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We compiled a database for the Campi Flegrei seismic events that occurred from 2011 to 2018 at all stations available (merging permanent and temporary networks). Then we computed the two observables of the crustal anisotropy: time delay between fast and slow S-wave’s arrivals, and polarization direction of the fast S-wave. These results provide useful information about the amount of crustal anisotropy and the main direction, respectively, with this latter representing a proxy for the local stress field. We could thus obtain a picture of their spatial and temporal distributions to be compared with other geophysical and geochemical observations. In particular we could identify common features, such as change points, to several time series. This helps us in building a more complete interpretation of the volcanic system changes that were occurring during the recent ongoing unrest phase, which started in 2005.

Volcanic degassing, a persistent manifestation of active volcanoes, provides crucial information on the dynamics of the magmatic feeding systems, and allows identifying the phases of volcanic unrest in the runup to volcanic eruptions. While thus determining volcanic degassing rates is a central topic in modern Volcanology, direct volcanic gas flux observations by classic spectroscopic techniques are challenged by (i) the need of adequate illumination (by sunlight) and clear weather conditions (ii) difficulties in robustly estimating plume speed velocity and transport direction, and (iii) a variety of optical and radiative transfer issues. Because of these, volcanic gas flux records are often sparse and incomplete, and affected by intrinsic noise that may prevent from fully resolving the gas emission changes associated with changing volcanic activity. To overcome such limitations, measuring the infrasound produced by the expansion of over-pressurized volcanic gas in the atmosphere, using infrasonic arrays, offers as a promising alternative/complementary tool to quantify and locate degassing at active volcanoes. Here, we report on 2-year long (April 2017—March 2019) period of combined measurements of the SO2 flux and of volcano-acoustic emissions produced by regular mild persistent strombolian activity and passive degassing of Stromboli Volcano (Sicily, Italy). These were obtained by a permanent monitoring SO2 camera and a five-element short-aperture (~300 m) infrasonic array. Our results highlight substantial temporal changes in degassing activity, that reflect the recurrent episodes of activations/inactivation of multiple distinct degassing sources within the crater area, as coherently tracked by SO2 and infrasound together. A simple waveform modeling of the infrasonic record, assuming a monopole acoustical source, suggests that infrasonic degassing, comprising of explosive events and continuous puffing activity, dominates the total persistent degassing budget as tracked by the SO2 camera.

Hydrothermal activity is a natural manifestation of volcanic degassing at calderas, testified by the presence of fumarolic fields, boiling pools, steaming ground and soil diffuse degassing, which are of interest for volcano monitoring and surveillance as they can be related to the magma dynamics within the caldera reservoirs. Campi Flegrei (Italy) is a half submerged resurgent caldera with a nested structure located at the western edge of the bay of Naples. Since its last eruption in 1538, several episodes of ground uplift accompanied by seismic swarms and intense degassing have been reported. The last uplift phase started in 2005 and is still ongoing. The Pisciarelli fumarolic field is a key area of the Campi Flegrei caldera where a continuous and vigorous degassing of hydrothermal fluids, of magmatic origin, takes place. Such fumarolic degassing is associated with a persistent harmonic tremor showing within the last decade an increasing amplitude trend that correlates well with the geochemical and geodetic unrest indicators of the caldera. In the framework of the DPC-INGV 2012-2021 Agreement and the LOVE-CF Project, we investigated the seismo-acoustic wavefield produced by fumarolic degassing with the aim of characterizing the source process that produces the harmonic tremor, and to propose a potential seismo-acoustic based tool to estimate the fumarolic gas fluxes in real time. At this aim, we performed a series of temporary geophysical experiments with the deployment of 4-element small aperture seismo-acoustic arrays equipped, at each array element, by a short-period three-component seismometer and a broadband infrasonic pressure sensor. We show that the harmonic tremor source is located within the fumarolic field at shallow depth (

Volcano seismicity is one of the key parameters to understand magma dynamics of erupting volcanoes. However, the physical process at the origin of the resulting complex and broadband seismic signals remains unclear. Syn-eruptive very long period (VLP) seismic signals have been explained in terms of the sudden expansion of gas pockets rising in the liquid melt. Their origin is linked to a magma dynamics which triggers the explosive process occurring before the explosive onset. We provide evidence based on acoustic, thermal, and ground deformation data to demonstrate that VLP signals at Stromboli are generated at the top of the magma column mainly after the explosion onset. We show that VLP amplitude and duration scale with the eruptive flux which induces a decompression of 103–104 Pa involving the uppermost ~ 250 m of the feeding conduit. The seismic VLP source represents the final stage of a ~ 200 s long charge and discharge mechanism the magma column has to release excess gas accumulated at the base of a denser and degassed magma mush. The position of the VLP seismic source coincides with the centroid of the shallow mush plug and tracks elevation changes of the magma free surface.

Open-vent volcanic activity is typically sustained by ascent and degassing of shallow magma, in which the rate of magma supply to the upper feeding system largely exceeds the rate of magma eruption. Such unbalance between supplied (input) and erupted (output) magma rates is thought to result from steady, degassing-driven, convective magma overturning in a shallow conduit/feeding dyke. Here, we characterize shallow magma circulation at Stromboli volcano by combining independent observations of heat (Volcanic Radiative Power; via satellite images) and gas (SO2, via UV camera) output in a temporal interval (from August 1, 2018 to April 30, 2020) encompassing the summer 2019 effusive eruption and two paroxysmal explosions (on July 3 and August 28, 2019). We show that, during the phase of ordinary strombolian explosive activity that preceded the 2019 effusive eruption, the average magma input rate (0.1-0.2 m3/s) exceeds the magma eruption rate (0.001-0.01 m3/s) by ∼2 orders of magnitude. Conversely, magma input and output rates converge to an average of ∼0.4 m3/s during the summer 2019 summit effusion, implying an overall suppression of magma recycling back into the feeding system, and hence of excess degassing. We find that, during the effusive eruption, the peak in SO2 emissions lags behind the thermal emission peak by ∼27 days, suggesting that magma output, feeding the lava flow field, initially dominates over magma input in the conduit. We propose that this conduit mass unloading, produced by this initial phase of the effusive eruption, leads to an overall decompression (of up to 30 Pa/s) of the shallow plumbing system, ultimately causing ascent of less-dense, volatile-rich magma batch(es) from depth, enhanced explosive activity, and elevated SO2 fluxes culminating into a paroxysmal explosion on August 28. Our results demonstrate that combined analysis of thermal and SO2 flux time-series paves the way to improved understanding of shallow magmatic system dynamics at open-vent volcanoes, and of the transition from explosive to effusive activity regimes.

International audience; ASHER, a new sensor for the characterization of tephra fallout in real time, was designed and developed for easy field deployment during volcanic eruptions. It can provide information on the accumulation rate of tephra fallout in real time as well as grain-size and settling velocity of falling particles. Particle detection is achieved with a laser barrier, with size and settling velocity being calculated from the amplitude and duration of obscuration peaks. The sampling rate (31,500 Hz), laser thickness (0.5 mm) and operation (ON/OFF state and dual acquisition mode) are adapted to minimize the noise level and allow detection of particles as small as ~100 μm. Additional measurements of weight and level of accumulated material within a removable collector allow broadening of the ASHER operation to accumulation rate from 10-2 to 103 g m-2s-1. Detailed calibration tests were performed in laboratory conditions on single grains of known shape and density along with a high-speed camera to test the capability to measure grain size and terminal velocity, and during two field campaigns at Stromboli and Etna volcanoes to test the operation in the field. Long-term field deployment has shown that combining the optical barrier with an automatic collector allows for a better characterization of tephra fallout, providing an estimate of density, and, therefore, it optimizes sensor operation and minimizes false alerts. Moreover, the low power requirements and onboard processing allows to operate the sensor remotely and solely on solar power in a remote location. Although technical improvements in sensor sensitivity and processing are still possible, the results presented suggest that ground sensors for real-time detection and analysis of tephra could potentially contribute to understanding the dynamics of explosive eruptions and could be successfully integrated into monitoring systems of active volcanoes.

At open-vent basaltic volcanoes, resolving the activity escalation that heralds larger, potentially harmful eruptions is challenged by the persistent mild ordinary activity, which often masks the precursory unrest signals related to heightened magma transport from depth. Gas (SO2 and CO2) fluxes at surface are controlled by rate of magma transport and degassing within the magma plumbing system, and thus constitute key parameters to infer deep magma budget and dynamics. Here, we use several year-long (2014-present) gas observations at Etna and Stromboli volcanoes, in Sicily, to provide new evidence for the utility of long-term instrumental gas monitoring in real-time detecting the early phase of unrest prior eruption, and for characterizing syn-eruptive dynamics. To this aim, we use information from a gas monitoring network of permanent ultraviolet (UV) cameras and automatic Multi-Gas instruments that, combined with geophysical observations, allow characterizing changes in degassing and eruptive dynamics at high temporal/spatial resolution. Our results show that the paroxysmal (lava fountaining) explosions that periodically interrupted persistent open-vent activity on Etna (during 2014-2020) were accompanied by systematic, repetitive SO2 emission patterns prior, during, and after eruptions. These allow us identifying the characteristic pre- syn- and post- eruptive degassing regimes, and to establish thresholds in the SO2 flux record that mark phases of unrest. On Stromboli, the much improved temporal/spatial resolution of UV cameras allows resolving the escalation of regular strombolian activity, and its concentration toward its North-east crater, that heralds onset of effusive eruptions. During effusive eruption, although magma level drops in the conduit and explosive summit activity ceases, UV camera observations can still detect explosive gas bursts deep in the conduit while no infrasonic activity is detected. Combining the UV camera-derived SO2 fluxes with CO2/SO2 ratio records measured by the Multi-Gas, the CO2 flux can be inferred. We find that such CO2 flux time-series can allow tracking degassing of deeply stored mafic magma months before Stromboli’s eruptions. We finally show that remotely sensed gas emission and thermal activity can be combined together to characterize the dynamics of shallow magmatic system prior to and during unrest, ultimately helping to define timing of magma re-charging events driving the eruptions.

Description, Escalating CO2 emissions precede basaltic explosive eruptions on time scales of weeks to months., The ordinarily benign activity of basaltic volcanoes is periodically interrupted by violent paroxysmal explosions ranging in size from Hawaiian to Plinian in the most extreme examples. These paroxysms often occur suddenly and with limited or no precursors, leaving their causal mechanisms still incompletely understood. Two such events took place in summer 2019 at Stromboli, a volcano otherwise known for its persistent mild open-vent activity, resulting in one fatality and damage to infrastructure. Here, we use a post hoc analysis and reinterpretation of volcanic gas compositions and fluxes acquired at Stromboli to show that the two paroxysms were preceded by detectable escalations in volcanic plume CO2 degassing weeks to months beforehand. Our results demonstrate that volcanic gas CO2 is a key driver of explosions and that the preparatory periods ahead of explosions in basaltic systems can be captured by precursory CO2 leakage from deeply stored mafic magma.

Two paroxysmal explosions occurred at Stromboli volcano in the Summer 2019, the first of which, on July 3, caused one fatality and some injuries. Within the 56 days between the two paroxysmal explosions, effusive activity from vents located in the summit area of the volcano occurred. No significant changes in routinely monitored parameters were detected before the paroxysmal explosions. However, we have calculated the polarization and the fractal dimension time series of the seismic signals from November 15, 2018 to September 15, 2019 and we have recognized variations that preceded the paroxysmal activity. In addition, we have defined a new parameter, based on RSAM estimation, related to the Very Long Period events, called VLP size, by means of which we have noticed significant variations through the whole month preceding the paroxysm of July 3. In the short term, we have analyzed the signals of a borehole strainmeter installed on the island, obtaining automatic triggers 10 minutes and 7.5 minutes before the July 3 and the August 28 paroxysms, respectively. The results of this study highlight mid-term seismic precursors of paroxysmal activity and provide valuable evidence for the development of an early warning system for paroxysmal explosions based on strainmeter measurements.

In summer 2019, two paroxysmal explosions occurred in Stromboli. The first one occurred on July 3, when the Strombolian ordinary eruptive activity did not show a significant intensification. The explosion formed an eruptive column more than 3 km high. A pyroclastic flow ran down the “Sciara del Fuoco” slope causing a victim and some injuries. Moreover, the pyroclastic flow spread over the sea surface for about one kilometer. On August 28, a second paroxysmal explosion occurred, similar to the previous one. Also in this case the eruption formed an eruptive column of more than 3 km and a pyroclastic flow that expanded along the “Sciara del Fuoco” slope and traveled about 1 km on the sea surface. In the period between the two paroxysms, effusive activity occurred from the summit crater area. The eruptive phase of summer 2019, which began with the paroxysm of 3 July, was not preceded by significant changes in the routinely monitored parameters, such as the hourly frequency (daily average) of the VLP events (typical of Stromboli) and the amplitude of the seismic signal (RSAM). For this reason, we have analyzed the seismic and dilatometric data, which were recorded by the INGV geophysical network in the period November 2018 - September 2019, focusing our attention on other parameters that can give indications on the activity state of the volcano. In particular, we analyzed the data of the broadband seismic stations, equipped with the Guralp CMG40T sensors, and the data of one Sacks-Evertson borehole strainmeter. We defined the "VLP size", which takes into account the waveform of the VLP events, in terms of both amplitude and duration. We also applied time varying Fractal Dimension (FD) analysis to the seismograms of a seismic station close to the crater area and we analyzed the polarization of the same signal. We carried out the polarization analysis both without applying a filter and by filtering the seismic signal in the typical frequency bands of the Stromboli volcanic tremor (1-3 Hz) and of the VLPs (0.5-0.05 Hz). We found that the "VLP size", the FD and the polarization parameters showed significant changes about one month before the paroxysm of July 3. In the short term, we applied an appropriately tuned STA/LTA algorithm to the data of the borehole strainmeter, which is installed on the island at about 2km from the craters, and we obtained an automatic detection of the paroxysmal events 10 and 7.5 minutes before the explosion of July 3 and August 28, respectively.

Understanding the mechanisms that cause effusive eruptions is the key to mitigating their associated hazard. Here, we combine results from permanent ultra-violet (UV) cameras, and from other geophysical observations (seismic very long period, thermal, and infrasonic activity), to characterize volcanic SO2 flux regime in the period prior, during, and after Stromboli's August-November 2014 effusive eruption. We show that, in the two months prior to effusion onset, the SO2 flux levels are two times average level. We explain this anomalously high SO2 regime as primarily determined by venting of rapidly rising, pressurized SO2-rich gas pockets, produced by strombolian explosions being more frequent and intense than usual. We develop a procedure to track (and count), in the UV camera record, the SO2 flux pulses produced by individual explosions and puffing activity (active degassing). We find that these SO2 pulses are far more numerous (67 ± 47 events/hour) before the effusion onset than during normal activity (20 ± 15 events/hour). This observation, combined with geophysical evidence, demonstrates an elevated gas bubble supply to the shallow conduits, causing elevated explosive and puffing activity. This increase (≥0.1 m3s-1) in magma transport rate in the north-east feeding conduits finally triggers effusion onset. Active degassing remains elevated also during the effusive phase, supporting the persistence of explosive and puffing activity during the effusive eruption, deep in the volcanic conduit. Our results demonstrate that permanent UV cameras can valuably contribute to monitoring at high sampling frequency gas dynamics and fluxes, thus opening the way to direct comparison with more established geophysical observations.

We used a one-year long SO2 flux record, which was obtained using a novel algorithm for real-time automatic processing of ultraviolet (UV) camera data, to characterize changes in degassing dynamics at the Mt. Etna volcano in 2016. These SO2 flux records, when combined with independent thermal and seismic evidence, allowed for capturing switches in activity from paroxysmal explosive eruptions to quiescent degassing. We found SO2 fluxes 1.5−2 times higher than the 2016 average (1588 tons/day) during the Etna’s May 16−25 eruptive paroxysmal activity, and mild but detectable SO2 flux increases more than one month before its onset. The SO2 flux typically peaked during a lava fountain. Here, the average SO2 degassing rate was ~158 kg/s, the peak emission was ~260 kg/s, and the total released SO2 mass was ~1700 tons (in 3 h on 18 May, 2016). Comparison between our data and prior (2014−2015) results revealed systematic SO2 emission patterns prior to, during, and after an Etna’s paroxysmal phases, which allows us to tentatively identify thresholds between pre-eruptive, syn-eruptive, and post-eruptive degassing regimes.

The persistent open-vent activity of basaltic volcanoes is periodically interrupted by spectacular but hazardous paroxysmal explosions. The rapid transition from quiescence to explosive eruption poses a significant challenge for volcanic hazard assessment and mitigation, and improving our understanding of the processes that trigger these paroxysmal events is critical. Although magmatic gas is unquestionably the driver, direct measurements of a paroxysm’s gas flux budget have remained challenging, to date. A particularly violent paroxysmal sequence took place on Etna on December 2015, intermittently involving all summit craters, especially the Voragine (VOR) that had previously displayed no activity for several years. Here, we characterize the volcano’s SO2 degassing budget prior to, during and after this paroxysmal sequence, using ground-based (UV-Camera) and satellite (OMI) observations, complemented with ground-and space-borne thermal measurements. We make use of the high spatial resolution of UV-cameras to resolve SO 2 emissions from the erupting VOR crater for the first time, and to characterize temporal switches in degassing activity from VOR to the nearby New Southeast Crater (NSEC). Our data show that onset of paroxysmal activity on December 3–5 was marked by visible escalation in VOR SO 2 fluxes (4,700–8,900 tons/day), in satellite-derived thermal emissions (2,000 MW vs. ∼2–11 MW in July-November 2015), and in OMI-derived daily SO 2 masses (5.4 ± 0.7 to 10.0 ± 1.3 kilotonnes, kt; 0.5 kt was the average in the pre-eruptive period). Switch in volcanic activity from VOR to NSEC on December 6 was detected by increasing SO 2 fluxes at the NSEC crater, and by decaying SO 2 emissions at VOR, until activity termination on December 19. Taken together, our observations infer the total degassed SO 2 mass for the entire VOR paroxysmal sequence at 21,000 ± 2,730 t, corresponding to complete degassing of ∼1.9 ± 0.3 Mm 3 of magma, or significantly less than the measured erupted magma volumes (5.1–12 Mm 3 ). From this mismatch we propose that only a small fraction of the erupted magma was actually emplaced in the shallow plumbing system during (or shortly prior) the paroxysmal sequence. Rather, the majority of the erupted magma was likely stored conduit magma, having gone through extensive degassing for days to weeks prior to the paroxysm.

Mass Flow Rate is one of the most crucial eruption source parameter used to define magnitude of eruption and to quantify the ash dispersal in the atmosphere. However, this parameter is in general difficult to be derived and no valid technique has been developed yet to measure it in real time with sufficient accuracy. Linear acoustics has been applied to infrasonic pressure waves generated by explosive eruptions to indirectly estimate the gas mass erupted and then the mass flow rate. Here, we test on Stromboli volcano (Italy) the performance of such methodology by comparing the acoustic derived results with independent gas mass estimates obtained with UV cameras, and constraining the acoustic source by thermal imagery. We show that different acoustic methods give comparable total gas masses in the 2 to 1425 kg range, which are fully consistent with the gas masses derived by UV cameras and previous direct SO 2 measurements. We show that total erupted gas mass, estimated by infrasound is not simply a function of the initial pressure, but rather the full infrasonic waveform should be considered. Thermal imagery provides evidence that infrasound is generated during the entire gas thrust phase. We provide examples to show how total gas masses derived by infrasonic signals can be affected by large uncertainties if duration of the signal is neglected. Only when duration of infrasound is included, the best correlation (0.8) with UV cameras and the 1:1 direct linear proportionality is obtained. Our results open new perspective for remotely derived gas mass and mass flow rates from acoustic signals.

We report on a systematic record of SO2 flux emissions from individual vents of Etna volcano (Sicily), which we obtained using a permanent UV camera network. Observations were carried out in summer 2014, a period encompassing two eruptive episodes of the New South East Crater (NSEC) and a fissure-fed eruption in the upper Valle del Bove. We demonstrate that our vent-resolved SO2 flux time series allow capturing shifts in activity from one vent to another and contribute to our understanding of Etna's shallow plumbing system structure. We find that the fissure eruption contributed ~50,000 t of SO2 or ~30% of the SO2 emitted by the volcano during the 5 July to 10 August eruptive interval. Activity from this eruptive vent gradually vanished on 10 August, marking a switch of degassing toward the NSEC. Onset of degassing at the NSEC was a precursory to explosive paroxysmal activity on 11-15 August.

In this study we analyse the main potential seismic sources in some axial and frontal sectors of the Northern Apennines, in Italy. This region was hit by a peculiar series of earthquakes that started in 1916 on the external thrust fronts near Rimini. Later, in 1917-1921, seismicity (up to Mw approximate to 6.5) shifted into the axial zone and clearly migrated north-westward, along the belt of active normal faults. The collection of fault-slip data focused on the active normal faults potentially involved in this earthquake series. The acquired data allowed us to better characterize the geometry and kinematics of the faults. In a few instances, the installation of local seismic networks during recent seismic sequences allowed the identification of the causative faults that are hinted to be also responsible for past earthquakes, particularly in the Romagna region and north-eastern Mugello. The Coulomb stress changes produced by the historical earthquakes generally brought closer to failure all the faults that supposedly caused the main seismic events of 1916-1921. However, the stress change magnitude is generally small and thus the static stress interaction among the main seismic sources is not supported by a significant seismic correlation. Significant stress change loading may be instead inferred for the triggering of a number of seismic events on neighbouring normal faults by the Garfagnana 1920 earthquake. In addition, the computation of the seismic stress changes suggests that seismic events with magnitude >= 6 may transmit stresses from the axial normal faults to specific external thrusts and vice versa. It is possible that a correlation may be made between loading applied by the major 1917-1920 extensional ruptures and the increased seismicity on the distal external thrusts. (C) 2016 Elsevier B.V. All rights reserved.

Effusive eruptions at open-conduit volcanoes are interpreted as reactions to a disequilibrium induced by the increase in magma supply. By comparing four of the most recent effusive eruptions at Stromboli volcano (Italy), we show how the volumes of lava discharged during each eruption are linearly correlated to the topographic positions of the effusive vents. This correlation cannot be explained by an excess of pressure within a deep magma chamber and raises questions about the actual contributions of deep magma dynamics. We derive a general model based on the discharge of a shallow reservoir and the magmastatic crustal load above the vent, to explain the linear link. In addition, we show how the drastic transition from effusive to violent explosions can be related to different decompression rates. We suggest that a gravity-driven model can shed light on similar cases of lateral effusive eruptions in other volcanic systems and can provide evidence of the roles of slow decompression rates in triggering violent paroxysmal explosive eruptions, which occasionally punctuate the effusive phases at basaltic volcanoes.

Identifying and characterizing strombolian activity from space is a challenging task for satellite-based infrared systems. Stromboli volcano is a natural laboratory that offers a unique opportunity for refining thermal remote-sensing applications that involve transient phenomena and small to moderate hot-spots. A new simple and fast algorithm gave us the opportunity to revisit the MODIS-derived thermal output at Stromboli volcano over the last 13 years. The new algorithm includes both night-time and daytime data and shows high performance with the detection of small-amplitude thermal anomalies ( 1000 MW). The ...

We show two examples of how integrated analysis of thermal and infrasound signal can be used to obtain, in real time, information on volcanic activity. Soufriere Hills Volcano (SHV) on Montserrat offers the opportunity to study a large variety of processes related to lava-dome activity, such as pyroclastic density currents (PDCs) and large Vulcanian eruptions. Infrasound and thermal analysis are used to constrain the propagation of PDCs and their velocities, which are calculated here to range between 15 and 75 m s −1 . During the Vulcanian eruption of 5 February 2010, infrasound and thermal records allow us to identify an approximately 13 s seismic precursor possibly related to the pressurization of the conduit before the explosion onset. The associated very long period (VLP) seismic signal is correlated with the gas-thrust phase detected by thermal imagery, and may reflect a change in the upward momentum induced by the mass discharge. Moreover, from infrasound and thermal analysis, we estimate a gas-thrust phase lasting 22 s, with an initial plume velocity of approximately 170 m s −1 and a mean volumetric discharge rate of 0.3×10 5 –9.2×10 5 m 3 s −1 . This information provided in real time gives important input parameters for modelling the tephra dispersal into the atmosphere.

Volcanic explosions release large amounts of hot gas and ash into the atmosphere to form plumes rising several kilometers above eruptive vents, which can pose serious risk on human health and aviation also at several thousands of kilometers from the volcanic source. However the most sophisticate atmospheric models and eruptive plume dynamics require input parameters such as duration of the ejection phase and total mass erupted to constrain the quantity of ash dispersed in the atmosphere and to efficiently evaluate the related hazard. The sudden ejection of this large quantity of ash can perturb the equilibrium of the whole atmosphere triggering oscillations well below the frequencies of acoustic waves, down to much longer periods typical of gravity waves. We show that atmospheric gravity oscillations induced by volcanic eruptions and recorded by pressure sensors can be modeled as a compact source representing the rate of erupted volcanic mass. We demonstrate the feasibility of using gravity waves to derive eruption source parameters such as duration of the injection and total erupted mass with direct application in constraining plume and ash dispersal models.

[1] Infrasonic and seismic waveforms were collected during violent strombolian activity at Yasur Volcano (Vanuatu). Averaging ~3000 seismic events showed stable waveforms, evidencing a low-frequency (0.1–0.3 Hz) signal preceding ~5–6 s the explosion. Infrasonic waveforms were mostly asymmetric with a sharp compressive (5–106 Pa) onset, followed by a small long-lasting rarefaction phase. Regardless of the pressure amplitude, the ratio between the positive and negative phases was constant. These waveform characteristics closely resembled blast waves. Infrared imagery showed an apparent cold spherical front ~20 m thick, which moved between 342 and 405 m/s before the explosive hot gas/fragments cloud. We interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front. We suggest that violent strombolian activity at Yasur was driven by supersonic dynamics with gas expanding at 1.1 Mach number inside the conduit.

Non-destructive measurements of deformations are a quite common application of holography but due to the intrinsic limits in the interferometric technique, those are generally confined only to small targets and in controlled environment. Here we present an advanced technique, based on Mid Infrared Digital Holography (MIR DH), which works in outdoor conditions and provides remote and real-time information on the oscillation modes of large engineering structures. Thanks to the long wavelength of the laser radiation, large areas of buildings can be simultaneously mapped with sub-micrometric resolution in terms of their amplitude and frequency oscillation modes providing all the modal parameters vital for all the correct prevention strategies when the functionality and the health status of the structures have to be evaluated. The existing experimental techniques used to evaluate the fundamental modes of a structure are based either on seismometric sensors or on Ground-based Synthetic Aperture Radar (GbSAR). Such devices have both serious drawbacks, which prevent their application at a large scale or in the short term. We here demonstrate that the MIR DH based technique can fully overcome these limitations and has the potential to represent a breakthrough advance in the field of dynamic characterization of large structures.

Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long-period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate. During the 2014 flank eruption of Stromboli, this magma input preceded the effusive eruption by several months. When the new lateral effusive vent opened on the Sciara del Fuoco slope, the effusion was accompanied by a large ground deflation, a deepening of the VLP seismic source, and the cessation of summit explosive activity. Such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent. We show how this model successfully reproduces the measured rate of effusion, the observed rate of ground deflation, and the deepening of the VLP seismic source. This study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open-conduit system and to track magma drainage during the effusive crisis, with a great impact on hazard assessment.

The Stromboli volcano erupted on February 27, 2007, after an intense Strombolian activity lasted about 2 months. The eruption was characterized by a series of rapidly evolving phenomena, like the propagation of an effusive fracture along the crater rim, the opening of lateral effusive vents, an unusually large effusive flux (> 10 m 3 /s), the collapse of the crater system, and a major strombolian explosion. This eruption was monitored by an integrated network of multiparameter instruments: broad-band seismometers, infrasonic array, thermal cameras and bore-hole tiltmeters. All the information collected and processed in real-time allowed to draw a clear picture of the eruption dynamics. Thermal imagery documented in real-time how the effusive fracture propagated at 12:24 GMT of February 27 along the northern rim of the NE crater towards the NE direction. The infrasonic array showed that explosive and degassing activity ceased at 10:32 GMT, a couple of hours before the opening of the effusive fractures. The end of the explosive activity coincided with an intense phase of high-frequency (4–22 Hz) tremor which showed two episodes of harmonic spectrum gliding from 4–7 Hz to ~ 22 Hz. The end of this phase was characterized at 18:26 GMT by a strong infrasonic signal located by the array in the Sciara del Fuoco and associated with the landslide induced by the opening of a lateral effusive vent at 400 m of elevation. This eruption provided a clear example on how the strategy of integrating in near-real-time geophysical information can pinpoint the transition from explosive to effusive activity of the shallow volcanic system at Stromboli.

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 (10 4 –10 7 J) is lower than that for events at Santiaguito and Fuego (10 8 –10 9 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.

International audience; After 3 years of mild gases emissions, the Ubinas volcano entered in a new eruptive phase on September 2nd, 2013. The MIROVA system (a space-based volcanic hot-spot detection system), allowed us to detect in near real time the thermal emissions associated with the eruption and provided early evidence of magma extrusion within the deep summit crater. By combining IR data with plume height, sulfur emissions, hot spring temperatures and seismic activity, we interpret the thermal output detected over Ubinas in terms of extrusion rates associated to the eruption. We suggest that the 2013–2014 eruptive crisis can be subdivided into three main phases: (i) shallow magma intrusion inside the edifice, (ii) extrusion and growing of a lava plug at the bottom of the summit crater coupled with increasing explosive activity and finally, (iii) disruption of the lava plug and gradual decline of the explosive activity. The occurrence of the 8.2 Mw Iquique (Chile) earthquake (365 km away from Ubinas) on April 1st, 2014, may have perturbed most of the analyzed parameters, suggesting a prompt interaction with the ongoing volcanic activity. In particular, the analysis of thermal and seismic datasets shows that the earthquake may have promoted the most intense thermal and explosive phase that culminated in a major explosion on April 19th, 2014. These results reveal the efficiency of space-based thermal observations in detecting the extrusion of hot magma within deep volcanic craters and in tracking its evolution. We emphasize that, in combination with other geophysical and geochemical datasets, MIROVA is an essential tool for monitoring remote volcanoes with rather difficult accessibility, like those of the Andes that reach remarkably high altitudes.

Effusive eruptions are explained as the mechanism by which volcanoes restore the equilibrium perturbed by magma rising in a chamber deep in the crust. Seismic, ground deformation and topographic measurements are compared with effusion rate during the 2007 Stromboli eruption, drawing an eruptive scenario that shifts our attention from the interior of the crust to the surface. The eruption is modelled as a gravity-driven drainage of magma stored in the volcanic edifice with a minor contribution of magma supplied at a steady rate from a deep reservoir. Here we show that the discharge rate can be predicted by the contraction of the volcano edifice and that the very-long-period seismicity migrates downwards, tracking the residual volume of magma in the shallow reservoir. Gravity-driven magma discharge dynamics explain the initially high discharge rates observed during eruptive crises and greatly influence our ability to predict the evolution of effusive eruptions.

Thermal activity is a common precursor to explosive volcanic activity. The ability to use these thermal precursors to monitor the volcano and obtain early warning about upcoming activity is beneficial for both human safety and infrastructure security. By using a very reliably active volcano, Stromboli Volcano in Italy, a method has been developed and tested to look at changes in the frequency of small scale explosive activity and how this activity changes prior to larger, ash producing explosive events. Thermal camera footage was used to designate parameters for typical explosions at Stromboli (size of spatter field, cooling rate, frequency of explosions) and this information was applied to characterize explosions in satellite imagery. Satellite data from The National Aeronautics and Space Administration's Moderate Resolution Imaging Spectroradiometer (MODIS) and US/Japan designed Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) for numerous periods in 2002 to 2009 were analyzed for thermal features which were used to calculate an estimate of the level of activity during the given time period. The results at Stromboli showed a high level of small scale explosions which stop completely prior to large paroxysmal eruptive episodes. This activity also corresponds well to seismic and infrasonic records at Stromboli, indicating that this thermal infrared monitoring method may be used in conjunction with other detection methods where available, and also indicates that it may be a useful method for volcano monitoring when other methods (e.g. seismic instrumentation, infrasound arrays, etc.) are not available.

Due to its massive air traffic impact, the 2010 eruption of Eyjafjallajokull was felt by millions of people and cost airlines more than U.S. $1.7 billion. The event has, thus, become widely cited in renewed efforts to improve real-time tracking of volcanic plumes, as witnessed by special sections published last year in Journal of Geophysical Research, (117, issues D20 and B9).

The persistent mild explosive activity of Stromboli is explained in terms of the dynamics of large gas slugs that ascend the magma conduit to burst at the free surface. This simple physical model has now strong evidence from both geophysical and geochemical viewpoints. In recent years, combined analyses of geophysical data, such as infrasound and thermal, integrated with seismological information, have improved constraint on conduit dynamics. We now know that gas expansion, preceding the explosion onset by 2 to 20 s, occurs at a depth of ∼260 m within the conduit. Explosions repeat at a typical rate of ∼13 events/h with gas jet velocities of 10―130 m/s. The time delay between the infrasound and thermal onset indicates a depth of

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 (54–142 m/s) based on the acoustic signal recorded during the 2010 Eyjafjallajokull eruption from 4 to 21 May. Exit velocities are converted into MER using additional information on vent diameter (50±10 m) and mixture density (5.4±1.1 kg/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.

International audience; Masses erupted during normal explosions at Stromboli volcano (Italy) are notoriously difficult to measure. We present a method that uses thermal infrared video for cooling bomb fields to obtain the total power emitted by all hot particles emitted during an explosion. A given mass of magma (M) will emit a finite amount of thermal power, defined by M cp(Te−T0), cp and Te being magma specific heat capacity and temperature, and T0 being ambient temperature. We use this relation to convert the total power emitted by the bomb field to the mass required to generate that power. To do this we extract power flux curves for the field and integrate this through time to obtain total power (E). This is used to estimate mass (Q) in Q=E/cp(Te−T0). When applied to individual bombs we obtain masses of between 1 and 9 kg per bomb, or a volume of 970 and 6500 cm3. These volumes equate to spheres with diameters 12 and 27 cm. For the entire bomb field we obtain volumes of 7-28 m3. We calculate masses for 32 eruptions and obtain typical bomb masses of between 103 and 104 kg per eruption. In addition, we estimate that between 102 and 103 kg of gas and ash are emitted as part of a mixed plume of bombs, gas and ash. We identify two types of eruption on the basis of the erupted bomb masses and the ratio of the plume's gas-and-ash component to the bomb component. The first type is bomb-dominated, is characterized by bomb masses of 104 kg and has ash-gas/ bomb ratios of ∼0.02. The second type is ash-and-gas dominated, is characterized by erupted bomb masses of 103 kg and has ash-gas/bomb ratios of around one, and as high as two. There is no correlation between the quantity of bombs and quantity of gas-ash erupted. In addition, while source pressure for each explosion correlates with the quantity of gas and ash erupted, the mass of bombs emitted varies independently of pressure.

[1] Explosive activity at Stromboli volcano is analyzed using a high-frame rate (50 Hz) thermal camera and differential pressure transducers. We develop a thermal image-based decomposition method to derive vertical and horizontal exit velocities of the explosive cloud. Peak vertical velocity ranges between 23 and 203 m/s, slightly higher than previous estimates and rapidly decreasing to a constant value of 30–50 m/s within the first ∼0.1 s. Plume velocities are consistent with an elongated cloud expanding much faster vertically than horizontally and indicating the interaction with the conduit wall. Considering a vent radius of ∼2 m we estimate a volumetric flux of 200–600 m3/s, which converts to total volumes of gas-particles of 103–104 m3 for a single eruption. These volumes are proportional to the thermal energy recorded by the camera, providing a means to convert thermal radiance to volumes. Comparing the thermal onset of the explosions with the arrival time of the acoustic pressure, we demonstrate that infrasound is propagating 0.14−1.7 s ahead of the explosive front. The time difference between thermal and acoustic onsets constrains the infrasonic source within the conduit at 15–35 m below the crater rim. Peak amplitudes of acoustic pressure show a power law relationship (p ∼ U2) with the exit vertical velocities consistent with the energy balance of a two-phase flow rapidly accelerated in the conduit by gas decompression. Our results support monopole isotropic acoustic radiation of a source embedded within the conduit walls and indicate that explosive dynamics undergo strong accelerations of 103–104 m/s2.

Twelve years of night-time MODIS (Moderate Resolution Imaging Spectroradiometer) observations, has been analysed to detect and quantify the radiative heat power emitted by Stromboli volcano (from March 2000 to September 2011). Using an accurate background subtraction of the MODIS signal at 4 μm, we were able to discriminate two main regimes of thermal radiation, related to different levels of volcanic activity. Effusive eruptions (occurred on December 28, 2002 and February 27, 2007) radiated at an average of ~ 186 MW with a frequency of alert detection of 50–95%. Conversely, during the typical strombolian activity, an average of ~ 9 MW is radiated, with a frequency of alert detection of 0–45%. Although during the effusive eruptions the radiative power is basically controlled by the lava discharge rate, our results suggest that during non-effusive periods (strombolian regime) both the intensity and the frequency of MODIS alerts are controlled by the height of the magmatic column feeding the activity at the surface. In particular we found that a radiative power of ~ 50 MW corresponds to a high magma column which is exposed, in the vent area, at the same rate in which the deeper gas-rich magma is typically supplied within the feeding system of Stromboli (~ 0.3 m3 s− 1). In this condition the magmatic system approaches steady state regimes. Above this threshold a transition from strombolian to effusive regimes may occur as shown by the detection of ~ 50 MW, 8–10 days before the onset of both the last two major flank eruptions. These values were reached after 1–2 months of gradual increase of the radiative power which was likely associated the rising of the magma column within the shallowest portion of the conduit. In addition our data suggest that over the years 2000–2011 several cycles of rise and fall of the magma column have occurred, which however did not culminate into an effusive eruption but only into recurrent episodes of sustained spattering or fountaining and summit overflows. These fluctuations has substantially increased in frequency and intensity after the 2007 eruption thus suggesting that this event has perturbed in some way the shallow plumbing system of Stromboli. We stress that the detection of a radiative power higher than 50 MW is a clear evidence of a very high magma column, which may prelude the onset of an effusive eruption and/or periods of sustained vent activity. In conclusion, we suggest that systematic analysis of MODIS data can be used to detect variations in the intensity of strombolian activity and may considerably improve volcano surveillance at Stromboli, as well as at other open-system volcanoes.

Basaltic volcanoes are dominated by lava emission and mild explosive activity. Nevertheless, many basaltic systems exhibit, from time to time, poorly documented and little-understood violent explosions. A short-lived, multiblast explosive crisis (paroxysmal explosion) occurred on 15 March 2007 during an effusive eruptive crisis at Stromboli (Italy). The explosive crisis, which started at 20:38:14 UT, had a total duration of ∼5 min. The combined use of multiparametric data collected by the permanent instrumental networks (seismic, acoustic, and thermal records) and a field survey carried out immediately after the event enabled us to constrain the eruptive dynamics and quantify physical parameters. The eruption consisted of three major pulses: In the first, lithic blocks and ash were ejected at speeds of 100–155 m/s and 130–210 m/s, respectively. The high solid load of the eruptive jet resulted in the partial collapse of the column with the formation of a small-volume pyroclastic density current. The second, 12 s long pulse emitted 2.2–2.7 × 107 kg of tephra (mass discharge rate = 1.9–2.3 × 106 kg/s), forming a 3 km high convective plume, dispersing tephra up to the west coast, and a dilute density current with limited dispersal downslope of the craters. A final, 30 s long phase formed a scoria flow with a volume of 1.5–1.7 × 104 m3 (mass discharge rate = 5.9–6.7 × 105 kg/s), a total runout of ∼200 m, and a velocity of 45 m/s. The total gas volume involved in the explosion was 1.3–1.9 × 104 m3 with an initial overpressure of 7.9 ± 0.4 MPa. We compared the 15 March 2007 event with historical paroxysms, in particular with that of 5 April 2003, which was remarkably similar.

Answering the question whether a regional earthquake can trigger or enhance volcanic activity requires a systematic measure of volcanic activity. One such measure is heat fl ux. The availability since A.D. 2000 of a satellite-derived heat fl ux inventory for global volcanism allows us to explore how earthquakes and volcanic activity may be linked. Examination of 7 yr of global volcanic heat fl ux data reveals 37 volcanic responses to regional earthquakes. Each response is expressed by an increase in heat fl ux within 1‐21 days of the triggering earthquake. Whether a volcano responds depends on earthquake magnitude, distance to the epicenter, and orientation of the earthquake focal mechanism in respect to the volcano: the focal mechanism has to align with the responding system. Of the 7 global major increases of seismic energy during 2000‐2006, 4 were followed by a global volcanic heat fl ux increase. The largest response involved a 300% increase and followed the largest earthquake in the period, the 2004 SumatraAndaman (Indonesia) earthquake, moment magnitude, M W = 9.3.

We examine the tectonic evolution and structural characteristics of the Quaternary intermontane Mugello, Casentino, and Sansepolcro basins, in the Northern Apennines fold-and-thrust belt. These basins have been classically interpreted to have developed under an extensional regime, and to mark the extension-compression transition. The results of our study have instead allowed framing the formation of these basins into a compressive setting tied to the activity of backthrust faults at their northeastern margin. Syndepositional activity of these structures is manifested by consistent architecture of sediments and outcrop-scale deformation. After this phase, the Mugello and Sansepolcro basins experienced a phase of normal faulting extending from the middle Pleistocene until Present. Basin evolution can be thus basically framed into a two-phase history, with extensional tectonics superposed onto compressional structures. Analysis of morphologic features has revealed the occurrence of fresh fault scarps and interaction of faulting with drainage systems, which have been interpreted as evidence for potential ongoing activity of normal faults. Extensional tectonics is also manifested by recent seismicity, and likely caused the strong historical earthquakes affecting the Mugello and Sansepolcro basins. Qualitative comparison of surface information with depth-converted seismic data suggests the basins to represent discrete subsiding areas within the seismic belt extending along the axial zone of the Apennines. The inferred chronology of deformation and the timing of activity of normal faults have an obvious impact on the elaboration of seismic hazard models.

We used a network of stations to perform systematic radon surveys at Stromboli volcano. The time series of periodic measurements show that monthly average 222Rn emissions reflect changes in volcanic activity and exhibit increasing trends prior and during the last major eruptive cycles. Maps of radon emissions indicate that diffuse degassing is operative at Stromboli volcano. Concentrated degassing essentially occurs in the summit area and within a sector proximal to the two major NE trending faults. These sites were chosen for deploying the two real-time stations that are currently operating at Stromboli. In these devices, the 222Rn electronic dosimeters are connected to a radiomodem for wireless data transfer to a receiving station at the volcano observatory. Radon activity, soil temperature and atmospheric pressure data are sampled and instantaneously transferred via web so that they can be checked remotely. Collected time series reveal an overall inverse correlation between radon emissions and seasonal temperature variations. Radon emissions in sectors of diffuse degassing are modulated by tidal forces as well. Radon activities recorded at the summit station, located along the fracture zone where the gas flux is concentrated, are positively correlated with changes in atmospheric pressure and confirm the occurrence of the “atmospheric stack effect”. We finally emphasize that real-time radon monitoring is an innovative technique that may be systematically applied in volcano surveillance.

Explosive volcanic eruptions typically show a huge column of ash and debris ejected into the stratosphere, crackling with lightning. Yet equally hazardous are the fast moving avalanches of hot gas and rock that can rush down the volcano's flanks at speeds approaching 280 kilometers per hour. Called pyroclastic flows, these surges can reach temperatures of 400°C. Fast currents and hot temperatures can quickly overwhelm communities living in the shadow of volcanoes, such as what happened to Pompeii and Herculaneum after the 79 C.E. eruption of Italy's Mount Vesuvius or to Saint-Pierre after Martinique's Mount Pelee erupted in 1902.