Servicio Nacional de Geología and Minería (SERNAGEOMIN) Observatorio Volcanológico de Los Andes del Sur (OVDAS) reported that increasing seismicity at Nevados de Chillán and increased activity of the hydrothermal system prompted an Alert Level increase to Yellow, on a three-color scale. During 1-31 December the seismic network recorded 1,259 seismic events, including 186 volcano-tectonic events with a maximum local M (LM) of 1.8 located 17 km NNW at a depth of 4 km, 1,030 long-period earthquakes with a maximum LM 1.7, 40 short episodes of spasmodic tremor, and three tornillo-type events. The webcam recorded a small white plume rising from the crater on 9 December.

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)