Dario Delle Donne

High‐frame rate thermal imagery of Strombolian explosions: Implications for explosive and infrasonic source dynamics

[1]xa0Explosive 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.

Earthquake-induced thermal anomalies at active volcanoes

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.

Dynamics of Strombolian Activity

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 <120 m below the vent. Infrasonic pressure and thermal transient amplitudes increases as the time delay of their onsets increases. We show how coupled fluctuations in these parameters are compatible with the migration of the magma column in response to increased gas flux. Infrasound and thermal data reveal that activity is also characterized by a persistent bursting of small gas bubbles, occurring every 1―2 s. This activity (puffing) releases more gas (∼100 t/d) than normal Strombolian explosions (∼35 t/d) and represents a major aspect of the volcano dynamics. Explosions and puffing show significant short-term variability. This can be explained in terms of changing gas supply to the shallow feeding system.

Tracking Pyroclastic Flows at Soufrière Hills Volcano

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 volcanos 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 Italys Mount Vesuvius or to Saint-Pierre after Martiniques Mount Pelee erupted in 1902.

Volcano seismicity and ground deformation unveil the gravity-driven magma discharge dynamics of a volcanic eruption

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.

Chapter 9 Thermal, acoustic and seismic signals from pyroclastic density currents and Vulcanian explosions at Soufrière Hills Volcano, Montserrat

Abstract 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. Soufrière 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 0.3×105–9.2×105 m3 s−1. This information provided in real time gives important input parameters for modelling the tephra dispersal into the atmosphere.

Tracking dynamics of magma migration in open-conduit systems

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.

Remote monitoring of building oscillation modes by means of real-time Mid Infrared Digital Holography

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.

Forecasting Effusive Dynamics and Decompression Rates by Magmastatic Model at Open-vent Volcanoes

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.

Dynamic triggering of mud volcano eruptions during the 2016-2017 central Italy seismic sequence

On 24 August 2016 a seismic event (Mw 6.0) was the first of the long Central Italy sequence (ongoing at the end of 2017) of medium-to-high magnitude earthquakes, with nine Mw ≥5 up to October 2017, and with about 74.000 seismic events registered after one year. The largest was the Mw 6.5 30 October 2016 event near Norcia. After the major seismic events, seventeen mud volcanoes erupted around Monteleone di Fermo village (Marche region). Mud volcano eruptions generally occurred a few hours to a few days after the main earthquakes, suggesting a seismic triggering. We analyzed the peak ground velocities (PGV) and dynamic stresses during the three largest earthquakes. We also evaluated the static stress changes in order to assess the potential influence of normal stress changes on the feeder system of the activated mud volcanoes. We find a correlation with dynamic stresses, whereas static stress changes are negligible or negative (with values reaching -0.44 bar, clamping feeder dykes). We conclude that seismic shaking (up to ~3.9 bar during Norcia earthquake) is the dominant driver for these eruptions. Finally, we evaluated the response ratio as a function of the dynamic stress. It increases exponentially with peak dynamic stress varying from 0.4 bar, to >50% for peak dynamic stress >2 bar, indicating a link between earthquake shaking and mud volcano activity.