Twenty years (1990–2010) of geodetic monitoring of Galeras volcano (Colombia) from continuous tilt measurements

Although geodetic monitoring techniques have continually evolved and improved, especially since the introduction of continuous GPS and InSAR, our experience indicates that electronic tiltmeters are very sensitive and can measure changes in ground deformation that would go unobserved by either GPS or InSAR. This characteristic makes tiltmeters very effective in monitoring short term, transient deformation episodes that may be associated with magma intrusions. To support our observation, we present the case study of 20 years of tilt measurements from Galeras (Colombia). Galeras – an andesitic stratovolcano that is part of the Galeras Volcanic Complex – is one of the most active volcanoes in Colombia. Historic activity is centered on a smallvolume cone inside the youngest amphitheater, which breaches the west flank of the volcano. At least 30 confirmed eruption periods have been recorded in the past 480 years, with episodes of unrest ranging from weak fumarolic activity and ash emissions to larger explosive events. The most recent eruption periods, monitored by electronic tiltmeters since 1988, have been characterized by minor explosive eruptions, and the emplacement of three crater domes and small pyroclastic flow deposits. In particular, we focus on three episodes of unrest that occurred in 1991, 2006 and 2008, when the deformation was clearly associated with shallow magma intrusions, and the emplacement and destruction of crater domes. The depth of the intrusions varied from a few hundred meters (August 2005) to two kilometers (January 2009), while the volume change ranged from 10,000 m3 (May-October 2009) to 1,000,000 m3 (January 2009). A comparison with seismic data indicates that the deformation sources were located within the cloud of hypocenters of the volcano-tectonic earthquake events. A lack of a clear correlation between the volume change (and depth) of the sources and the total SO2 flux could indicate that the unrest at Galeras was related to a larger intrusive event with only a small part of the magma erupted in the form of tephra and lava domes.