Anomalous Diffuse CO2 Emission prior to the January 2002 Short-term Unrest at San Miguel Volcano, El Salvador, Central America

On January 16, 2002, short-term unrest occurred at San Miguel volcano. A gas-and-steamash plume rose a few hundred meters above the summit crater. An anomalous microseismicity pattern, about 75 events between 7:30 and 10:30 hours, was also observed. Continuous monitoring of CO2 efflux on the volcano started on November 24, 2001, in the attempt to provide a multidisciplinary approach for its volcanic surveillance. The background mean of the diffuse CO2 emission is about 16 g m)2 d)1, but a 17- fold increase, up to 270 g m)2 d)1, was detected on January 7, nine days before the January 2002 shortterm unrest at San Miguel volcano. These observed anomalous changes on diffuse CO2 degassing could be related to either a sharp increase of CO2 pressure within the volcanic-hydrothermal system or degassing from an uprising fresh gas-rich magma within the shallow plumbing system of the volcano since meteorological fluctuations cannot explain this observed increase of diffuse CO2 emission.