Estudio infrasónico de volcanes activos de los Andes del Sur aplicado al monitoreo remoto

Abstract

Volcanic activity produces a wide variety of acoustic signals. Infrasound is sound below 20 Hz, capable of propagating hundreds of kilometers in the atmosphere while preserving information about the source. For this reason, recording these signals is a widely used monitoring tool of surface activity, both local (<20 km) and regional (>100 km). The path of propagation is controlled by the atmospheric structure, mainly by temperature gradients and the horizontal components of the wind. The circulation conditions that prevail over Patagonia enhance the propagation of infrasound of eruptive sources located in the volcanic arc towards the east. For this reason, in this Thesis we deployed two small-aperture infrasound arrays (<300 m) in the Andean region of Neuquén, to test their performance as a monitoring tool for volcanic processes. The first infrasound array installed in Argentina was deployed in Caviahue in 2014, 13 km from the Copahue volcano (CPH, aperture 280 m). Applying the multichannel semblance analysis algorithm, we detected strombolian eruptions of the Villarrica volcano 192 km away (2015 and 2016), the subplinian eruption of the Calbuco volcano 410 km away (2015), and signals associated with the sustained emission of ash in the Copahue volcano (2016). During the summer of 2017 – 2018, the second array (GND 120 m) was installed 3 km away from the Lanín volcano. Using the same algorithm, the continuous tremor emitted by the crateric lava lake of the Villarrica volcano 47 km from the array was detected on several occasions. In addition, we detected 88 events associated with surficial mass movements (∼1 event/day) that occurred on the slopes of the Lanín volcano and on the flanks of the hills that border the Malleo river. Eruptive source parameters were derived from the acoustic analysis of the infrasound signals. Therefore, it was necessary to quantify signal amplitude losses during propagation due to interaction with topography and absorption in the atmosphere. For local propagation, we estimated amplitude losses caused by diffraction around topographic barriers between Copahue and CPH. In the Calbuco – CPH regional case, we used the software InfraGa to model the ray tracing through an atmospheric structure reconstructed with numerical weather models (analysis and re–analysis) and empirical models with the software AVOG2S. From this modeling we could simulate the main waveguides at a regional level. The comparison with the infrasound data showed discrepancies that were interpreted as the result of processes not considered by the method used. These effects could be due to the interaction of the wave with the topography and with small-scale atmospheric structures. The activity of Copahue was characterized by the continuous emission of low – altitude plumes that dispersed large amounts of ash to the southeast, tens of kilometers away. The analysis of the acoustic data suggests a process of rigid fragmentation of low magma overpressure, which is almost completely dissipated during the passage of the eruptive mixture through the ash – filled crater, which would explain the low amplitude that characterizes these signals. The estimated material exit velocity (10 – 20 m/s) is consistent with the emission of low-altitude plumes. On the other hand, the subplinian eruption of the Calbuco volcano generated two plumes over 13 km high above the volcano, with ashes dispersed over northern Patagonia. The analysis of these eruptive tremors, allowed us to estimate the first infrasound derived source parameters, such as the height of the plume (18 – 25 km), the exit velocity of the material (35 – 150 m/s) and the radius of the conduit (25 – 35 m). The heights obtained and the temporal variations of the acoustic parameters are consistent with observations from other studies. This work proves that the application of infrasound arrays in the Andes of Neuquén is useful to monitor and investigate both regional and local volcanic processes, as well as surficial mass movements. The latter is of great relevance in a context of climate change that promotes the destabilization of slopes due to the alteration of the natural cycles of freezing – thawing and precipitations.