High-frequency seismicity decreased in September compared to August. Only ten events occurred W of the crater at depths <6 km, of which seven were <1 km. Magnitudes varied between 0.4 and 1.7. A swarm of small earthquakes registered on 15 September had characteristics similar to those observed before the 16 July eruption. There were no significant deep tremor episodes. Few long-period events were recorded, but they were more common during the first 2 weeks of the month. COSPEC measurements of SO₂ flux in September varied between 50 and 450 t/d, similar to August values. The electronic tiltmeter [at Crater Station] was generally stable tangentially, with a slight deflation (-3.9 µrad), and had fluctuating radial values with a cumulative inflation (+5.9 µrad). The [Peladitos station] was also relatively stable, with changes of -3.8 and +9.0 µrad for the tangential and radial components, respectively.

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: M. Calvache, INGEOMINAS - Observatorio Vulcanológico del Sur.