Seismicity in August was at the lowest levels since monitoring began, but there was an increase in high-frequency events. On 1 August, a high-frequency earthquake centered 7 km W of the active cone was felt in the city of Pasto. During August, four swarms were detected, similar to those recorded a few hours before the 16 July eruption. The swarms were centered NW of the crater at depths of 2.5-5.0 km (figure 59); magnitudes were generally ~0.4, with the strongest shocks reaching M 2.3. The number and energy release of long-period earthquakes were the lowest since February 1989.

Figure 59. Epicenters (top) and depths (bottom) of seismic activity at Galeras, August 1992. Courtesy of INGEOMINAS.

Deformation measured by two electronic tiltmeters near the summit (Crater and Peladitos), and the short leveling-line vector (Las Piedras), was low compared to that of September and October 1991, associated with dome emplacement. A strong offset at Peladitos on 9 August coincided with one of the swarms of high-frequency events. Deformation remained relatively stable at Crater until a sharp change occurred on 24 August (figure 60).

Figure 60. Deformation data from the Crater and Peladitos electronic tiltmeters, 0.9 km NE and 1.5 km SE, respectively, of the crater at Galeras, August 1992. Courtesy of INGEOMINAS.

COSPEC data showed SO₂ emissions of ~35 t/d, the lowest values since monitoring began in early 1989. Ash emission associated with a long-period seismic event was observed at 1535 on 11 August. Geologists noted that pulses of great activity, as from 1924-36, alternate with periods of minimum activity, and that the present decline in activity is consistent with the historical behavior of Galeras.

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: J. Romero, INGEOMINAS - Observatorio Vulcanólogico del Sur.