OVSICORI-UNA reported that an eruption from Turrialba's West Crater on 21 May was preceded by seismic activity characterized by more than 150 volcanic earthquakes per day since 18 April.

Increased gas emissions were detected on 20 May, producing a sky-blue plume visible from nearby areas. Hybrid earthquakes also increased and became numerous at 0452 on 21 May. Continuous harmonic tremor followed and then increased at 0720. Eruptions from West Crater occurred at 0830 and after 1100 from two vents which opened in January 2010 (Boca 2010, on the W wall) and January 2012 (Boca 2012, on the E wall). The eruptions generated ash plumes that rose more than 500 m; ashfall was reported in the area of Picada (N), and in San José (35 km WSW) and Heredia (38 km W) of Ipís de Guadalupe, Goicoechea (28 km WSW), la Fazio, Zetillal (43 km W), San Isidro-San Pedro de Coronado, and San Luis de Santo Domingo (28 km W). At around 1200 ash emissions ceased and seismicity decreased.

Geological Summary. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)