RESUMEN
Lava fountains are a common manifestation of basaltic volcanism. While magma degassing plays a clear key role in their generation, the controls on their duration and intermittency are only partially understood, not least due to the challenges of measuring the most abundant gases, H2O and CO2. The 2021 Fagradalsfjall eruption in Iceland included a six-week episode of uncommonly periodic lava fountaining, featuring ~ 100-400 m high fountains lasting a few minutes followed by repose intervals of comparable duration. Exceptional conditions on 5 May 2021 permitted close-range (~300 m), highly time-resolved (every ~ 2 s) spectroscopic measurement of emitted gases during 16 fountain-repose cycles. The observed proportions of major and minor gas molecular species (including H2O, CO2, SO2, HCl, HF and CO) reveal a stage of CO2 degassing in the upper crust during magma ascent, followed by further gas-liquid separation at very shallow depths (~100 m). We explain the pulsatory lava fountaining as the result of pressure cycles within a shallow magma-filled cavity. The degassing at Fagradalsfjall and our explanatory model throw light on the wide spectrum of terrestrial lava fountaining and the subsurface cavities associated with basaltic vents.
RESUMEN
Earthquakes are frequently accompanied by public reports of audible low-frequency noises. In 2018, public reports of booms or thunder-like noises were linked to induced earthquakes during an Engineered Geothermal System project in the Helsinki Metropolitan area. In response, two microphone arrays were deployed to record and study these acoustic signals while stimulation at the drill site continued. During the 11 day deployment, we find 39 earthquakes accompanied by possible atmospheric acoustic signals. Moment magnitudes of these events ranged from [Formula: see text] to 1.87 with located depths of 4.8-6.5 km. Analysis of the largest event revealed a broadband frequency content, including in the audible range, and high apparent velocities across the arrays. We conclude that the audible noises were generated by local ground reverberation during the arrival of seismic body waves. The inclusion of acoustic monitoring at future geothermal development projects will be beneficial for studying seismic-to-acoustic coupling during sequences of induced earthquakes.
RESUMEN
We combine geophysical and experimental observations to interpret preeruptive unrest at Volcán de Colima in 1998. 17,893 volcanic earthquakes were detected between 1 October and 31 December 1998, including 504 clusters. Using seismic ambient noise interferometry, we observe a drop in velocity prior to the eruption linked to damage accumulation during magma ascent. This is supported by experimental observations where static stress causes a velocity decrease prior to failure. Furthermore, we observe acoustic emission clusters during the experiments, with lower porosity samples producing higher numbers of repeaters. This behavior introduces tensile failure as an additional viable mechanism for clusters during magma ascent. The findings suggest that preeruptive magma ascent may be monitored to variable degrees of accuracy via descriptions of damage accumulation and associated seismic velocity changes.