Sedimentation process of ashfall during a Vulcanian eruption as revealed by high-temporal-resolution grain size analysis and high-speed camera imaging

Abstract

We here examined the sedimentation process of falling ash particles during a short-lived Vulcanian eruption at Sakurajima volcano, Japan, using high-speed camera imaging of airborne ash particles, as well as grain size analyses of ash samples collected at high temporal resolution. Ashfalls from a sequence of two Vulcanian eruptions at 10:44 and 11:26 JST (Japan Standard Time) on 26 March 2016 were investigated. The field site for imaging and sampling was located approximately 3.5 km from the vent. The ash particles collected exhibit bimodal grain size distributions (GSDs). The median particle size of the main subpopulation decreased throughout the ashfall owing to sizedependent segregation of ash particles from a single source released from a constant height. The volume fraction of the subordinate subpopulation is interpreted to represent a component of ash aggregates and shows an increase in its volume fraction after the eruption onset. High-speed camera imaging reveals that the mean particle density decreased over time as the ashfall continued. These data suggest an increasing contribution of low-density aggregates during the course of the ashfall. The temporal changes in the density of airborne ash particles and in the GSD of ash samples demonstrate that the sedimentation process during a Vulcanian eruption at Sakurajima volcano is characterized by sedimentation of ash particles from transient eruption plumes at constant height, in which ash aggregation increases over time. Moreover, time series analysis of the ash falling rate shows that secondary thickening occurred during these Vulcanian eruptions and that this can be triggered by the settling of ash aggregates.

Physical and Aerodynamic Characterization of Particle Clusters at Sakurajima Volcano (Japan)

“…All these values are in good agreement with the results of and Miwa et al (2020) for similar eruptions of Sakurajima volcano, even though Miwa et al (2020) also report some higher values of terminal velocity (i.e., 6.5 m/s), corresponding however to unlikely density for volcanic particles (i.e., 4,400 kg/m 3 ).…”

“…Their main properties can only be derived from the analysis of the HS-HR camera footages. The analysis of the video footages returned an unprecedented and statistically solid database of the physical parameters governing the sedimentation dynamics of aggregates and allowed better constraints on their internal structure and characteristics with respect to previous studies (e.g., Bonadonna et al, 2011;Taddeucci et al, 2011;Brown et al, 2012;Miwa et al, 2020). The presented results, although specific for Sakurajima volcano, provide a first comprehensive database of physical parameters of particle clusters never collected before with such a detail.…”

“…Obtained dataset on particle settling dynamics and textural features resulted in a larger database compared with previous studies (e.g., Taddeucci et al, 2011;Miwa et al, 2020). Results of HS-HR video analysis (in terms of the aggregate terminal velocity, density and size) are shown in Figure 5 (and summarized in Supplementary Table S3).…”

“…As a result, many efforts have been made to improve the description of aggregates and our understanding of aggregation processes (e.g., Gilbert and Lane, 1994;James et al, 2002;James et al, 2003;Durant et al, 2009;Bonadonna et al, 2011;Rose & Durant, 2011;Taddeucci et al, 2011;Brown et al, 2012;Van Eaton et al, 2012;Van Eaton and Wilson, 2013;Burns et al, 2017;Vogel et al, 2019), as well as to provide increasingly accurate numerical descriptions for more effective hazard assessments (Cornell et al, 1983;Veitch and Woods, 2001;Bonadonna et al, 2002a;Textor et al, 2006;Costa et al, 2010). Despite the importance of aggregation, due to the low preservation potential of particle clusters in tephra-fallout deposits, only a few examples exist that document the fundamental physical and aerodynamic parameters of ash aggregates, such as their bulk density, terminal velocity and size distribution of the constitutive particles (Bonadonna et al, 2002b;Bonadonna et al, 2011;Taddeucci et al, 2011;Van Eaton et al, 2012;Burns et al, 2017;Miwa et al, 2020). The lack of detailed and ground-based data for ash aggregates importantly affects the reliability of numerical model results (Brown et al, 2012;Durant et al, 2015).…”

“…Recently, the direct filming of aggregates during tephra fallout has emphasized the importance of a field-based approach to study particle clusters. In fact, such a strategy allows for physical and aerodynamic parameters of particle clusters to be derived before their disruption at the moment of impact with the ground (Taddeucci et al, 2011;Miwa et al, 2020). For this reason, in this study, we have adopted the state-of-the-art technique involving a high-speed, high-resolution (HS-HR) camera coupled with ground observations and ash collection on adhesive paper aimed at the detailed characterization of aggregates .…”