Report on Poas (Costa Rica) — March 2000

Bulletin of the Global Volcanism Network, vol. 25, no. 3 (March 2000)
Managing Editor: Richard Wunderman

Poas (Costa Rica) Conspicuous plumes and abundant low-frequency earthquakes in late 1999

Please cite this report as:

Global Volcanism Program, 2000. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 25:3. 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)

Comparative quiet continued at Poás; however, in addition to the fumarolic degassing often seen, seismicity was relatively high during the reporting period, June 1999-January 2000, when low-frequency earthquakes typically registered over 4,000 times per month (table 9). For comparison, a relative high in 1999 occurred in May when low-frequency events occurred ~1,400 times, and during a high in January 1998, when there were over 2,500 events. On 18 July 1999 an MR 3.1 earthquake occurred with 6 km focal depth and an epicenter 5 km NW of the active crater.

Table 9. A summary of seismic, temperature, and lake height data for Poás during July 1999-January 2000. Lake heights are with respect to the previous month and positive upwards (rising lake levels). The stated temperature for the pyroclastic cone's degassing refers to the value at an accessible point where the measurements are taken regularly. The seismic station POA2 lies 2.8 km SW of the active crater. "NR" indicates information absent and not reported. Courtesy of OVSICORI-UNA.

[Skip text table]
    Month        Low-frequency    Mid-frequency       Total
                  earthquakes      earthquakes     earthquakes

    Jun 1999        2,571               NR              NR
    Jul 1999        2,996               64            3,061
    Aug 1999        4,548              184            4,736
    Sep 1999        5,264              274            5,542
    Oct 1999        4,743              190            4,987
    Nov 1999        4,142              221            4,393
    Dec 1999        4,278              405            4,679
    Jan 2000          NR                NR              low

    Month        Crater lake      Surface elevation change
                 temperature           of crater lake

    Jun 1999         NR                      NR
    Jul 1999        32°C                  -47 cm
    Aug 1999        37°C                  -32 cm
    Sep 1999        40°C                     NR
    Oct 1999        38°C                     NR
    Nov 1999        33°C                  101 cm
    Dec 1999        35°C                  156 cm
    Jan 2000        40°C                 ~140 cm

Tremor, which was seldom reported in 1999, took place for less than about 0.5 hours a day during October-November 1999. In contrast, tremor averaged only 0.1 hours a day during December 1999. In contrast, tremor durations of 20 to 70 hours were common in early 1998. Also appearing in the month of October 1999 were 5 unusual low-frequency events in conjunction with tremor; these low-frequency earthquakes had periods of 40-175 seconds.

During August -October, the pyroclastic cone's degassing led to unusually high plumes reaching 0.7 to 2 km above the crater floor. December plume heights ranged between 0.7 and 1 km. Some of the hottest temperatures were measured near the pyroclastic cone: up to 95°C during December-January and often over 92°C when reported during other months in late 1999.

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, V. Barboza, R. Sáenz, E. Malavassi, R. Van der Laat, T. Marino, J. Barquero, and E. Hernández, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: