During May, water in the crater lake returned to the level of the previous summer. Fumarolic emissions N of the lake decreased, while subaqueous fumaroles in the SE, E, and N parts of the lakes remained active. Small landslides occurred along the crater's E, N, and SW walls. A monthly total of 126 earthquakes was recorded (at station IRZ2, 5 km W of the crater), with a M 1.8 event centered 3.6 km SW of the crater, at 1 km depth, on 5 May. Low-frequency seismicity continued through May.

Geological Summary. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the 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 main crater, which contains a small lake. The first well-documented 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. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI.