Activity remained at low levels through April and May, similar to January-March of this year. Seismicity was characterized by small-magnitude "butterfly-type" events near the active cone, principally shallow earthquakes associated with rock fractures and fluid movement. It is possible that this activity is influenced by the gravitational field associated with tides (lunar-solar attraction) and by external agents such as rain. Sporadic long-period events are associated with fluid movement, and high-frequency events are associated with rock fractures.

Shallow "butterfly-type" earthquakes were frequent until mid-April, then decreased during May to an average of <10 earthquakes/day toward the middle of the month. High-frequency earthquakes reached a maximum of 3/day and were located mainly 3-4 km W and N of the summit at depths of 2-7 km. On 12 May, one of these earthquakes (M 1.9), was felt in Jenoy, 8 km N of the volcano. Five small-magnitude "screw-type" events were registered from 1 to 12 May. A tremor pulse on 27 May that lasted for ~15 minutes was possibly caused by magma-water interaction; it occurred during a time of strong rains in the region.

Electronic tiltmeters installed on the volcanic structure did not register any deformation in April or May. The SO₂ measurements taken from the gas column during April revealed continued low emission levels. COSPEC measurements of SO₂ in May were also low, with a variation of 50-798 t/d. Most fumarolic activity was toward the W side of the main crater.

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: INGEOMINAS, Pasto.