Report on Poas (Costa Rica) — January 1995

Bulletin of the Global Volcanism Network, vol. 20, no. 1 (January 1995)
Managing Editor: Richard Wunderman

Poas (Costa Rica) January seismicity highest in five months

Please cite this report as:

Global Volcanism Program, 1995. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 20:1. 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)

Fumarolic activity continued at Poás in the reestablished crater lake. OVSICORI-UNA reported that in January the lake had a temperature of 50°C, a turquoise-green color, and its level dropped by 40 cm with respect to December. During January evaporative clouds rose to as high as 50 m above the lake. The N part of the dome continued to slide. No significant changes were registered by study of either leveling lines, dry tilt, or the distance measuring network over the crater surface.

During January, low- and medium-frequency earthquakes took place more often than in the five previous months (table 6). As partly summarized in 19:11, during 1994 low-frequency earthquakes peaked in March (7,118 events), medium-frequency earthquakes peaked in June (556 events), and high-frequency earthquakes peaked in August (31 events). The greatest number of seismic events took place on 12 January in the low-frequency range, and on 16 January in the medium-frequency range (figure 57).

Table 6. Poás seismic data, January 1994-August 1995 (from the station POA2, located 2.7 km SW of the active crater). NR = Not Reported. Courtesy of OVSICORI-UNA.

[Skip text table]
                       Frequency            Hours
    Month       Low      Medium     High     of
              (<2 Hz) (2.1-3.0 Hz) (>3 Hz)  Tremor

    Jan 1994   4,040       94         0       NR
    Feb 1994   4,786       59         0       NR
    Mar 1994   7,118      114         0       NR
    Apr 1994   6,916       23         7       NR
    May 1994   5,336       99         9       NR
    Jun 1994   5,472      556         1       NR
    Jul 1994   4,994      374        15       NR
    Aug 1994   3,639      102        31       NR
    Sep 1994   1,524       31        24       NR
    Oct 1994   3,630       88         0       NR
    Nov 1994   3,563       91         1       NR
    Dec 1994   2,539       14         2       NR

    Jan 1995   4,347      198         0        0
    Feb 1995   4,710      207        20        0
    Mar 1995   6,881      113         1        0
    Apr 1995   5,798      367         2       11
    May 1995   3,142      715         0        5
    Jun 1995   2,258      322        28        0
    Jul 1995     NR        NR        NR       NR
    Aug 1995   5,486      165         0        9

Figure (see Caption) Figure 57. Low- and medium-frequency earthquakes at Poás during January 1995. Courtesy of OVSICORI-UNA.

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: Erick Fernández, Jorge Barquero, Rodolfo Van der Laat, Franklin de Obaldia, Tomás Marino, Vilma Barboza, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.