Report on Irazu (Costa Rica) — October 1994
Bulletin of the Global Volcanism Network, vol. 19, no. 10 (October 1994)
Managing Editor: Richard Wunderman.
Irazu (Costa Rica) Eighteen shallow earthquakes M <=2
Please cite this report as:
Global Volcanism Program, 1994. Report on Irazu (Costa Rica) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:10. Smithsonian Institution.
9.979°N, 83.852°W; summit elev. 3432 m
All times are local (unless otherwise noted)
During October, the crater lake at Irazú remained high, covering the crater floor with yellow-colored water. In addition to active flank fumaroles on the NW, subaqueous fumaroles bubbled consistently in the N, NW, W, SW, and SE parts of the lake, near the crater wall. Rockslides were seen coming down the N, SW, and E crater wall. Seismic events in October totaled 18 earthquakes with S minus P values of 2-3 seconds; some events reached M 2 with epicenters <3 km from the crater and focal depths of 4.0-4.5 km. Geodetic and leveling surveys in September found no significant changes.
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. Fernández, J. Barquero, and V. Barboza, OVSICORI-UNA; G. Soto and F. Arias, ICE; Mauricio Mora, Escuela Centroamericana de Geología, Univ de Costa Rica.