Low seismicity during January and February was characterized by events NNE of the crater at depths of 2- 8 km. A source at 4-10 km depth generated 30 events with M <= 1.6 (figure 81). Long-period events remained comparatively infrequent: The average occurrence was one event/day, a trend similar to previous months.

Figure 81. Seismicity around Galeras volcano during January and February 1997. Major towns are also shown. Courtesy of OVP-INGEOMINAS.

Galeras deformation stations located 2, 4, and 7 km from the active crater typically inflated by 15 µrad.

Geological Summary. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: Observatorio Vulcanologico y Sismologico de Pasto, INGEOMINAS, Carrera 31, No. 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: http://mafalda.univalle.edu.co/~ovp).