Transport of ballistic projectiles during the 2015 Aso Strombolian eruptions

Abstract

Large pyroclasts-often called ballistic projectiles-cause many casualties and serious damage on people and infrastructures. One useful measure of avoiding such disasters is to numerically simulate the ballistic trajectories and forecast where large pyroclasts deposit. Numerical models are based on the transport dynamics of these particles. Therefore, in order to accurately forecast the spatial distribution of these particles, large pyroclasts from the 2015 Aso Strombolian eruptions were observed with a video camera. In order to extrapolate the mechanism of particle transport, we have analyzed the frame-by-frame images and obtained particle trajectories. Using the trajectory data, we investigated the features of Strombolian activity such as ejection velocity, explosion energy, and particle release depth. As gas flow around airborne particles can be one of the strongest controlling factors of particle transport, the gas flow velocities were estimated by comparing the simulated and observed trajectories. The range of the ejection velocity of the observed eruptions was 5.1-35.5 m/s, while the gas flow velocity, which is larger than the ejection velocity, reached a maximum of 90 m/s, with mean values of 25-52 m/s for each bursting event. The particle release depth, where pyroclasts start to move separately from the chunk of magmatic fragments, was estimated to be 11-13 m using linear extrapolation of the trajectories. Although these parabolic trajectories provide us with an illusion of particles unaffected by the gas flow, the parameter values show that the particles are transported by the gas flow, which is possibly released from inside the conduit.

Combined approach to estimate the depth of the magma surface in a shallow conduit at Aso volcano, Japan

“…Therefore, the depth of the magma-air interface was most likely not very shallow. Tsunematsu et al (2019) estimated the depth to be 11-13 m using ballistic trajectories at Aso volcano. Since such a shallow depth should not provide acoustic resonance, this result might be an underestimation.…”

“…Since such a shallow depth should not provide acoustic resonance, this result might be an underestimation. Tsunematsu et al (2019) proposed a possible scenario in which the depth of explosion is different from the depth where the magma fragments begin to form parabolic trajectories. In this scenario, the fragments initially rise with the gas from the explosion source.…”

“…Although the best approach is to measure this depth from the crater rim directly, this is not possible at most volcanoes, because the magma surface in the conduit is not visible. In many cases, various records obtained at the time of the Strombolian explosions, such as the arrival-time delay between seismic and infrasound signals (Hagerty et al 2000;Ruiz et al 2006;Petersen and McNutt 2007;Sciotto et al 2013;Richardson et al 2014), the arrival-time delay between infrasound and thermal signals (Ripepe et al 2002;Gurioli et al 2014), the peak frequency and Q value of infrasound signals (Johnson et al 2018a, b;Watson et al 2019Watson et al , 2020, the peak frequency and overtones of infrasound signals (Fee et al 2010), the ballistic trajectories (Dürig et al 2015;Tsunematsu et al 2019), and the ejection velocity of particles (Taddeucci et al 2012;Cigala et al 2017;Salvatore et al 2020), have been used to estimate this depth. The methods to estimate the depth of the magma-air interface using these records have some unknown parameters other than the depth.…”

Linking gas and particle ejection dynamics to boundary conditions in scaled shock-tube experiments

“…Accordingly, reliably deciphering free magma surface and conduit/vent geometry is difficult and still affects estimations in natural eruptions with large uncertainty (e.g. Gaudin et al 2014;Dürig et al 2015;Salvatore et al 2018;Tsunematsu et al 2019). Our scaled experiments demonstrated an empirical correlation between initial conditions and gas and particle spreading angle resulting in a complex interplay between 1) particle inertia, 2) particle load, 3) vent geometry (gas expansion dynamics) and 4) effective overpressure at the vent.…”

Special issue “Advancement of our knowledge on Aso volcano: current activity and background”

“…The increase in resistivity can be ascribed to a decrease in the amount of conductive groundwater in the upper part of an aquifer located below the elevation of 800 m, while the decrease in resistivity implies that enhanced fluid temperature and pressure changed the subsurface hydrothermal system and formed a temporal fluid reservoir at the shallow level during the magmatic eruption period. Tsunematsu et al (2019) conducted a video camera observation of the Strombolian eruptions at Aso volcano in 2015 and analyzed the video images to investigate the gas flow effect on the particle transport of large pyroclasts (> 10 cm). Using the obtained trajectory data, the features of Strombolian activity such as ejection velocity, explosion energy, and particle release depth were investigated.…”