ANOMALOUS DIFFUSE SOIL FLUX EMISSIONS OF CARBON DIOXIDE BEFORE VOLCANIC CRISIS AT SANTA ANA VOLCANO, EL SALVADOR, CENTRAL AMERICA

Santa Ana volcano has presented anomalous emissions of diffuse soil CO2 degassing before the seismic-volcanic crisis of March and August-September, 2007. Measurements of soil CO2 efflux along a profile located at an arc of the crater rim to the southeast has been measured using a Licor-820 spectrophotometer coupled to an accumulation chamber (Chiodini ,1998) several times after the October 1, 2005 eruption. The location of this profile was chosen because it intercepts a fault that bisects the volcano with a SE-NW strike. Previous investigations show that the average CO2 flux emitted along this profile in May, 2001 was 15 g/m2day, (Salazar et al., 2004). This value increased to 420 g/m2day seven days before the eruption of October 1, 2005 (Barahona et al.,2007). During the calm post-eruptive period of 2006, the average CO2 flux along this profile decreased to an average of 31 g/m2day. A seismic-volcanic crisis occurred at Santa Ana between March 8 and 20, 2007. Nine days before this crisis the emissions of CO2 along the studied profile increased to 196 g/m2day. At the same time a change in color of the water in the crater lake was observed as well as an approximately 96% decrease in water volume due to evaporation. On March 11, 2007, the RSAM increased up to 38 units. On March 12, 2007, 550 long period seismic events were detected. On March 14, the seismic network detected 13 volcano-tectonic seismic events (SNET, 2007). A new seismic-volcanic crisis was registered during August, 30 to September 18, 2007 with a maximum of 41 units in RSAM on September 9, 2007; volcano-tectonic seismic events were not detected during this period of time (SNET, 2007). Eighteen days before the start of this crisis, the average CO2 efflux along the studied profile presented an increase to 176 g/m2day. The values of soil CO2 emissions along the SE profile along the crater rim prior to these two seismic-volcanic crisis were 12 times greater than the average level in May 2001, and 2 times lower than the average CO2 emissions at the same profile before the October 1, 2005 eruption. These results suggest that the emissions of soil CO2 at Santa Ana volcano crater rim can be used as precursors of volcanic activity. At other volcanoes of the world, identification of permeable structures close to the main vent and monitoring of the diffuse gases at those locations could give important information about the volcano activity.