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Laguna del Maule Volcanic Complex (LMVC, Southern Andes of Chile) has been experiencing large rates (ca. 30 cm/yr) of surface uplift as detected since 2008 by satellite geodetic measurements. Previous works have modeled the source of this deformation as an inflating rectangular sub-horizontal sill underlying LMVC at 5 km depth, which is supposedly related to an active process of magmatic replenishment of a shallow silicic reservoir. However little is known about the tectonic context on which this activity is taking place, particularly its relation with crustal seismicity that could help understanding and monitoring the current deformation process. Here we present the first detailed characterization of the seismic activity taking place at LMVC and integrate it with structural data acquired in the field in order to illuminate the possible connection between the ongoing process of surface uplift and the activation of crustal faults. Our main finding is the recognition of repetitive volcano-tectonic (VT) seismic swarms that occur periodically between 2011 and 2014 near the SW corner of the sill modeled by InSAR studies. A cross-correlation analysis of the waveforms recorded for these VT events allows identifying three different seismic families. Families F1 and F3 share some common features in the stacked waveform and its locations, which markedly differ from those of family F2. Swarms belonging to this later family are more energetic and its energy was increasing since 2011 to a peak in January 2013, which coincide with maximum vertical velocities detected by local GPS stations. This points to a common process relating both phenomena. The location of VT seismic swarms roughly coincides with the intersection of a NE-SW lineament with a WNW-ESE lineament. The former shows clear field evidences of dextral strike-slip that are fully consistent with one nodal plane of focal mechanism for well-recorded F2 events. The conjugate nodal plane of these focal mechanisms could coincide with the WNW-ESE lineament, for which our field reconnaissance suggests a dominant normal motion. Events belonging to families F1 and F3 are also dominantly strike-slip but with some mixture with thrust and normal components. Our results, in conjunction with results of previous authors, suggest a complex mechanical interaction between the arrangement of crustal faults forming the structural framework on which the magmatic plumbing system of LMVC is emplaced and the inflating source at depth. [ABSTRACT FROM AUTHOR]

• Surface subsidence found in Cerro Pabellon geothermal plant. • Slow rates of subsidence. • Different sources of deformation may explain the location of the subsidence within the aquifer. In 2017, Cerro Pabellón in northern Chile became the first high-enthalpy geothermal power facility to go into service in South America. We evaluated Interferometric Synthetic Aperture Radar (InSAR) data containing 201 SAR scenes from the Sentinel-1A/B satellite mission between 2014 and 2021 to quantify the time-dependent surface deformation in the vicinity of the Cerro Pabellón geothermal plant. Using ascending and descending satellite orbits and the InSAR small baseline subset (SBAS) approach, we detected subsidence rates up to 6.3 mm/yr along the line of sight near the geothermal area, which began in 2017 and persisted through 2020 at which time the deformation rate decreased. We believe that geothermal activity may be responsible for the observed deformation based on the timing of the deformation – there was no detectable deformation in the area until the start of operations of the power plant. We invert the source of ground deformation using multiple analytic elastic half-space models (one or more spheroi ds or a sill-like source). The depth of the inferred sources is between 370 and 660 meters and is similar to the depth of the reservoir as determined by surface geophysics and well data. InSAR time series have not yet detected any tectonic deformation on the faults that encircle the graben where the geothermal plant is located, nor at the graben's periphery. While the seismic records from a regional network near the geothermal field, analyzed between March 2017 and August 2020, show microseismicity in the graben near the power plant, accurate positions cannot be computed due to sparse and inadequate seismic data. Therefore, any possible correlation between the observed seismicity and the geothermal operations remains uncertain. This study emphasizes the need for an improved seismic network to obtain a better understanding of the geothermal-tectonic interplay. [ABSTRACT FROM AUTHOR]

Llaima Volcano (38.692°S, 71.729°W) is one of the most active centers in South America, with ∼56 eruptions since 1852 CE. During the last eruptive cycle (2007–2009), the volcano displayed typical features of open conduit systems, with frequent long-period volcanic seismicity (LP). After the M W 8.8 Maule megathrust earthquake (27 February 2010), Llaima volcano located 305 km to SE of the epicenter, has experienced one of the longest periods of quiescence since 1852, with reduced activity observed since the beginning of its continuous instrumental monitoring in the year 2007. To better understand this behavior, we track the repose time between eruptions in the 20th century to depict the current period of quiescence and provide details of the pattern of volcano seismicity after the earthquake. The number of LP events decreased ∼90% shortly after the earthquake. In turn, crustal faults nearby (∼20 km south of the main crater) showed an increased level of seismicity lasting for approximately six months after the mainshock before returning to background levels. In order to propose driving mechanisms for such behavior, we computed the static changes of the stress tensor on chosen receiver structures. The acting receiver structure differs from those related to the long-term stress regime controlled by regional tectonics and was inferred from the pre-earthquake shallow seismicity (2007–2009). LP and volcanotectonic seismicity (VT) before the earthquake were located in a NW-SE pattern, in contrast to the NE-trending alignment of flank vents (and hence dike swarms) usually interpreted as an indicator of the maximum horizontal stress axis. From the static stress transfer imparted by the earthquake, we find that an opening event would have occurred at a NW-trending receiver structure coupled with closing of other potential structures, which promoted magma storage along this blind structure but precluding further propagation to the surface. Our results thus show a rare example of earthquake-induced suppression of volcanic activity, where the geometry of the fault-fracture network that shape the plumbing system likely plays a major role. • Large quiescence of an active volcano following of Maule earthquake M W 8.8 (27 February 2010), Chile • Coulomb stress calculation earthquake-volcano interaction • Dynamic and static stress transfer by Maule earthquake M W 8.8 Chile • Seismic behavior after the last eruptive period • Eruptive periods since 1860 to Llaima volcano [ABSTRACT FROM AUTHOR]

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. • Descriptions and evaluations of the seismic and eruptive behavior of Llaima volcano. • LP seismicity and Tremor are events recorded during all eruptive period and absence of volcano-tectonics seismic events. • Coexistence of Strombolian and Hawaiian type eruptions, and gentle lava flow. • Fractures on the flanks (SW and SE) and many vents inside the crater. • Paroxismal eruptive phases with sudden ending. Open conduit system behavior. [ABSTRACT FROM AUTHOR]

Automatic pattern recognition applied to seismic signals from volcanoes may assist seismic monitoring by reducing the workload of analysts, allowing them to focus on more challenging activities, such as producing reports, implementing models, and understanding volcanic behaviour. In a previous work, we proposed a structure for automatic classification of seismic events in Llaima volcano, one of the most active volcanoes in the Southern Andes, located in the Araucanía Region of Chile. A database of events taken from three monitoring stations on the volcano was used to create a classification structure, independent of which station provided the signal. The database included three types of volcanic events: tremor, long period, and volcano–tectonic and a contrast group which contains other types of seismic signals. In the present work, we maintain the same classification scheme, but we consider separately the stations information in order to assess whether the complementary information provided by different stations improves the performance of the classifier in recognising seismic patterns. This paper proposes two strategies for combining the information from the stations: i) combining the features extracted from the signals from each station and ii) combining the classifiers of each station. In the first case, the features extracted from the signals from each station are combined forming the input for a single classification structure. In the second, a decision stage combines the results of the classifiers for each station to give a unique output. The results confirm that the station-dependent strategies that combine the features and the classifiers from several stations improves the classification performance, and that the combination of the features provides the best performance. The results show an average improvement of 9% in the classification accuracy when compared with the station-independent method. [ABSTRACT FROM AUTHOR]

This paper proposes a computer-based classifier to automatically identify four seismic event classes of the Llaima volcano, one of the most active volcanoes in the Southern Andes, situated in the Araucanía Region of Chile. A combination of features that provided good recognition performance in our previous papers concerning the Llaima and Villarica (located 100 km south of Llaima) volcanoes is utilized in order to train the classifiers. These features are extracted from the amplitude, frequency and phase of the seismic signals. Unlike the previous works where fixed length windows were used to obtain the seismic signals, this paper employs signals of variable lengths that span the entire seismic event. The classifiers are implemented using support vector machines. A confidence analysis is also included to improve reliability of the classification. Results indicate that the features used for recognition of the events of Villarica volcano also provide good recognition results for the Llaima volcano, yielding classification exactitude of over 80%. [ABSTRACT FROM AUTHOR]

Volcanoes are often monitored by geophysical and geochemical instruments that aim to track and anticipate their eruptive activity. However, assessment of the state of the volcano at any given time, and its evolution towards eruption or change in eruptive activity is notoriously difficult. Once explosive activity has begun, the study of volcanic ash can provide crucial insights on whether the activity is mainly driven by the hydrothermal system, shallow gas accumulation, or/and a new stalled intrusion close to the surface. We present the results of a study of ash componentry from part of the current volcanic crisis that started in December 2015 at Nevados de Chillán Volcanic complex (Chile) which we integrate with seismic and visual data. We identified three main stages: (i) an early one that lasted for about a year and includes two months of increased seismicity and significant amount of juvenile ash fragments, and thus suggesting some explosions were fed by a shallow magma intrusion. (ii) A second one which lasted for about six months with cycles of quiescence and explosions, and a predominance of lithic particles in the ash, suggesting that the explosions were probably driven by the dynamics in the upper part of the system, including shallow gas accumulation or/and the hydrothermal system, rather than by fresh magma intrusion. (iii) Finally, after about two years of unrest and intermittent explosions, seismicity increased again and the ash became dominated by juvenile particles, and led to the extrusion of a dome. The timing and sequence of events that we report is broadly similar to other volcanoes that have produced dome eruptions such as Soufriere Hills (Montserrat), Unzen (Japan) and Sinabung (Indonesia). Our study highlights the usefulness of integration of volcanic ash studies with other monitoring data and importance of integration of many case studies to gain a more comprehensive understanding of the processes and evolution of dome-forming eruptions. [Display omitted] • Ash particles are classified using binocular and Scanning Electron Microscopy observations. • We find temporal changes in juvenile ash component, crystallinity, shape and alteration degree. • Correlation of ash time-series with seismic and visual data suggests three stages including dome extrusion. • We compare this succession to other dome volcanoes including Sinabung, Soufriere Hills and Unzen. [ABSTRACT FROM AUTHOR]