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 ...

This paper describes ash-venting activity at Soufrière Hills Volcano, Montserrat that was precursory to the onset of three phases of lava extrusion in 2005, 2008 and 2009, and similar ash venting that occurred during the fifth phase of lava extrusion.We describe in detail a style of mild, tephra-generating activity termed ash venting and its associated tephra products. The nature of the seismicity associated with ash venting is compared with that of explosive activity. All explosive events, from small explosions to large Vulcanian explosions, have impulsive, low-frequency onsets. These are absent in ash-venting events, which have subtle, emergent onsets.Microscope and grain-size analyses show that ash-venting events and large Vulcanian explosions generate tephra that is similar in grain size (in medial and distal regions), although phreatic events in 2005 were finer grained. Ash-venting products are either composed of fine-grained, variably altered pre-existing material or juvenile material. There is a general correlation between the length of the pause and the length of the period of precursory activity prior to lava extrusion following it. Syn-extrusive ash venting is frequently associated with short-term increases in extrusion rate and is considered to be related to shear-induced fragmentation at the conduit margin.