Report on Poas (Costa Rica) — September 1995

Bulletin of the Global Volcanism Network, vol. 20, no. 9 (September 1995)
Managing Editor: Richard Wunderman

Poas (Costa Rica) Over 9,000 seismic events in September, most of them low-frequency; no tilt

Please cite this report as:

Global Volcanism Program, 1995. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 20:9. Smithsonian Institution.

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Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)

During September, 9,144 seismic events took place, the most for any month in 1994 or thus far in 1995. These events were predominantly low-frequency (8,854 events, figure 58); <300 were medium-frequency events. The day with the largest number of events, the 13th, was followed four days later by M 3.1 and 2.7 earthquakes. These struck at 2 and 5 km depths, and 2.3 and 6.7 km SW of the main crater, respectively. During September, short intervals of tremor registered for a total of 19 hours, the longest duration of any month in 1995. Other geophysical measurements during September, specifically dry tilt and the distance net surrounding the active crater, failed to show significant changes.

Figure (see Caption) Figure 58. Low-frequency seismicity at Poás, January-September 1995. Data were collected at station POA2 located 2.7 km SW of the active crater. Courtesy of OVSICORI-UNA.

The level of the sky blue lake within the N crater climbed 0.6 m in September with respect to August. The lake's temperature was 34°C. Fumaroles on the W lake terrace increased their output, but they only generated gas columns <50 m high. Although weaker than these fumaroles, two new fumaroles appeared on the terrace to the NW and SW of the lake. Other new fumaroles were seen along the N wall of the pyroclastic cone; fumarolic gases discharging from the cone reached 93°C. Constant bubbling continued to issue from points in the central and W lake. The fumarolic area on the SW and S wall maintained a 90-95°C temperature and discharged gas columns that rose as high as 100 m. Mass wasting of unstable hydrothermally altered rocks in this area covered some fumarolic vents and opened new ones.

Geologic Background. The broad, well-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, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 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 the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.