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Precursors and build-up May 2008-4 June 2011
In May 2008, Puyehue had increased seismic activity, but no eruption followed. Seismic activity increased again in late April and early May 2011. A column of gases was observed rising from Puyehue on 26 April and the alert level of SERNAGEOMIN was raised to 3 (yellow). On 5 May, SERNAGEOMIN first detected earthquakes typical for magma moving at depth. The earthquakes were located 2-5 km beneath the southeast flank of the volcano. Seismic activity continued to rise during May. On 2 June, seismic activity raised sharply and the alert was raised to 4 (yellow, "eruption likely in the near future, days to weeks"). On 3 June, a seismic swarm with 1450 earthquakes was measured, with more than 300 quakes being larger than magnitude 2. There were still no visual changes at the volcano.
Early on 4 June, red alert (5 out of 8) was given ("eruption imminent") after a further increase in seismicity, and evacuations of the areas around the volcano began. The seismic activity had evolved to tremor of high energy, interpreted as magma movements inside Cordón Caulle at depths between 1-4 km. In the last 6 hours before the eruption, an average of 230 earthquakes per hour were registered, 12 of which were greater than magnitude 4 and 50 greater than M3. The largest was a felt earthquake at 8:36 local time with a magnitude of 4.4. Residents reported up to 20 felt earthquakes on Friday evening and Saturday morning. About 3500 people were evacuated.
Start of the eruption (4 June)
The explosive eruption began in the afternoon of Saturday 4 June, and produced a 12 km high ash column drifting east into Argentinia.
Heavy ash fall ("like a snow storm" according to interviews) occurred in the city of Bariloche in Argentina and its airport was closed. The Argentine government sent military to assist residents, traffic and businesses affected by the ashfall. The border crossing between Chile and Argentina at Cardenal Samore was closed due to poor visibility in the ash rain.