Seismic activity increased significantly in August, reaching the highest number of events (>150/day), the greatest reduced displacement (>800 cm²), and the highest released energy (~5.0 x 10⁸ ergs; see figure 52) by long-period events since monitoring began in February 1989. Explosions and continuous ash emission from the crater were accompanied by periodic ejections of incandescent blocks up to tens of centimeters in diameter. Incandescence was visible within the crater at dispersed sites. Although weather conditions impeded direct observations, it was possible to confirm that many of the long-period earthquakes and tremor episodes had associated surface activity. SO₂ flux was low, ranging from 7 to ~370 t/d.

Substantial deformation changes were measured by the electronic tiltmeter [at Crater Station], with a resultant vector of 231 µrad of inflation (118° azimuth) in the 2 weeks ending 14 August. Lower levels of deformation, 3.7 µrad at 183° azimuth, were measured [at Peladitos Station].

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.

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