During July and August 1999, low-intensity seismic activity continued, similar to that of previous months (BGVN 24:07). Fifty-six volcano-tectonic (VT) earthquakes were registered during this period compared to 90 during the previous two months. The depths of these VT events were between 0.35 and 19 km below the summit, and the total energy released was estimated as 4.82 x 1013 ergs. The largest magnitude event, on the morning of 16 July, had a coda magnitude 1.7 and depth of 8 km.

Additionally, 20 long-period events and 10 tremor episodes were recorded with an energy release of 5.38 x 1012 ergs. Dominant frequencies during the tremor episodes were 2.0-4.0 Hz. The tremor event on 23 July had a small amplitude with respect to the long coda, a quasi-monocromatic frequency of ~2.01 Hz, and an energy release of 2.09 x 1012 ergs. Periodic fumarole temperature measurements taken during the two-month period in the active crater registered a range of 130-394°C.

Radon-222 emissions measured in the soil at six stations were not significantly different from values in previous months. As in the May-June period, the greatest emissions occurred at the Sismo2 station (~5 km NE of the summit) attaining a maximum value of 2,297 pCi/l.

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