Diffuse Carbon Dioxide Degassing Monitoring at Santa Ana-Izalco-Coatepeque Volcanic System, El Salvador, Central America

Santa Ana volcanic complex (0.22 Ma), located 40 Km west of San Salvador, comprises Santa Ana, Izalco, and Cerro Verde stratovolcanoes, the Coatepeque collapse caldera, as well as several cinder cones and explosion craters. Most recent activity has occurred at Izalco (1966) and Santa Ana which shows a permanent acidic crater lake with an intense fumarolic activity. In addition, Santa Ana exhibits a SO₂-rich rising plume though no local seismicity has been reported. Weak fumarolic activity is also present at two locations within the Santa Ana volcanic complex: the summit crater of Izalco and Cerro Pacho at Coatepeque caldera. Other important structural features of this volcanic complex are two fault/fissure systems running NNW-SSE that can be identified by the alignment of the stratovolcanoes and numerous cinder cones and explosion craters. In January 2001, a 7.6 magnitude earthquake occurred about 150 Km SE of Santa Ana volcano. A soil gas and CO₂ efflux survey was performed to evaluate the impact of this seismic event upon the diffuse degassing rates in Santa Ana volcanic complex in March 2001. A total of 450 soil gas and diffuse CO₂ efflux measurements were carried out covering an area of 209.5 Km². CO₂ efflux ranged from non-detectable values to 293 gm^-2d^-1, with a median of 8.9 gm^-2d^-1 and an upper quartile of 5.2 gm^-2d^-1. The CO₂ efflux spatial distribution reveals the existence of areas with CO₂ efflux higher than 60 gm^-2d^-1 associated to the fault/fissure systems of NNW-SSE orientation. One of these areas, Cerro Pacho, was selected for the continuous monitoring of diffuse CO₂ efflux in late May 2001. Secular variations of diffuse CO₂ efflux ranged from 27.4 to 329 gm^-2d^-1 with a median of 130 gm^-2d^-1 and a quartile range of 59.3 gm^-2d^-1. An increasing trend of 43 gm^-2d^-1 was observed between May and August 2001 overlapped to high-frequency minor fluctuations related to meteorological variables' changes. However, a larger observation time-span is needed to understand the influence of the rainy-season and meteorological parameters in the observed CO₂ efflux time series.