Search

Debris flows constitute a severe natural hazard and studies are performed to investigate triggering mechanisms and to identify and evaluate early warning systems. We present a seismoacoustic analysis of debris flow activity at Illgraben, Switzerland, with infrasound data collected with a small aperture array. Events are recorded as emergent signals of long duration, with seismic and infrasound amplitudes scaling with the flow discharge. The spectral content is stable and peaking at 8 Hz for the seismic and at 5 Hz for the infrasound that suggests two separate processes of elastic energy radiation, most likely bed‐load transport for the seismic and waves at the free surface for the infrasound. Although amplitude and frequency content of the infrasound signal are well within the processing limits, most of the signal is not showing any correlation among the array elements. We suggest that this is a consequence of the contribution of multiple sources of infrasound acting with variable amplitude and phase along the surface of the debris flow. At Illgraben, coherent infrasound is recorded only from fixed sources, corresponding to check dams within the channel. Here infrasound radiation is increased and the dams turn into predominant sources of energy. This allows to unambiguously identify the occurrence of debris flow at Illgraben with the infrasound array, from a remote and safe position and with a timing that is similar to the early warning system based on in‐channel sensors. This clearly shows how infrasound arrays could be used as an efficient early warning systems. Plain Language Summary: Debris flows constitute a severe natural hazard in alpine environments, and studies are currently performed to investigate triggering mechanisms and to identify and evaluate early warning systems. We present a seismoacoustic analysis of debris flow activity at Illgraben, Switzerland, with infrasound data collected for the first time with an array of infrasound sensors deployed outside the channel in a safe position. This seimoacoustic experiment is providing interesting constraints on the mutual radiation of energy in the ground and in the atmosphere by the flow. Moreover, the use of the infrasound antenna allowed for the first time to carefully analyze the mechanism of infrasound radiation that we model as an extended source of energy with variable phase. Moreover, the array is able to detect the debris flow when it crosses dams or topography steps, even if this occurs at large distance (>2 km) and despite not being visible from the recording site. For the specific case of Illgraben, this allows to unambiguously identifying the occurrence of debris flow with a timing that is similar to the early warning system based on in‐channel sensors. This clearly shows how infrasound arrays could be used as new early warning systems for debris flow activity. Key Points: Infrasound array analysis shows that debris flows can be modeled as an extended source of infrasound with variable phaseSeismic and infrasound spectra of debris flows at Illgraben are decoupled and suggest two different mechanisms of energy radiationWhen sharp topography is present, infrasound arrays can be used as an efficient early warning systems for debris flows remotely [ABSTRACT FROM AUTHOR]

Explosive volcanic eruptions can eject large amounts of ash into the atmosphere, posing a serious threat to populations living near the volcano. The abrupt occurrence of such events requires a rapid response and proper volcanic hazard evaluation. Current monitoring procedures still require human intervention, which often results in significant delays between the occurrence of an eruption and notifications being dispatched. We show how dedicated infrasound array processing can be used to detect and notify the authorities, automatically and in real time, of the onset of explosive eruptions. Conceptually, our method relies on the strong coupling between infrasound and the explosive process, and it is not based on probabilistic considerations but on the ability infrasound has to detect the early stage of the explosive phase. This procedure has been tested for the last 8 years, and it is currently applied to issue early warnings for explosive eruptions at Etna Volcano. We show that the system is able to provide a prealert ~1 hr before the eruption, and it has a 96.6% success rate, with only 1.7% false positive alerts and no false negative alerts. This is, to our knowledge, the first example of an operational early warning system totally based on an unsupervised algorithm that provides automatic notifications of eruptions to a government agency. We show that the same early warning concept might be applicable to arrays at large distances (>500 km), suggesting that infrasound could be successfully used to issue automatic notifications of ongoing eruptions at regional to global scales. Plain Language Summary: Most of the volcanic eruptions are rapidly evolving phenomena, strongly limiting the possibility to prompt activate emergency plans. We still lack the possibility to notify volcanic eruptions automatically, making our society highly exposed to the effects of large explosive eruptions. We present the first early‐warning system based on the acoustic waves generated by volcanic eruptions. Our approach relies on the strong coupling between sound and explosive process, and, as for the earthquake, it is not based on probabilistic consideration but on a quasi‐deterministic approach. In the last 8 years, the system was providing a prealert notification ~1 hr before the eruptive onset with 96.6% rate of success, 1.7% positive false alerts, and no negative false alerts. This is, in our knowledge, the first example of an operational early‐warning system totally based on automatic and unmanned algorithm that provides automatic notification of eruption to government agency automatically and without man supervision. Early warnings can be applied even at regional distances (˃500 km), and notification of ongoing volcanic eruptions would be of valuable importance for aviation safety especially for the many active volcanoes worldwide that are still lacking of geophysical monitoring systems. Key Points: We present the first example of operational early warning for volcanic eruptions based on automatic and unsupervised algorithmThe infrasound array processing detects in real‐time explosive eruptions with a 96.6% of success and no false negative alertsThe early‐warning algorithm automatically delivers prealert notification ~1 hr before the occurrence of the eruptive onset [ABSTRACT FROM AUTHOR]

Abstract: Risk assessment of snow avalanches is mostly related to weather conditions and snow cover. However, a robust risk validation requires avalanche activity data, in order to compare predictions to actual events. For this purpose in December 2009 we installed a temporary 4-element, small aperture (150m), infrasound array in the northwestern Italian Alps. The array was installed south of Mt. Rosa, at an elevation of 2000ma.s.l. in the valley of Gressoney, Italy, where natural avalanches are expected and avalanche control by explosives is regularly performed. A multi-channel correlation analysis is carried out on the continuous infrasound data set recorded by the array as a function of apparent velocity, back-azimuth and frequency of recorded infrasound. This allowed detectionof infrasonic signals propagating across the array from the background noise. During the 5-month-long experiment, 343 infrasonic events have been detected and characterized. These include sharp infrasonic transients (99 events) produced by explosions during avalanche control as well as longer lasting signals (244 events) possibly caused by avalanches. Although only few of these events were validated by direct avalanche observation, obtained results are promising and encouraging application of infrasound for long-term avalanche observation on wide areas, as the peak in avalanche activity in winter 2009–2010 was observed shortly after the infrasound events peaked as well. [Copyright &y& Elsevier]

Abstract: Eruptive plume dynamics is a direct expression of explosive style, where duration and altitude of eruptive clouds and dispersion of erupted products is proportional to the degree of fragmentation. We present an analysis of infrasonic and thermal records for explosions at Villarrica (Chile), Stromboli (Italy), Santiaguito and Fuego (Guatemala) volcanoes. Across these four systems magma composition spans from basaltic to dacitic and explosive activity is typically described as ranging from Strombolian to Vulcanian. We use this analysis to provide a quantitative, geophysically-based description of, and discrimination between, the different explosive styles that characterize the four volcanoes. While infrasound is directly related to the emission of over-pressurized gas, and thus solely reflects the plume emission, both plume emission and ascent are detected thermally. Thus, the two data sets together provide a complete description of the plume dynamics. In particular, while infrasound solely reflects the gas-thrust phase driving plume emission, thermal radiation energy is also affected by buoyancy during plume ascent. Thermal radiation energy estimated for explosions at Stromboli and Villarrica (104–107 J) is lower than that for events at Santiaguito and Fuego (108–109 J), but infrasonic energies overlap. This suggests a greater contribution of buoyancy for eruptive clouds at Santiaguito and Fuego when compared with Stromboli and Villarrica. We further investigated the plume dynamics by comparing infrasonic energy, which reflects gas-thrust (E GT ), with the difference between thermal radiation and infrasonic energies, which mostly reflects buoyancy (E B ). Our data distribution reveals two separate clusters. Explosions at Stromboli and Villarrica share low values of buoyancy, pointing to a gas-thrust dominated emission, efficient coupling of the infrasonic source to the atmosphere, and a Strombolian-type source process to generate a plume rich in coarse fragments. In contrast, explosions at Santiaguito and Fuego share large buoyancy (E B ), and are consistent with Vulcanian-type events in which a large part of infrasonic energy is spent in magma fragmentation leading to ascent of buoyant, ash-rich plumes. We demonstrate that the comparison between thermal radiation and infrasonic energies of explosions allows a quantitative discrimination between, and characterization of, Strombolian- and Vulcanian-type explosions, which leads us to suggest that such an approach may be used as an index to further (and quantitatively) discriminate between eruptive styles. [Copyright &y& Elsevier]

Abstract: During operational ash-cloud forecasting, prediction of ash concentration and total erupted mass directly depends on the determination of mass eruption rate (MER), which is typically inferred from plume height. Uncertainties for plume heights are large, especially for bent-over plumes in which the ascent dynamics are strongly affected by the surrounding wind field. Here we show how uncertainties can be reduced if MER is derived directly from geophysical observations of source dynamics. The combination of infrasound measurements and thermal camera imagery allows for the infrasonic type of source to be constrained (a dipole in this case) and for the plume exit velocity to be calculated (54–142m/s) based on the acoustic signal recorded during the 2010 Eyjafjallajökull eruption from 4 to 21 May. Exit velocities are converted into MER using additional information on vent diameter (50±10m) and mixture density (5.4±1.1kg/m3), resulting in an average ∼9×105 kg/s MER during the considered period of the eruption. We validate our acoustic-derived MER by using independent measurements of plume heights (Icelandic Meteorological Office radar observations). Acoustically derived MER are converted into plume heights using field-based relationships and a 1D radially averaged buoyant plume theory model using a reconstructed total grain size distribution. We conclude that the use of infrasonic monitoring may lead to important understanding of the plume dynamics and allows for real-time determination of eruption source parameters. This could improve substantially the forecasting of volcano-related hazards, with important implications for civil aviation safety. [Copyright &y& Elsevier]