OVDAS-SERNAGEOMIN reported that seismicity at Copahue increased at 1250 on 9 September, characterized by increased amplitude of continuous tremor. Anomalies in the seismic signals suggested elevated surface activity, though cloudy weather prevented visual observations. According to the Buenos Aires VAAC diffuse steam emissions recorded by the webcam on 10 September rose to 3.5 km (11,000 ft) a.s.l. and possibly contained minor amounts of ash. The Alert Level remained at Yellow (second lowest level on a four-color scale); SERNAGEOMIN recommended no entry into a restricted area within 500 m of the crater. ONEMI maintained the Alert Level at 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: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Oficina Nacional de Emergencia-Ministerio del Interior (ONEMI), Buenos Aires Volcanic Ash Advisory Center (VAAC)