Temporal Evolution of Diffuse CO2 Degassing of Taal Volcano In Response to Nearby Large Magnitude Earthquake Events

Taal is one of the most active volcanoes in the Philippines known for its explosive phreatic and phreatomagmatic eruptions. Monitoring the changes in flux rates and concentration of diffuse degassing are important geochemical tools in studying volcanic unrest since the emission rate of diffuse CO₂ can increase months before an eruption. Previous published work in monitoring diffuse degassing at Taal involved discrete measurements of CO₂ flux from the MCL and geothermal field soils only. This study includes three years' worth of continuous hourly soil CO₂ flux data from a geochemical station situated at a fumarolic area on Taal's northern flank. In addition, CO₂ efflux surveys at the Main Crater Lake (MCL) and earthquake data were correlated to the processed flux data from the monitoring station. Results highlight two important seismic events: an earthquake swarm, 13 of which have magnitudes 4.0 or greater, that originated around 30 km south of the volcano on April 4 to 11, 2017 and a M6.3 earthquake that occurred approximately 50 km east of Taal on August 10, 2017. An increase in CO₂ flux from both the soil and the MCL preceded the earthquake swarm. CO₂ emission rates continued to increase after April 2017. When the M6.3 tremblor struck on August 10, the flux sharply rose until it peaked at 2,677 t/d on November 1, 2017 with the emission decreasing considerably after. The coeval increases in CO₂ observed at both the MCL and soil efflux show that the main CO₂ flux changes were not driven by meteorological variables but were clearly associated with fluid pressure fluctuations in the volcanic system due to the changes in subsurface permeability after the strong earthquake events. This study contributes to understanding the relationship of volcanic degassing to nearby tectonic earthquakes which has significant implications to monitoring earthquake triggered eruptions.