Concentration and carbon-isotopic change of dissolved gas from Murono mud volcano in Tokamachi City, Niigata Prefecture (central Japan), just before and after the 2014 Kamishiro Fault Earthquake

Dissolved gas concentrations, compositions, and carbon isotopic values are compared between times immediately prior to and following the 2014 Kamishiro Fault Earthquake at the Murono mud volcano (Tokamachi City, Niigata Prefecture, central Japan). Immediately following the earthquake, concentrations of methane, carbon diox-ide, ethane, and propane increased, while carbon isotopic values of methane and ethane decreased, on average. Carbon isotopic values of methane and gas compositions (C 1 /[C 2 +C 3 ] ratio) indicate that the dissolved methane of the mud volcano originates from the thermo-genic decomposition of organic matter. As the carbon isotopic values of methane and ethane are significantly more positive than those of natural gas from the Niigata Oil and Gas Field, the dissolved gases of the mud volcano are thought to be derived from the deepest source rocks of the Niigata Oil and Gas Field. However, the supply of ethane generated from less mature source rocks possibly increased following the earthquake. Our results reveal that a change in behavior of fluid deep below the mud volcano was likely caused by fluctua tions in volumetric strain related to the earthquake. Theoretical strain at the mud volcano can be estimated as 687×10 – 8 , which is much higher than estimates from previous studies for the lower lim-its of strain triggered by anomalous activity at mud volcanoes. Murono mud volcano is an important case study for investigating the correlation between mud volcanoes and earthquakes, because it fre-quently experiences the large volumetric strain of earthquakes that are comparable to the 2014 Kamishiro Fault Earthquake.