The Observatorio Vulcanológico y Sismológico de Pasto (OVSP) reported that during 1 January-31 March 2002, volcanic activity at Galeras was at a low level, similar to that reported in recent years. During the report period, a total of 43 volcano-tectonic events were recorded with a total seismic energy of 1.25 x 10¹⁵ erg, based on their durations. The majority of these events were located toward the NE of the volcanic edifice and W of the active crater (figure 99).

Figure 99. Epicenters of volcano-tectonic earthquakes that occurred in the Galeras region during January-March 2002. Courtesy OVSP.

On 7 January a small earthquake swarm, including eight small-magnitude events, was located beneath the western part of the active crater. Two earthquakes were felt in Pasto and Jenoy. The first occurred on 12 March at 0128 with a local magnitude of 2.4; it was located 5 km NE of the active crater and 8 km below the summit. The second occurred on 14 March at 1753 with a local magnitude of 3.1; it was located 6 km NE of the active crater and at a depth of 8 km below the summit.

Three long-period events and 37 tremor episodes with a total energy of 3.08 x 10¹³ erg were registered. Some of the tremor episodes were of harmonic type, with dominant frequencies between 2.2 and 2.5 Hz (figure 100), with the source located in the crater region at shallow levels.

Figure 100. Digital record of a tremor episode at Galeras on 5 January 2002 at broad-band station Crater (1.6 km S of the active crater) and its normalized spectrum. Courtesy OVSP.

On 12 January a tornillo event was registered (figure 101) with a spectrum that showed frequency picks at 2.4, 6.9, and 8.9 Hz. The energy of this event, based on its duration, was 3.04 x 1013 erg.

Figure 101. Digital record of tornillo event registered on 12 January 2002 at broad-band station Anganoy (0.9 km E of the active crater) and its normalized spectrum. Courtesy OVSP.

On 5 January 2002 three seismic events occurred that were associated with rockfalls that resulted from the physical instability of the volcanic edifice in the W sector, rains, and earthquakes near to that unstable region. Similar activity had occurred on 6 December 2001.

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: Patricia Ponce, Observatorio Vulcanológico y Sismológico de Pasto (OVSP), INGEOMINAS, Carrera 31, 18-07 Parque Infantil, P.O. Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).