SERNAGEOMIN reported that two long-period earthquake signals were recorded at Nevados de Chillán at 0813 on 3 October and 1630 on 4 October, and both were followed by a dense ash emission. The ash plume from the first event rose 760 m above the summit and drifted SSW, while the ash plume from the second event rose as high as 1.9 km and also drifted SSW. The Alert Level remained at Yellow, the second lowest level on a four-color scale. ONEMI maintained an Alert Level Yellow (the middle level on a three-color scale) for the communities of Pinto and Coihueco, and reminded residents not to approach the crater within 2 km.

Geological Summary. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The dominantly andesitic Cerro Blanco (Volcán Nevado) stratovolcano is located at the NW end of the massif. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The Volcán Nuevo lava-dome complex formed during 1906-1945 on the NW flank of Viejo. The Volcán Arrau dome complex was then constructed on the SE side of Volcán Nuevo between 1973 and 1986, and eventually exceeded its height. Smaller domes or cones are present in the 5-km valley between the two major edifices.

Source: Servicio Nacional de Geología y Minería (SERNAGEOMIN)