Seismicity has generally remained low following the 7 June eruption. In the first half of July, "screw-type" long-period events were detected at a rate of ~1/day. Shallow "butterfly-type" events (hybrid between high-frequency and long-period) at depths of 1.2-1.5 km and M 0.6-1.4 began increasing in mid-July, reaching a maximum of 66 events on 20 July. A M 2.6 long-period event also occurred on 17 July, centered within the E part of the volcano at depths greater than the "screw-type" or "butterfly-type" events. Most of the seismicity was registered at the seismic station 0.9 km NE of the summit. After this brief increase, seismic activity decreased to low levels by the end of the month and remained low through August. Most events registered in August were "butterfly-type" earthquakes, the greatest number of which were detected on 7-12 August, with a maximum of 255 events/day. After this swarm the number of daily events returned to previous values. Only 8 "screw-type" events occurred in August, with durations of 32-100 seconds. Seismicity in August was characterized by high-frequency events, generally located at shallow levels NW of the active crater, with magnitudes between 0.7 and 1.6 and depths of 0.3-8.0 km.

Observations during overflights in July and August revealed continued minor degassing at most of the fissures in the active crater, with the exception of the "Florencia" fumarole and the "Chava" fissure SW of the crater rim. Weak emissions were producing columns rising <10 m above the vents. COSPEC measurements of SO₂ flux were low, ranging from 31-233 t/d. Rockslides from the inner walls of the main crater were seen during a 3 July overflight. Electronic tiltmeters continued to show great stability in July and August, similar to recent months.

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.