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Report on Irazu (Costa Rica) — May 1991


Bulletin of the Global Volcanism Network, vol. 16, no. 5 (May 1991)
Managing Editor: Lindsay McClelland.

Irazu (Costa Rica) Tectonic earthquake swarm

Please cite this report as:

Global Volcanism Program, 1991. Report on Irazu (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199105-345060


Costa Rica

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

All times are local (unless otherwise noted)

A large swarm of tectonic earthquakes was recorded just S of the crater from 2 January through the end of February. On 25 May, a rapid increase in the number of tectonic earthquakes marked the start of a second swarm in the same zone. A shock located about 1 km E of the crater was felt on 28 May (M 3.5), and two others centered near the crater were felt on 5 June at 0534 (M 3.5) and 0540 (M 3.2). Scientists believe that the seismicity may represent reactivation of the fault zone involved in the M [7.6] earthquake that occurred about 90 km ESE on 22 April. No changes in surface activity at the volcano were reported.

Geological Summary. 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: R. Barquero, ICE; Mario Fernández, Red Sismológica Nacional (RSN), Univ de Costa Rica; ACAN news service, Panamá City, Panamá.