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

Abstract

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 21 . 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 21 and a mean volumetric discharge rate of 0.3 Â 10 5 -9.2 Â 10 5 m 3 s 21 . This information provided in real time gives important input parameters for modelling the tephra dispersal into the atmosphere.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.Lava dome eruptions represent a style of volcanism of distinctive interest because of their potential consequences, such as the generation of pyroclastic density currents (PDCs) down the volcano's flanks and large explosive eruptions following partial-dome collapse. Lava domes are formed by silica-rich lava that is too viscous to flow and, instead, builds-up over the point of extrusion. The hazards from lava-dome eruptions are well known owing to unpredictable transitions from the slow extrusion of viscous lava to vigorous explosions, and to the propensity of lava domes to suddenly collapse, spawning devastating PDCs down the flanks of volcanic edifices (Young et al. 1998;. As a lava dome grows, parts of it may collapse because of gravitational instability or as the result of gas explosions within the dome itself, thus forming PDCs. The ability to detect and track the propagation of these fast-moving density currents is of utmost importance in understanding the evolution of lava dome eruptions and mitigating related hazards.Methods for the location of PDCs, based on seismic amplitudes, have been proposed in the past (e.g. Jolly et al. 2002) and have proved quite effective for the characterization of large collapse events. These techniques require dense seismic networks and optimal azimuthal coverage; both conditions are rarely met on lava dome volcanoes where seismic networks are sparse because of the restrictions imposed by rough terrain and eruption hazards. The use of seismic amplitudes requires understanding of local site effects and frequency-dependent attenuation, thus introducing another layer of complexity. Finally, these methods are ...

Infrasound array criteria for automatic detection and front velocity estimation of snow avalanches: towards a real-time early-warning system

“…4). The stable position of the infrasound source during the second phase could be explained as the rapid deceleration of the avalanche flow induced by the change of topographic slope (Delle Donne et al, 2014). The third phase, which strongly recalls infrasonic waveforms recorded for snow avalanches at the Vallée de la Sionne test site and interpreted as being produced by the dynamics of avalanche deposition in the run-out zone (Kogelnig et al, 2011).…”

“…In this work we present results on automatic avalanche identification and evaluation of the avalanche front velocity obtained with an infrasound array, which operated during the 2012-2013 winter season near Ischgl (Paznaun valley, Austria), monitoring events from the Grosstal avalanche channel where spontaneous and controlled events typically occur every year (Jöbstl et al, 2014). The Grosstal avalanche channel is monitored permanently with a pulsed Doppler radar (Kogelnig et al, 2012), and we use it here to evaluate results obtained with the infrasound array.…”

“…The Grosstal avalanche channel, positioned on the northern flank of the Paznaun valley near the town of Ischgl (Austria), is typically characterized by the occurrence of several events/year (Jöbstl et al, 2014), with avalanches reaching the Silvretta road every 10 years (Fig. 1).…”

Chapter 1 An overview of the eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010

“…Infrasound arrays deployed at St George's Hill, Lees Yard and the Waterworks showed how the timing and directionality of explosive signals could be interpreted . Integration of infrasound with thermal camera data has allowed the dynamics of Vulcanian explosions and PDC emplacement to be measured (Delle Donne et al 2014). Gravity surveys in the north of the island revealed signals that were Fig.…”

An inclined Vulcanian explosion and associated products

“…1) recording images at two frames per second documented the 11 February 2010 dome collapse event. The FLIR A20 camera records thermal wavelengths between 7.5 and 13 µm with a resolution of 0.1 °C and has an angular field of view of 34 × 25°, yielding a 352 × 244 pixel image (see also Delle Donne et al 2014). Recorded values correspond to brightness temperature at these frequencies and are not calibrated for atmospheric attenuation to retrieve absolute surface temperature; we thus analyse these data in relative terms.…”