Crustal seismic velocity responds to a magmatic intrusion and seasonal loading in Iceland's Northern Volcanic Zone

Impact of temperature and relative humidity variations on coda waves in concrete

The microstructure of concrete can be affected by many factors, from non-destructive environmental factors through to destructive damage induced by transient stresses. Coda wave interferometry is a technique that is sensitive enough to detect weak changes within concrete by evaluating the ultrasonic signal perturbation compared to a reference state. As concrete microstructure is sensitive to many factors, it is important to separate their contributions to the observables. In this study, we characterize the relationships between the concrete elastic and inelastic properties, and temperature and relative humidity. We confirm previous theoretical studies that found a linear relationship between temperature changes and velocity variation of the ultrasonic waves for a given concrete mix, and provide scaling factors per Kelvin for multiple settings. We also confirm an anti-correlation with relative humidity using long-term conditioning. Furthermore, we explore beyond the existing studies to establish the relationship linking humidity and temperature changes to ultrasonic wave attenuation.

Spatio-temporal velocity variations observed during the pre-eruptive episode of La Palma 2021 eruption inferred from ambient noise interferometry

On Sept. 19th, 2021, a volcanic eruption began on the island of La Palma (Canary Islands, Spain). The pre-eruptive episode was characterized by seismicity and ground deformation that started only 9.5 days before the eruption. In this study, we applied seismic interferometry to the data recorded by six broadband seismic stations, allowing us to estimate velocity variations during the weeks preceding the eruption. About 9.5 days before the eruption, we observed a reduction in the seismic velocities is registered next to the eruptive centers that opened later. Furthermore, this zone overlaps with the epicenters of a cluster of volcano-tectonic earthquakes located at shallow depth (< 4 km) and detached from the main cluster of deeper seismicity. We interpret the decrease in seismic velocities and the occurrence of such a shallow earthquake cluster as the effect of hydrothermal fluid released by the ascending magma batch and reaching the surface faster than the magma itself.

Tracking supercritical geothermal fluid distribution from continuous seismic monitoring

Continuous seismic monitoring could play a pivotal role in deep geothermal energy exploration. We monitored seismicity near geothermal production areas of the Kuju volcanic complex with a dense seismic network and automated event detection. Most events were shallow (less than 3 km below sea level) and distributed along a boundary between regions of high and low resistivity and S-wave velocity, interpreted as a lithological boundary or related fracture zone. Deeper events located on top of subvertical conductors may reflect fracturing associated with magmatic fluid intrusion. A correlation may exist between seismicity and heavy rainfall three days prior to increased pore pressure in pre-existing fractures. Our findings support the presence of supercritical geothermal fluids and demonstrate the importance of continuous seismic monitoring in supercritical geothermal energy exploration.

Hidden pressurized fluids prior to the 2014 phreatic eruption at Mt Ontake

A large fraction of volcanic eruptions does not expel magma at the surface. Such an eruption occurred at Mt Ontake in 2014, claiming the life of at least 58 hikers in what became the worst volcanic disaster in Japan in almost a century. Tens of scientific studies attempted to identify a precursor and to unravel the processes at work but overall remain inconclusive. By taking advantage of continuous seismic recordings, we uncover an intriguing sequence of correlated seismic velocity and volumetric strain changes starting 5 months before the eruption; a period previously considered as completely quiescent. We use various novel approaches such as covariance matrix eigenvalues distribution, cutting-edge deep-learning models, and ascribe such velocity pattern as reflecting critically stressed conditions in the upper portions of the volcano. These, in turn, later triggered detectable deformation and earthquakes. Our results shed light onto previously undetected pressurized fluids using stations located above the volcano-hydrothermal system and hold great potential for monitoring.

Phreatic volcanic eruptions can be unexpected and devastating. Here the authors, using seismic-based methodologies, find that pressurized fluids accumulated 5 months before the deadly phreatic eruption at Mt Ontake; a period previously considered as completely quiescent.