According to ONEMI, OVDAS-SERNAGEOMIN reported that during 1-31 March there were 83 volcano-tectonic events recorded at Copahue, and 204 earthquakes indicting fluid movement. Tremor levels increased on 24 March, the same day as the phreatic explosion, though by the next day decreased to baseline levels. Webcams recorded gas plumes rising from El Agrio crater as high as 1 km. During an overflight on 3 April scientists observed the crater lake, and continuous white gas plumes rising almost 400 m. The Alert Level remained at Yellow (second highest level on a four-color scale); SERNAGEOMIN recommended no entry into a restricted area within 1 km of the crater. ONEMI maintained an Alert Level Yellow (the middle level on a three-color scale) for the municipality of Alto Biobío.

Geological Summary. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Sources: Oficina Nacional de Emergencia-Ministerio del Interior (ONEMI), Servicio Nacional de Geología y Minería (SERNAGEOMIN)