Following the earthquake swarm in late November and early December 1993, seismic activity returned to the low levels of previous months. During January, activity consisted primarily of rock-fracture events located at superficial levels and others at depths of <3 km around the active crater. The largest event (M 2.0), on 3 January, was located at a depth of 2.9 km. The deepest were at 2.5-8 km, centered ~ 3.5 km NNE of the active crater where the November and December 1993 swarms were located. One event was felt on 17 January (M 2.6), centered on the NNE source at a depth of 5.2 km. High-frequency butterfly-type events showed a periodicity similar to tidal patterns.

Long-period events associated with fluid movements, though few in number (14), were a new type of event detected in January. These events had durations of 7-64 seconds, with maximum amplitudes between 5.2 and 27.8 mm, and frequencies of 1-10 Hz. Variations in background tremor continued to be registered at the Crater-2 station located 1.6 km from the active crater.

Deformation data did not indicate variations on the surface of the volcano. However, on 18 January there were increases in both the tangential and radial components at the Peladitos tiltmeter (1.6 km E of the active crater) of 14.2 µrad, which may have been caused by electronic problems.

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.