During October, tremor at Poás reached 101 hours; the last time tremor rose over 12 hours/month was May-September 1994, an interval when tremor ranged between 49 and 307 hours/month. The number of minor earthquakes, which were predominantly of low frequency, continued to climb during the month of October, reaching 9,838 events. This was a value ~8% larger than the total for September, the previous month with the most seismic activity in 1995.

The crater lake has risen consistently: by ~5 m during June-October (ICE), and by ~30 cm in the last month (OVSICORI-UNA). During October 1995, the fumarole on the W terrace appeared to have decreased its emissions compared to recent months (< 50-m-high steam plumes), and others on the lake's NW and SW sides also had diminished output. Fumaroles on the S and SW crater wall produced steam columns reaching 100 m tall. During October, bubbling in the lake still continued. During October OVSICORI-UNA scientists measured the temperatures at several sites: pyroclastic cone, 93°C; fumaroles on the S and SW sides of the crater, 95-97°C; the lake in the inactive crater (Lake Botos), 15°C; and the lake in the active crater, 30°C.

Head scarps of landslides that emanate from the dome and flow toward the lake displayed ongoing mass wasting; ICE workers mentioned that this mass wasting may have been triggered by recent heavy rains. In addition, ICE reported that on 17 September (at 0548) a M 3.9 earthquake struck; it had a depth of 5 km and an epicenter 1.6 km SW of the main crater. At the summit, the earthquake's intensity was MM III-IV.

Geological Summary. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernandez, E. Duarte, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA); Mauricio Mora, Escuela Centroamericana de Geologia, Universidad de Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles: OSIVAM, Instituto Costarricense de Electricidad (ICE).