Forefront of studies on mud volcanoes: Advances in studies on inland mud volcanoes in the last 8 years

Inland mud volcanoes are distributed along anticline axes and faults. Based on studies of inland mud volcanoes in Japan and Taiwan over the last eight years, we discuss the relationship between mount shape and the geological structure that controls the distribution of mud volcanos and the ascent of erupting fluid composed of mud, groundwater, and gas. Mud volcanoes distributed along anticline axes (AMVs) are characterized by intense eruptions with topographic depressions several meters to one km in diameter and several tens of meters deep. These features arise from hydro-fracturing of the impermeable cap rock under high gas pressure, which is due to degassing in the saline mud chamber beneath the cap rock. The δ18O values of groundwater erupted from AMVs are 0‰-2‰. Thermogenic hydrocarbon gas from AMVs originates deep underground and ascends and mixes with biogenic gas under the cap rock before erupting at the surface. The mounts form low mud pools because of the high water content of the fluid. If the cap rock is not distributed at a crest of anticline, the ascending fluid erupts gently under the cap rock. Mud volcanoes distributed along faults (FMVs) are characterized by the gentle eruption of fluid that originates deep underground. The δ18O values of groundwater erupted from FMVs are 3-8‰. Fluid erupted from FMVs originates deeper compared with fluid at FMVs and ascends through faults at fracture zones. AMVs form high cones up to several tens of meters high as a result of the low water content of fluid from the clay-rich fault gauge. Thermogenic hydrocarbon gas at FMVs originates from microbial decomposition deep in the fracture zone. AMVs cause serious damage to infrastructure and are a threat to human life.