ALBERT

Description: Volcanic explosions produce energy that propagates both in the subsurface as seismic waves and in the atmosphere as acoustic waves. We analyse thousands of explosions which occurred at different craters at Etna volcano (Italy) in 2018 and 2019. We recorded signals from infrasound sensors, geophones (GPH), broadband seismometers (BB) and Distributed Acoustic Sensing (DAS) with fibre optic cable. The instruments were deployed at Piano delle Concazze at about 2 to 2.5 km from the active craters, within (or onto) a ~300,000 m2 scoria layer deposited by recent volcanic eruptions. The DAS interrogator was setup inside the Pizzi Deneri Volcanic Observatory (~2800 m elevation). Infrasonic explosion records span over a large range of pressure amplitudes with the largest one reaching 130 Pa (peak to peak), with an energy of ca. 2.5x1011 J. In the DAS and the BB records, we find a 4-s long seismic “low frequency” signal (1-2 Hz) corresponding to the seismic waves, followed by a 2-s long “high-frequency” signal (16-21 Hz), induced by the infrasound pressure pulse. The infrasound sensors contain a 1-2 Hz infrasound pulse, but surprisingly no high frequency signal. At locations where the scoria layer is very thin or even non-existent, this high frequency signal is absent from both DAS strain-rate records and BB/GPH velocity seismograms. These observations suggest that the scoria layer is excited by the infrasound pressure pulse, leading to the resonance of lose material above more competent substratum. We relate the high frequency resonance to the layer thickness. Multichannel Analysis of Surface Wave from jumps performed along the fibre optic cable provide the structure of the subsurface, and confirm thicknesses derived from the explosion analysis. As not all captured explosions led to the observation of these high frequency resonance, we systematically analyze the amplitudes of the incident pressure wave versus the recorded strain and find a non-linear relationship between the two. This non-linear behaviour is likely to be found at other explosive volcanoes. Furthermore, our observations suggest it might also be triggered by other atmospheric pressure sources, like thunderstorms. This analysis can lead to a better understanding of acoustic-to-seismic ground coupling and near-surface rock response from natural, but also anthropogenic sources, such as fireworks and gas explosions.

Description: We demonstrate the capability of distributed acoustic sensing (DAS) to record volcano-related dynamic strain at Etna (Italy). In summer 2019, we gathered DAS measurements from a 1.5 km long fibre in a shallow trench and seismic records from a conventional dense array comprised of 26 broadband sensors that was deployed in Piano delle Concazze close to the summit area. Etna activity during the acquisition period gives the extraordinary opportunity to record dynamic strain changes (∼ 10−8 strain) in correspondence with volcanic events. To validate the DAS strain measurements, we explore array-derived methods to estimate strain changes from the seismic signals and to compare with strain DAS signals. A general good agreement is found between array-derived strain and DAS measurements along the fibre optic cable. Short wavelength discrepancies correspond with fault zones, showing the potential of DAS for mapping local perturbations of the strain field and thus site effect due to small-scale heterogeneities in volcanic settings.

Description: Mount Etna (Sicilia, Italy) is one of the most active volcanoes worldwide, located at the boundary between the African and the Eurasian plates. It is characterized by the occurrence of many phenomena such as lava flows, ash eruptions, earthquakes. Its eastern flank is also characterized by a complex system of active faults, associated with an eastward flank movement, up until the submarine environment. As Etna flanks are densely inhabited areas, we aim at better understanding the link between these phenomena to better assess associated hazards and risks. Since 2018, we have been measuring several locations with Distributed Fiber Optic Sensing yearly, enabling us to observe strain at meter-scale spatial interval and on a broad frequency range. We show records and present results from selected cables. Close to the summit active craters, we interrogate dedicated cables, and could analyze the ground response in association with explosions and volcanic tremor. In urban areas, we interrogate telecommunication cables and record local earthquakes. In the submarine area, we interrogate a cable which crosses the North Alfeo fault with several different optical techniques. In the southern flank of the volcano, we show volcanic signals from a cable deployed in a borehole. We also demonstrate how simultaneous multi-fiber measurements can help constrain earthquake hypocenter location. We discuss opportunities and challenges of using fiber optic cable in various environments such as the Etna volcano and beyond, for an integrated vision from deep processes, their interaction with the sub-surface dynamics and the volcano-tectonic structures.

Description: Mountain regions are especially sensitive to climatic changes. The complex local topography modulates meteorological and climatic patterns and simultaneously creates challenges in modeling and observing local-scale features. In this study, we present an extensive evaluation of an ensemble of regional climate models (RCMs) from EURO-CORDEX and bias adjusted simulations from CORDEX-Adjust, both at 12 km resolution, for the European Alps. These are compared with two higher resolution simulations from convection-permitting models at 4 and 2 km. The evaluation focuses on elevation-dependent biases in temperature and precipitation and multi-decadal time periods. We use APGD and EOBS as reference observations in addition to high-resolution national datasets. Preliminary results show a large variation in the RCMs’ capability to match observed temperature and precipitation elevation gradients, while bias adjustment can remove the overall cold and wet bias of the 12km RCMs in the Alps. Our results inform the application of both raw and bias-adjusted RCM simulations for impact analyses in the complex Alpine terrain. They furthermore provide a reference for the evaluation of upcoming CMIP6-based RCM ensembles at convection-permitting and conventional resolution.