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Report on Irazu (Costa Rica) — March 2000

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

Irazu (Costa Rica) Microseisms during June-November 1999 take place 12-61 times a month

Please cite this report as:

Global Volcanism Program, 2000. Report on Irazu (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 25:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200003-345060.

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Irazu

Costa Rica

9.979°N, 83.852°W; summit elev. 3432 m

All times are local (unless otherwise noted)


Seismic monitoring disclosed many days during June-November 1999 without any detectable microseisms. Monthly averages for July-November 1999 (table 6) tallied 12 to 61 microseisms a month, an average of roughly 0.5-2 microseisms per day. Other monitoring data were absent. The last eruption at Irazú consisted of a small phreatic explosion in December 1994 (BGVN 19:12).

Table 6. Monthly microseisms at Irazú as recorded at station IRZ2, located 5 km SW of the active crater, July-November 1999. The two months with available RSAM data show the range of computed values (e.g., October estimates of 7 to 101 units). Courtesy of OVSICORI-UNA.

Month Number of microseisms RSAM
Jul 1999 61 --
Aug 1999 17 --
Sep 1999 35 --
Oct 1999 29 7-101
Nov 1999 12 8-136

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

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.