Precursory Diffuse CO2 Emission Signature of the January 2002 Short-term Unrest at San Miguel Volcano, El Salvador, Central America

San Miguel is one of the most active volcanoes in El Salvador and rises from near sea level to 2,132 meters. San Miguel has erupted at least 29 times since 1699 and its historical eruptions consisted mainly of relatively quiescent emplacement of lava flows or minor explosions that produced modest tephra falls (Mayer-Abich, 1956). In January 2002 a short-term volcanic unrest characterized by a gas-and-steam plume containing a little ash rising with a mushroom-like profile a few hundred meters above the summit crater occurred at San Miguel volcano (BGVN 27:02, 2002). In addition, an anomalous microseismicity pattern, about 75 events between 7:30 and 10:30 hours, was also observed on January 16, 2002. This style of increased seismicity and gas emission is within the range of normal activity at San Miguel according to SNET, since intermittent periods of vigorous steam-and-gas emission have been commonly reported in recent years (BGVN 27:02, 2002). Continuous geochemical monitoring of diffuse CO₂ degassing is performed on the eastern flank of San Miguel volcano since November 24, 2001, to provide a multidisciplinary approach for San Miguel's volcanic surveillance program. At the observation site, the background mean of the diffuse CO₂ emission rate is about 16 gm^-2d^-1, but a 17-fold increase, up to 270 gm^-2d^-1, was detected on January 7, nine days before the January 2002 short-term unrest at San Miguel. This observed anomalous changes on diffuse CO₂ degassing rate might be related to a sharp increase of CO₂ pressure within the volcanic-hydrothermal system of San Miguel since meteorological fluctuations cannot explain this observed significant increase of diffuse CO₂ emission nine days before the short-term unrest at San Miguel volcano on January 16, 2002.