Tide-modulated gas emissions and tremors off SW Taiwan

Escape of methane and carbon dioxide from the seafloor has great impacts not only on the geosphere but also on the biosphere, the hydrosphere and the atmosphere. However, its mechanism is still poorly understood. For that, we have deployed 8 Ocean bottom Seismometers around an active mud volcano off SW Taiwan. We find that the gas emissions from the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially the diurnal and semi-diurnal constitutes. The daily activity of gas emissions and the intensity of tremors are directly proportional to the daily tidal range. Particularly, for each tidal day the gas emissions and long-duration tremors mainly occur at the rising slopes to the higher high tides and at the falling slopes to the lower low tides. It is then suggested that the upward flow of methane beneath the seabed is more active in those periods. Both the individual tremors and long-duration tremors occur as results of the gas expulsions out of the gas-bearing seabed. High-frequency components of the tremors quickly decay and are followed by low-frequency harmonic oscillations. Long-duration tremors can enhance seafloor instability and increase the potential for submarine landslides.