Two small pyroclastic eruptions in the first half of April produced columns 6 km high. The first, at 1603 on 4 April, ejected 18 x 10⁴ m³ of ash. Seismicity associated with the eruption reached M 3 and lasted for 123 seconds, saturating nearby stations (within 2 km) for the first 17 seconds. Analysis of records from stations >5 km away showed dominant frequencies of 4.9 and 12.6 Hz. Long-period seismicity increased slightly for 8 hours after the explosion. The second eruption occurred at 0321 on 13 April, with 21.7 x 10⁴ m³ of ash and blocks ejected. The long-period event associated with this eruption reached M 3.1 and lasted for 140 seconds, saturating nearby stations for the first 33 seconds. The dominant frequencies were 9.8 and 12.4 Hz. Small-magnitude long-period seismicity continued for 30 minutes.

Seven high-frequency events were registered on 1 April, with a maximum magnitude of 4.5. The earthquakes occurred at 0048 (M 4.2), 0159 (M 4.5), 0204 (M 4.0), 0303 (M 3.5), 0508 (M 3.1), 0839 (M 3.0), and 2145 (M 2.8). High-frequency seismicity increased again 26 April, peaked the morning of the 27th (figure 66), and was continuing in early May. Another earthquake, M 3.6, occurred at 1030 on 29 April. All of these earthquakes, as well as 67 other events, had epicenters 3 km N of the active crater at depths of 2-8 km below the summit (figure 67). There were ~300 earthquakes recorded in April 1993.

Figure 66. Daily number of earthquakes at Galeras, 1 January to 30 April 1993. Dashed line indicates long-period events; solid line indicates high-frequency events (very low until 27 April). Arrows at top indicate eruptions on 14 January, 23 March, 4 April, and 13 April. Courtesy of INGEOMINAS.

Figure 67. Locations of high-frequency earthquakes at Galeras, 26-30 April 1993. Courtesy of INGEOMINAS.

"Screw-type" events, monochromatic long-period events characterized by a long, slowly decaying coda, reappeared on 8 April. A total of 18 of these events was recorded in April, the most significant at 0619 and 1030 on 10 April and at 0926 on 29 April, about an hour before an M 3.6 earthquake. This type of seismic signal has usually preceded eruptions, but was absent before the 4 April eruption. However, relatively small earthquakes, "hybrids between high-frequency and long-period," were registered at stations close to the crater. This activity, similar to that observed before other eruptions at Galeras, was more noticeable during the first half of the month, with swarms on 1, 2, 6, 8, and 9 April.

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, Pasto.