarine volcanic areas.

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.

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