Comparison of Two Advection-Diffusion Methods for Tephra Transport in Volcanic Eruptions

In order to model the dispersal of volcanic particles in the atmosphere and their deposition on the ground, one has to simulate an advection-diffusion-sedimentation process on a large spatial area. Here we compare a Lattice Boltzmann and a Cellular Automata approach. Our results show that for high Peclet regimes, the cellular automata model produce results that are as accurate as the lattice Boltzmann model and is computationally more effectiv

New numerical solutions for the description of volcanic particle dispersal

When volcanoes erupt explosively, particles are ejected and deposit on the ground. Numerical models for particle transports are important for both the hazard assessment and the eruptive dynamics. Total grainsize distributions are necessary for numerical simulations of particle transport. Comparing field data and numerical simulations, we show the representative sampling distance required to have reasonable grainsize distributions. Numerical models of tephra dispersal require better parameterizations. We have firstly developed 2D model with Cellular Automata (CA) method and expanded it to 3D. In our 3D model, diffusion is described with a random velocity corresponding fluctuations of turbulence. The implementation of plume significantly improves the description of particle distributions. Ballistic trajectories are modeled in 3D with a Discrete Event Simulation (DES) method. Multiparticle simulations are implemented including particle-particle collisions. We show the effect of collisions on travel distance. This model is applied to the hazard assessment of Vulcano Island (Italy)

Transport of ballistic projectiles during the 2015 Aso Strombolian eruptions

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

Grain-size features of two large eruptions from Cotopaxi volcano (Ecuador) and implications for the calculation of the total grain-size distribution

Studies of grain-size distributions of explosive volcanic eruptions provide important insights into fragmentation mechanisms and eruptive conditions and are crucial to the modeling of tephra dispersal. As a result of sedimentation processes and plume dynamics, grain-size features vary significantly both in the downwind and crosswind directions and are difficult to characterize. We have analyzed grain-size features in the downwind and crosswind directions of the two largest eruptions of the last 2000years of Cotopaxi volcano activity (Ecuador). Crosswind grain-size variations are similar for both eruptions (i.e., layers 3 and 5), while at any given downwind distance from vent, the layer 3 deposit is coarser than the layer 5 one. This suggests that layers 3 and 5 were characterized by similar plume height but that layer 3 was advected by a stronger wind. In addition, both deposits are coarsest along the dispersal axis and become richer in ash in the crosswind direction showing a Gaussian decreasing rate. Deposit thickness also shows a Gaussian crosswind decay, but layer 3 is significantly thicker at all points than is layer 5 due to the former's larger erupted mass. Based on both quantitative analysis of field data and on numerical simulations, we show that tephra deposits associated with large explosive eruptions (i.e., plume height of 30km) should be sampled out to at least 200km from the vent (depending on wind speed and tropopause height) in order to derive complete grain-size distributions that are not depleted in fines. Eruptions occurring in a strong wind field at high latitudes (e.g., Iceland) require lesser representative-sampling distances because of the lower tropopause heights

Grain-size features of two large eruptions from Cotopaxi volcano (Ecuador) and implications for the calculation of the total grain-size distribution

Studies of grain-size distributions of explosive volcanic eruptions provide important insights into fragmentation mechanisms and eruptive conditions and are crucial to the modeling of tephra dispersal. As a result of sedimentation processes and plume dynamics, grain-size features vary significantly both in the downwind and crosswind directions and are difficult to characterize. We have analyzed grain-size features in the downwind and crosswind directions of the two largest eruptions of the last 2000 years of Cotopaxi volcano activity (Ecuador). Crosswind grain-size variations are similar for both eruptions (i.e., layers 3 and 5), while at any given downwind distance from vent, the layer 3 deposit is coarser than the layer 5 one. This suggests that layers 3 and 5 were characterized by similar plume height but that layer 3 was advected by a stronger wind. In addition, both deposits are coarsest along the dispersal axis and become richer in ash in the crosswind direction showing a Gaussian decreasing rate. Deposit thickness also shows a Gaussian crosswind decay, but layer 3 is significantly thicker at all points than is layer 5 due to the former's larger erupted mass. Based on both quantitative analysis of field data and on numerical simulations, we show that tephra deposits associated with large explosive eruptions (i.e., plume height of 30 km) should be sampled out to at least 200 km from the vent (depending on wind speed and tropopause height) in order to derive complete grain-size distributions that are not depleted in fines. Eruptions occurring in a strong wind field at high latitudes (e.g., Iceland) require lesser representative-sampling distances because of the lower tropopause heights

Avalanches on Mt. Fuji, Japan: Seismic detection and tracking combined with numerical simulations

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Seismic location and tracking of snow avalanches and slush flows on Mt. Fuji, Japan

Avalanches are often released at the dormant stratovolcano Mt. Fuji, which is the highest mountain of Japan (3776 m a.s.l.). These avalanches exhibit different flow types from dry-snow avalanches in winter to slush flows triggered by heavy rainfall in late winter to early spring. Avalanches from different flanks represent a major natural hazard as they can reach large dimensions with run-out distances up to 4 km, destroy parts of the forest, and sometimes damage infrastructure. To monitor the volcanic activity of Mt. Fuji, a permanent and dense seismic network is installed around the volcano. The small distance between the seismic sensors and the volcano flank (<10 km) allowed us to detect numerous avalanche events from the seismic recordings and locate them in time and space. We present the detailed analysis of three avalanche or slush flow periods in the winters of 2014, 2016, and 2018. The largest events (size class 4–5) are detected by the seismic network at maximum distances of about 15 km, and medium-size events (size class 3–4) within a radius of 9 km. To localize the seismic events, we used the automated approach of amplitude source location (ASL) based on the decay of the seismic amplitudes with distance from the moving flow. The recorded amplitudes at each station have to be corrected by the site amplification factors, which are estimated by the coda method using data from local earthquakes. Our results show the feasibility of tracking the flow path of avalanches and slush flows with considerable precision (on the order of magnitude of 100 m) and thus estimating information such as the approximate run-out distance and the average front speed of the flows, which are usually poorly known. To estimate the precision of the seismic tracking, we analyzed aerial photos of the release area and determined the flow path and run-out distance, estimated the release volume from the meteorological records, and conducted numerical simulations with Titan2D to reconstruct the dynamics of the flow. The precision as a function of time is deduced from the comparison with the numerical simulations, showing mean location errors ranging between 85 and 271 m. The average front speeds estimated seismically, which ranged from 27 to 51 m s−1, are consistent with the numerically predicted speeds. In addition, we deduced two scaling relationships based on seismic parameters to quantify the size of the mass flow events. Our results are indispensable for assessing avalanche risk in the Mt. Fuji region as seismic records are often the only available dataset for this natural hazard. The approach presented here could be applied in the development of an early-detection and location system for avalanches based on seismic sensors.publishedVersio

A numerical model of ballistic transport with collisions in a volcanic setting

Fragments associated with explosive volcanic eruptions range from microns to meters in diameter, with the largest ones following ballistic trajectories from the eruptive vent. Recent field observations suggest that bombs ejected during Strombolian eruptions may collide while airborne. We developed a Discrete Event Simulator to study numerically the impact of such collisions on hazard assessment. We show that the area where bombs can land might be significantly increased when collisions occur. As a consequence, if collisions are dominant, the deposition distance cannot be used to estimate important eruption parameters, such as exit speed

Applying a Cellular Automata Method for the Study of Transport and Deposition of Volcanic Particles

The prediction of volcanic tephra dispersal is important for hazard studies and risk assessments. We are interested in simulating a full tephra transport system taking into account turbulent flows and particle aggregation using cellular automata (CA). In this preliminary research, we apply a probabilistic transport CA to our problem. Results show good agreement with field data, indicating that CA are adequate to simulate tephra transport