In July, 2004, after 11 years of relative quiescence, Galeras Volcano (Colombia) renewed its eruptive activity with a sequence of explosions and ash emissions. Initial evidence of the activity transition appeared in gas measurements from the instruments of the multiparameter station (Seidl et al, 2003, Faber et al, 2003) in early June, followed by a strong increase in the shallow seismic activity below the active cone on June 27 (Gómez et al, 2004). As is the case at many other volcanoes, the most clear evidence for the transition came in the form of seismic swarms, which included both volcano tectonic and long-period events, and tremor. Eruptive activity commenced with two brief episodes of ash emission, on July 16 and July 21. Since then, ash emissions and explosions have been intermittent. An episode lasting from August 11 - 19 began with a large explosion and released more ash than any individual episode from 1989 to 1993. Sudden deformation, as well as changes in the electric and magnetic fields at the crater electromagnetic (EM) station and the gas parameters, such as CO2 concentration and fumarole temperature, accompanied the ash emissions on July 16 and July 21. While the EM and gas sensors were lost to ashfall by the end of July, monitoring continued via seismometers, tiltmeters and visual inspection.

Figure 13.33: Raw velocity seismograms of a tornillo recorded with a broadband seismometer.
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