Abstract We present here the first volcanic gas compositional time‐series taken prior to a paroxysmal eruption of Villarrica volcano (Chile). Our gas plume observations were obtained using a fully autonomous Multi‐component Gas Analyser System (Multi‐GAS) in the 3 month‐long phase of escalating volcanic activity that culminated into the 3 March 2015 paroxysm, the largest since 1985. Our results demonstrate a temporal evolution of volcanic plume composition, from low CO 2 /SO 2 ratios (0.65‐2.7) during November 2014‐January 2015 to CO 2 /SO 2 ratios up to ≈ 9 then after. The H 2 O/CO 2 ratio simultaneously declined to <38 in the same temporal interval. We use results of volatile saturation models to demonstrate that this evolution toward CO 2 ‐enriched gas was likely caused by unusual supply of deeply sourced gas bubbles. We propose that separate ascent of over‐pressured gas bubbles, originating from at least 20‐35 MPa pressures, was the driver for activity escalation toward the 3 March climax.
We synthesize the envelopes simulating a drastic change in the scattering properties of the medium. This requires the inclusion of boundary conditions in the a 2D Monte Carlo simulation of the scattered wave-field; these conditions are deterministic in nature, but produce stochastic effects, still described by the radiative transfer equations. The change produces a wave flux in a direction largely different from the incident one, a phenomenon evidenced in real seismic envelopes. Also, the model allows to mark the reach of diffusion in coda waves, at least in a medium without topography. This marker is important for many volcano-monitoring techniques as well as for interferometry. We check our models by comparison with true data recorded in the whole caldera; the result is a reliable first order model of the envelopes included in the caldera rim, or in its proximities. Outside the rim, the envelopes present greater complexities, requiring the introduction of a new model including turbulence. The synthetics can be employed to check tomography structures - especially for attenuation and for scattering tomography. Also, it can be a basement stone for overcoming linear optics in the description of scattering in highly heterogeneous area.
Resuming erupting activity at volcanoes that have been long quiescent poses a significant challenge to hazard assessment, as it require assessment of whether the change in activity is an isolated event or the beginning of a new eruptive sequence. Such inception is often poorly characterised as quiescent volcanoes tend to be poorly equipped and not extensively monitored, especially with respect to gas geochemistry. Here, we report gas composition and flux measurements from a newly opened vent at the very onset of eruptive activity at the Nevados de Chillán volcanic complex (Chile) in January-February 2016. The molar proportions of H2O, CO2, SO2, H2S and H2 gases are found to be 98.4, 0.97, 0.11, 0.01 and 0.5 mol% respectively. The mean SO2 flux recorded in early February 2016 during periods of eruptive discharge amounts to 0.4-0.6 kg s-1. Our results indicate that the new vent opening was propelled by magmatic gases, triggering repeated eruptions. Ash particles ejected by the first blast of 8 January are dominated by lithic fragments of dacitic composition. By contrast the ash ejected in a subsequent eruption contains both lithic fragments of dense dacite, and a fresher, sparsely vesicular material of basaltic andesite composition. By October 2017 the ejected ash is back to being dominated by the dense dacitic lithic material. Together with the seismic and deformation record, these observations point to the explosive activity resulting from a small intrusion of basaltic to andesitic magma at shallow level. The fate of this magma, whether stalling or eventually triggering a magmatic eruption, remains to be seen, but current observations suggest the former is most likely.
created for two areas, herein referred to as the test table grid region and the nearfield grid region. The test table grid includes the region within ~40 m from surface ground zero, with photographs collected at a flight altitude of 8.5 m above ground level (AGL). The near-field grid area covered a broader area, 90–130 m from surface ground zero, and collected at a flight altitude of 22 m AGL. The photographs, processed using Agisoft Photoscan® in conjunction with 125 surveyed ground control point targets, yielded a 6-mm pixel-size digital elevation model (DEM) for the test table grid region. This provided the ≤3 cm resolution in the topographic data to map in fine detail a suite of features related to the underground explosion: uplift, subsidence, surface fractures, and morphological change detection. The near-field grid region data collection resulted in a 2-cm pixel-size DEM, enabling mapping of a broader range of features related to the explosion, including: uplift and subsidence, rock fall, and slope sloughing. This study represents one of the first works to constrain, both temporally and spatially, explosion-related surface damage using a UAS photogrammetric platform; these data will help to advance the science of underground explosion detection.
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