Abstract

Llaima Volcano, one of the most active volcanoes in South America, has experienced intense activity during the last 100 years. The most recent eruptive activity occurred during the period 2007–2009 with at least six energetic eruptions, the January 1st 2008 (VEI 3) being the strongest episode. Most of the paroxysmal activity was characterized by a rapid increase in seismic energy (minutes to hours) and the absence of precursory signals, as well as by an accelerated drop of the seismic energy and eruption intensity at the end. Moreover, subtle changes in the increase of low-energy long-period (LP) seismicity and the occurrence of minor explosive activity (six months before the onset of the eruptive cycle), were the only remarkable changes observed in advance. This is the first study that includes a detailed description of the 2007–2009 eruptive period and is based on a temporal analysis of the seismic records, technical reports from the monitoring network and >2000 photos. This set allows a characterization of the coeval eruptive styles (including strombolian and hawaiian activity), morphological changes of the active vents inside the main crater and two fractures located in the SW and SE flanks. Ten phases were identified based on the eruptive style, Volcanic Explosivity Index (VEI) and the salient features of the seismic events. The seismicity that accompanied the eruptive phases was characterized by the predominance of a continuous tremor (TR) and discrete LP events, and a remarkable absence of volcano-tectonics (VT) earthquakes. A waveform cross-correlation analysis of LP events showed an overall low similarity between them, which suggests multiple sources. Our observations and analysis suggest that the plumbing system is composed of multiple independent structures, some of which reached the surface during this eruptive period, indicating that the upper part of the cone could be an unstable and essentially weak zone. The activity and seismicity observed are characteristic of an open-vent system in which the magma can ascend rapidly from several km depth, with little to no clear precursory activity. These results are in agreement with petrological studies of the products of these eruptions.