OVSICORI-UNA reported that at about 0530 on 28 April seismicity at Turrialba increased, though gas-and-steam emissions continued at normal levels. At 0300 on 30 April passive ash emissions began and rose over 500 m above West Crater. Continuous variable amplitude tremor and frequent small explosions were recorded during 30 April-1 May. At 0630 on 1 May residents in Santa Cruz de Turrialba (8 km SSE) reported hearing sounds from the volcano resembling a turbine. At the same time the seismic network recorded an increase in the amplitude of tremor, associated with an increase in gas and tephra emissions. Minor amounts of ash fell in La Central (4 km SW) and La Pastora. On 2 May frequent small explosions and sustained seismic tremor with significantly variable amplitude were recorded. Ash-and-gas emissions rose 500 m above the crater and drifted W, though periodically plumes with higher volumes of ash rose just over 1 km. Based on a news report, there were more than 200 explosions recorded during 29 April-2 May; an explosion at around 0600 on 2 May produced an ash plume that rose 2 km high. Seismic amplitude began decreasing during 0300-0700 on 3 May. Frequent explosions continued to produce ash plumes that rose as high as 1 km and drifted mainly N. Most tephra-fall occurred around West and Central craters.

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