On 6 August the Alert Level for Laguna del Maule was raised to Yellow (the second lowest level on a four-color scale) based on data analysis by Servicio Nacional de Geología y Minería (SERNAGEOMIN) and Servicio Nacional de Prevención y Respuesta ante Desastres (SENAPRED). The Servicio Geológico Minero Argentino (SEGEMAR) Observatorio Argentino de Vigilancia Volcánica (OAVV) also raised the Alert Level to Yellow (the second lowest level on a four-color scale). The number of seismic events, notably volcano-tectonic earthquakes, had progressively increased in recent weeks and were mainly located in the E and central parts of the complex. Several swarms were detected; in total, more than 11,000 volcano-tectonic earthquakes were recorded in July. Additionally, the rate of surface deformation had accelerated, reaching high levels. Vertical deformation rates using Global Navigation Satellite System (GNSS) data were 4.2 cm per month at the station located closest to Laguna del Maule lake during mid-May-July. Variations in horizontal displacement were around 2.1 cm/month, a value characterized as high. SENAPRED declared “Early Warning” for San Clemente enabling increased monitoring. The public was warned to stay 2 km away from the center of the activity, in the Troncoso-Nieblas sectors about 5 km SW of the lake’s shore, due to the potential for anomalous carbon dioxide emissions.

Geological Summary. The Laguna del Maule volcanic complex includes a 15 x 25 km caldera with a cluster of small stratovolcanoes, lava domes, and pyroclastic cones of Pleistocene-to-Holocene age. The caldera lies mostly on the Chilean side of the border, but partially extends into Argentina. Fourteen Pleistocene basaltic lava flows were erupted down the upper part of the Maule river valley. A cluster of Pleistocene cinder cones was constructed on the NW side of Maule lake in the northern part of the caldera. The latest activity produced an explosion crater on the E side of the lake and a series of Holocene rhyolitic lava domes and blocky lava flows that surround it.

Sources: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Servicio Geológico Minero Argentino (SEGEMAR)