OVSICORI-UNA reported that starting at about 1800 on 17 May the seismic network at Turrialba began recording very-long-period earthquakes, followed by sustained volcanic tremor with significant amplitude that began at 2200. At 1120 on 18 May an ash plume rose 600-700 m above the summit area and drifted SW. At 1430 tremor amplitude decreased, along with the emissions. A gas-and-vapor plume with low ash content rose as high as 300 m and drifted WNW. On 19 May vigorous gas emissions were observed, alternating with ash emissions at 0600, 0938, 1111, and 1405. The plumes rose 300-700 m and drifted S, SW, W, and NW; ashfall was reported in areas of Valle Central, including in Coronado, Guadalupe, and Heredia (38 km W). Tremor increased slightly at 1550 and an ash plume rose 1 km and drifted NNW. On 20 May at 0720 a Strombolian phase began, producing an ash-and-gas plume that rose 3 km and drifted W. The eruptive column collapsed, generating pyroclastic flows that reached the nearby ranches of La Silva and La Picada, Irazu volcano, and the Cráter Central. According to a news article, some airlines have canceled or delayed flights into the Juan Santamaría International Airport (48 km W).

Gas-and-ash emissions continued during 21-22 May; plumes rose as high as 600 m above the summit. Villagers reported ashfall in areas of San José (70 km W), Cartago (25 km SW), Alajuela (49 km W), Heredia (38 km W), Puriscal (65 km WSW), and Jaco (100 km SW). During 22-23 May tremor amplitude decreased. Ash plumes rose as high as 1 km and drifted W and SW on 23 May, causing ashfall in areas downwind including Tapezco (Zarcero-Alfaro Ruíz, 70 km WNW), Guácima de Alajuela (55 km WSW), Cartago (25 km SW), Alajuela, Heredia, Barva (39 km W), Finca Lara (17 km W), Finca Laguna (23 km WNW), Grecia, and Naranjo. Seismic tremor amplitude significantly decreased on 24 May, and ash was no longer visible in the emissions. Gas-and-steam plumes rose 500 m above the volcano.

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.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA), Tico Times