Long-term in situ monitoring at Dashgil mud volcano, Azerbaijan: a link between seismicity, pore-pressure transients and methane emission
Mud volcanism is a global phenomenon usually associated with compressional tectonics that favour extrusion of fluid- and clay mineral-rich sediment both on land and offshore. Methane, the dominant gas phase, is emitted at variable rates during and after emplacement of the mud domes. In case of continental mud volcanoes, the gas is directly released into the atmosphere, thereby contributing to global warming. Azerbaijan is one of the countries with one of the highest abundances of mud domes globally. One of the most prominent mud volcanoes, Dashgil, has been chosen for a case study because of its historic record of violent eruptions, continued activity, and well-documented regional geology in the Caucasus orogenic wedge adjacent to the Caspian Sea. Since 2003, gas flux has quantitatively measured at one of the two crater lakes and is characterized by valve-type behaviour and episodically violent degassing. In 2007, the large crater lake was additionally equipped with methane fluxmeters as well as an in situ pore-pressure probe into the conduit. Our data are complemented by regional seismicity, and exhibit the following results: (1) there seems to be a significant correlation between changes in pore pressure in the conduit feeding the main crater lake and the rate of gas escape; (2) changes in gas-flux rate appear to be independent of local seismicity, in particular since no larger EQs have been recorded since 2003; (3) despite discontinuous monitoring owing to technical failures, we observe an overall increase in methane emission with time; (4) nearby earthquake activity (<M4.6) can be correlated with pore-pressure transients recorded by the piezometer, which reach up to 2.4 kPa compared to the pre-seismic value; (5) from time to time, there are strong lake-level fluctuations decoupled from precipitation or evaporation, which are explained by subbottom hydraulic communication between the two crater lakes and adjacent gryphons. The wealth of observations leads us to conclude that monitoring gas flux and pore pressure provides important time series data for both methane emission into the atmosphere and pore pressure as a proxy for co-seismic strain.