Miwa et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0316-8(2020) 7:3

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Sedimentation process of ashfall during a
Vulcanian eruption as revealed by hightemporal-resolution grain size analysis andhigh-speed camera imaging
T. Miwa1* , Y. Iriyama1, M. Nagai1 and F. Nanayama2

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 analysesof 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 fractionof the subordinate subpopulation is interpreted to represent a component of ash aggregates and shows anincrease 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, inwhich 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 ofash aggregates.
Keywords: Volcanic ash, Aggregation, High-speed camera imaging, High-resolution temporal sampling, Grain size
distribution, Vulcanian eruption, Sakurajima volcano

Introduction
Explosive eruptions generate and disperse tephra that
deposits around a volcano. The characteristics of thesetephra deposits reflect their sedimentation process,which is controlled by eruptive style, wind conditions,and the aerodynamic properties of the particles (e.g.,
Iriyama et al. 2018; Koyaguchi and Ohno 2001a; Walker
1971; Wohletz et al. 1989). Many previous studies haveinvestigated deposit characteristics to evaluate the sedimentation process for Plinian and sub-Plinian eruptions,* Correspondence: miwao@bosai.go.jp
1
National Research Institute for Earth Science and Disaster Resilience,
Tennodai 3-1, Tsukuba 305-0006, Japan
Full list of author information is available at the end of the article

whereas the number of studies that have examined sedimentation processes for Vulcanian eruption is still limited (e.g., Bonadonna et al. 2002; Eychenne et al. 2012;
Miyabuchi et al. 2013). Vulcanian eruptions are commonly defined as short-lived explosive eruptions lastingfrom few seconds to few hours (Clarke 2013). These
eruptions often occur intermittently during persistent
volcanic activity (Clarke 2013; Hammer et al. 1999; Iguchi et al. 2013). Although the eruptive mass (< 0.1 km3)
and eruptive column height (typically < 10 km) of single
Vulcanian events are relatively small, a series of Vulcanian eruptions can produce thick, edifice-forming ash deposits (Andronico and Cioni 2002; Imura 1991, 1995).

© The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
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Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

High-temporal resolution grain size analysis of tephra
deposits can provide information on the process of sedimentation during Vulcanian eruptions. The tephra
source during a Vulcanian eruption is considered a
discrete “volcanic thermal” (Woods and Kienle 1994)
which refers to instantaneous emission of batch of volcanic cloud or a ring vortex at the head of a transientplume (Chojnicki et al. 2014; Chojnicki et al. 2015a;
Chojnicki et al. 2015b; Tournigand et al. 2017). We investigate tephra sedimentation by assessing temporalvariations in the grain size distribution (GSD) of tephra,
which reflects the sedimentation process (e.g., Iriyama
et al. 2018; Koyaguchi and Ohno 2001b). The deposit
from a single Vulcanian eruption is generally too thin to
examine its temporal variations from the internal stratigraphy (Imura 1991, 1995; Miura et al. 2012; Miyabuchi
et al. 2013). Therefore, high-temporal resolution sampling of falling ash is required to obtain the temporalvariation in GSD during a Vulcanian eruption (Bagheri
et al. 2016; Bonadonna et al. 2002). Bonadonna et al.
(2002) sampled falling ash particles at 5 min intervals
during a series of Vulcanian eruptions at Soufrière Hills
Volcano, Montserrat, and found a progression from a
unimodal to bimodal GSD in the ash samples. Previous
studies, however, have not performed a detailed examination of a polymodal GSD during a Vulcanian eruption,
including the use of deconvolution analysis to derive theorigin of particle size subpopulations within a tephra deposit (Evans et al. 2009; Eychenne et al. 2012; Wohletz
et al. 1989).
Ash aggregation alters the aerodynamic properties oftephra and affects the sedimentation process (e.g., Careyand Sigurdsson 1982; Costa et al. 2010). Ash aggregatesfrom dry eruptions are generally fragile and are rarely
preserved in the deposit after reaching the ground surface (Bonadonna et al. 2011; Gilbert et al. 1991; Jameset al. 2003; Sparks et al. 1997; Taddeucci et al. 2011).
Therefore, in situ observation of falling particles isneeded to quantify the properties (size, falling velocity,and density) of the aggregates (Bagheri et al. 2016; Taddeucci et al. 2011). For Vulcanian eruptions, Bagheri
et al. (2016) described the temporal variation in ash aggregates at Sakurajima volcano, based on high-speed,
high-resolution camera imaging. The effects of aggregation on the sedimentation process during a Vulcanianeruption are, however, still poorly constrained.
In this study, we investigated temporal variations inthe sedimentation process of falling ash particles during
a Vulcanian eruption at Sakurajima volcano, Japan. We
conducted a detailed grain size analysis of ash samples
collected at high temporal resolution and observed airborne ash particles using high-speed camera imaging.
The combination of these analyses provides detailed insights into the sedimentation process of ash particles,

Page 2 of 16

including the effects of ash aggregation, during a shortlived Vulcanian eruption.

Vulcanian activity at Sakurajima volcano
Sakurajima volcano (Fig. 1a) is an active andesitic volcano located in southern Kyushu, Japan. Large-scale
eruptions of Sakurajima have generated both tephra deposits and lava flows (Kobayashi et al. 2013). Recent Vulcanian activity at Sakurajima has been ongoing since
1955, and deposits have attained a maximum cumulative
thickness of 120 cm at a site 2 km west from the vents
since that time (Imura 1995). The main eruptive ventswitched from the Minamidake summit crater to theShowa crater in 2006. The Showa crater is located on
the southeastern edifice at an altitude of approximately
800 m.a.s.l. (Iguchi et al. 2010). The Japan Meteorological Agency (JMA) reported over 5500 explosions in
the Showa crater from 2006 to 2017. The volumes ofsome of the individual Vulcanian eruptions are estimated to range from 5.9 × 10−7 to 3.6 × 10−6 km3 (Oishi
et al. 2018). Their eruptive products are characterized byangular and dense clasts ranging in size from ash particles to volcanic bombs (Miwa et al. 2013).
Methods/Experimental
Studied eruption

We studied a sequence of Vulcanian eruptions from theShowa crater on 26 March 2016 (Fig. 1b). The firsteruption occurred at 10:44 and was followed by a second
eruption at 11:26 Japan Standard Time (JST). Figure 2
captures the development of the volcanic cloud during
this sequence of Vulcanian eruptions. The first eruption
comprised an instantaneous initial jet followed by anash-rich mushroom cloud. The volcanic plume ascended
vertically, reaching a maximum height of 3500 m.a.s.l.
180 s after the onset of the eruption (Report of eruptive
activity at Sakurajima volcano published by Japan Meteorological Agency (JMA); https://www.jma-net.go.jp/
kagoshima/vol/data/skr_exp_2016.html). The second
eruption generated a volcanic cloud consisting of an initial jet phase followed by a weak mushroom cloud. The
maximum height of the volcanic cloud during the second eruption was recorded as 2900 m.a.s.l. 180 s after
the start of the eruption (Report of eruptive activity at
Sakurajima volcano published by JMA; https://www.jmanet.go.jp/kagoshima/vol/data/skr_exp_2016.html). Since
the maximum height of the second eruption was smaller,
thermal energy and the ash content should be smaller.
The volcanic clouds from both the first and second
eruptions were spread by westerly wind with a velocityof ~ 10 m s−1 (Fig. 3) (Meteorological data set publishedby JMA; http://www.data.jma.go.jp/obd/stats/etrn/index.
php). Pyroclastic density currents were not observed
during the studied eruptions (Fig. 2).

Miwa et al. Progress in Earth and Planetary ScienceFig. 1 (See legend on next page.)(2020) 7:3

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Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

Page 4 of 16(See figure on previous page.)
Fig. 1 a Locations of Sakurajima volcano and the field observation site (N31.581622, E130.705992). The location map was based on the Digital
Topographic Map published by the Geospatial Information Authority of Japan and processed using Kashmir software. Ushine indicates location of
the time-lapse camera installed by JMA (see Fig. 2). b Photograph of the studied eruption, which started at 10:44 JST, 26 March 2016. View fromthe location of photo in this figure. The height of column in this photo is approximately 900 m. c Field observation site and view of
instrument setup

Fig. 2 Images obtained from the time-lapse camera installed by JMA in the Ushine area, which is located approximately 10 km east of the Showa
crater (see Fig. 1). The images were taken at 10:43–12:17 JST, 26 March 2016, with a frequency of one image every 2 min

Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

Page 5 of 16

Fig. 3 Height profile of a velocity and b direction of upper wind at Kagoshima region meteorological station. The dashed line in a is the result of
linear fitting of the relationship between wind velocity and height

Field observations

We conducted field observations consisting of highspeed camera imaging of airborne ash particles andhigh-temporal resolution sampling of falling ash (Fig.
1c). The field site was located approximately 3.5 km eastof the Showa crater (Fig. 1a) and was roughly under thedispersal axis of the volcanic cloud. The consideration
on location of the dispersal axis is also supported byeastward wind direction at height of 2000–3000 m a.s.l.
around Kagoshima area as reported by JMA (Fig. 3)(http://www.data.jma.go.jp/obd/stats/etrn/index.php).
High-speed camera imaging

To estimate the aerodynamic properties of falling ash
particles, we observed airborne ash particles using ahigh-speed camera (Fig. 1c and Additional file 1: Movie
S1, Additional file 2: Movie S2, and Additional file 3:
Movie S3). The monochromatic high-speed camera(HAS-L2M by Ditect Co. Ltd.) was used to obtain video
footage at 3000 fps, with an image size of 180 pixels ×
190 pixels. The lens used had a focal length of 50 mm
and an aperture of f/1.4, providing a focal distance of 50
cm, an image resolution of 0.095 mm/pixel, and 4.75mm of focal depth. The aperture was set as large as possible to reduce the focal depth of the images so as tominimize the particle size analysis error. Calibration
using checkerboard panels showed that the image distortion in the captured images was negligible for the analyses. The camera was set on a tripod to horizontally
capture the falling ash, and a white background was selected for the image background. During the eruption,
we recorded three footages from 10:57, 11:33, and 12:00.
These footages are labeled HSC1057, 1133, and 1200, respectively. Each piece of footage lasted for approximately
1 min, with the duration being limited by camera memory and computer processing power. The computertransfer time forced an interval of approximately 30 min
between recordings.
The captured footage was exported as a time series of
8-bit bitmap files, and the software package ImageJ wasused to estimate the size and vertical falling velocity of
in-flight particles. We identified just-focused particles in
the images and made binary images of the particles byapplying a tone threshold. Using an image resolution of
0.095 mm/pixel, the lengths (l) of major and minor axesof each filmed particle were estimated, and the 2D
equivalent diameter was calculated using
D¼
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
lminor  lmajor . Falling velocities of ash particles were
estimated using the change in vertical position ofMiwa et al. Progress in Earth and Planetary Science(2020) 7:3

Page 6 of 16

particle centroids using a time sequence of binary images at a frame rate of 3000 fps.
Errors on the estimated particle falling velocities were
estimated from the uncertainty in the particle centroid
positions. Using an exposure time of 333 μs with a falling velocity of 0.2–7.0 m/s, we multiplied the exposure
time and falling velocity to estimate the uncertainty ofthe particle centroid positions at 67–2300 μm. Thus, thepercentage error in the centroid positions relative to thefalling distance taken from high-speed camera imagery(18 mm) is estimated at 0.37–12% (100 × uncertainty/
height of the image), with faster-falling particles having alarger relative error which was estimated to ± 0.001–
0.910 m/s for the range of falling velocities.

sampling was held under calm weather condition, contamination from old deposit by wind resuspension ishighly unlikely and our careful cleaning of trays avoided
contamination from the old sampling. Unfortunately, theash sample collected during 11:30–11:35 was lost while
being transferred to a plastic bag for the sample.
The ash falling rates were calculated by dividing theaccumulated mass of fallen ash by the duration of sampling and the area of the plastic trays. The accumulated
mass was measured using precise scales (error of 1.0 ×
10−5 g; Shimadzu AUW220D) in National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan. The error in sampling duration is less than
10 s.

High-temporal resolution sampling

Grain size analysis

Additionally, we sampled falling ash particles at hightemporal resolution (Table 1) simultaneously with thehigh-speed camera imaging. The sampling was performed from 10:57 to 12:12 with time intervals of 1 minfor 10:57–11:07 and 5 min for 11:07–12:12. Two plastic
trays with opening area of 38 cm × 52 cm were used forthe sampling. We alternated the two trays to enable continuous sampling of the falling ash particles. Since theIn order to examine in detail the sedimentation processof falling ash particles, we analyzed the GSD of 22 ash
samples obtained at regular intervals throughout theashfall. The grain size analyses were performed using a
Partica LA-960 instrument from HORIBA at the Geological Survey of Japan, Tsukuba. The grain size analyzer
measures particle sizes ranging from 0.01 to 3000 μm
using laser diffraction. The ash samples were dispersed

Table 1 Summary of high-temporal sampling
Sample name

Beginning time (JST)End time (JST)Interval [min]Ash falling rate [g min−1 m−2]

sj0326-01

10:57

10:58

1

92.85

sj0326-02

10:58

10:59

1

54.92

sj0326-03

10:59

11:00

1

33.12

sj0326-04

11:00

11:01

1

13.14

sj0326-05

11:01

11:02

1

10.08

sj0326-06

11:02

11:03

1

7.98

sj0326-07

11:03

11:04

1

0.40

sj0326-08

11:04

11:05

1

1.13

sj0326-09

11:05

11:06

1

1.53

sj0326-10

11:06

11:07

1

2.77

sj0326-11

11:07

11:12

5

2.48

sj0326-12

11:12

11:17

5

1.38

sj0326-13

11:17

11:22

5

0.65
0.23

sj0326-14

11:22

11:27

5

No sample

11:27

11:32

5

sj0326-15

11:32

11:37

5

0.12

sj0326-16

11:37

11:42

5

0.15

sj0326-17

11:42

11:47

5

0.91

sj0326-18

11:47

11:52

5

3.26

sj0326-19

11:52

11:57

5

3.06

sj0326-20

11:57

12:02

5

0.92

sj0326-21

12:02

12:07

5

0.55

sj0326-22

12:07

12:12

5

0.25

Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

in distilled water before the analyses were performed.
The plastic bags were washed using distilled water to ensure measurement of very fine particles.
We first used the GRADISTAT program (Blott andPye 2001) to analyze GSDs. The median size (Md) andstandard deviation (Sd) were calculated using the graphical method of Folk and Ward (1957). Although theseparameters can be calculated only in lognormal GSDs
with a unimodal shape, we characterized the GSDs
which include bimodal and trimodal distributions byusing these parameters for convenience in presentation(Evans et al. 2009).
Multiple lognormal curves were fitted to examine subpopulations within the GSDs. The fitting was done using
PeakFit software (Hulinks Inc.). PeakFit deconvolved
subpopulations from the GSDs using a basic function
with a linear baseline based on least squares regression
between the results of analysis and the fitting calculation. We tested lognormal, Weibull, and Rosin-Rammler
distributions as the basic functions for fitting (Eychenne
et al. 2012; Sheridan et al. 1987; Wohletz et al. 1989)
and applied lognormal distributions because of theirsimple expression. The number of subpopulations wasdetermined by the number of apparent peaks found in
the GSDs. We named these subpopulations SP#1, SP#2,
and SP#3 in descending order of their peak amplitudes.
The Md and Sd values of the subpopulations wereexpressed using the Φ scale (= −log2d; d is particle sizein millimeters).

Results and discussion
Time series of the ash falling rate

The ash falling rate at the field site varied during theashfall (Fig. 4). The ashfall at the field site started at 10:
57 (13 min after the first eruption onset) and ceased at
12:12. The ash falling rate reached a maximum during
the first 1 min (10:57–10:58) and decreased during thePage 7 of 16

following 7 min (10:57–11:04). The rate then recoveredand showed a secondary peak 10 min after the onset ofashfall (11:07–11:08), followed by another decrease (11:
08–11:37). The rate gradually increased to reach a third
peak (11:37–11:52) and finally decreased until the end of
the ashfall (11:52–12:12). During the field observation,
we observed the fallout of ash aggregates which werebroken on the impact with the ground surface at least
between 11:45 and 11:46 (Additional file 4: Movie S4).
However, the type of aggregate observed in this study
could not be determined because we did not perform
high-speed and high-resolution camera imaging of the
impact of aggregates (Bagheri et al. 2016).
Aerodynamic properties of falling ash

Here, we present the results of high-speed camera imagingof airborne ash particles. Analysis of the images showsthat particle size ranged from 0.12 to 2.2 mm and that falling velocity ranged from 0.2 to 7.0 m/s (Fig. 5a). Size andfalling ash velocity decreased from HSC1057 to HSC1133
and HSC1200. The size-velocity relationship is consistent
with the results presented by Bagheri et al. (2016). Theirresults fall between our results for HSC1057 and others(HSC1133 and HSC1200) (Fig. 5a). The relationship is
also similar to that obtained for the 2010 eruption of
Eyjafjallajökull volcano (Taddeucci et al. 2011). Specifically, the size-velocity relationship found in HSC1057 overlaps the results for individual particles from
Eyjafjallajökull, and the relationship identified in HSC1133
and HSC1200 is similar to that for aggregated particles for
Eyjafjallajökull (Taddeucci et al. 2011).
The Reynolds number (Re: ratio of inertia to viscous
force) and drag coefficient (Cd: drag force of a fluid on amoving particle) of falling particles were calculated fromthe size and velocity of ash particles. For the calculation,
we applied the model of Wilson and Huang (1979)
using:

Fig. 4 Timeline of field observations and temporal variation in ash falling rate at the field site. Note that logarithmic scale in the vertical axis.
Numbers refer to sample numberMiwa et al. Progress in Earth and Planetary ScienceFig. 5 (See legend on next page.)(2020) 7:3

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Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

Page 9 of 16(See figure on previous page.)
Fig. 5 Aerodynamic properties of falling ash particles obtained using high-speed camera imaging. Black circles, triangles, and crosses show theresults of footages HSC1057, HSC1133, and HSC1200, respectively. a Relationship between falling velocity and diameter of ash particles. Curves
represent the model of Wilson and Huang (1979), and the corresponding density and form factor (FF = (lintermediate + lminor)/(2 × lmajor)) values areshown. Solid red curves were obtained based on density variation in the ash deposit measured using a helium pycnometer. Dashed black curves
were calculated by the given constant density. Gray square shading indicates the range in particle size and velocity of the aggregates in
Sakurajima volcano, as reported in Bagheri et al. (2016). Colorless and gray areas enclosed by gray dash lines indicate the individual andaggregate particles from Eyjafjallajökull volcano, respectively (Taddeucci et al. 2011). b Relationship between Reynolds number (Re) and dragcoefficient (Cd). Dashed curves show the model of Wilson and Huang (1979) with form factors of 0.35 and 0.75. Colorless and gray areas enclosed
by gray dash lines indicate the individual and aggregate particles from Eyjafjallajökull volcano, respectively (Taddeucci et al. 2011). c Density anddiameter of falling ash particles calculated from Eqs. (1), (2), and (3). The solid red line represents density variation in the ash deposit, as measured
using a helium pycnometer

Re ¼ ρvL=η
Cd ¼

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
24 −0:828
þ 2 1:07−FF
FF
Re

ð1Þ
ð2Þ

where ρ is the density of the medium; v is falling velocity; L is the characteristic length of the particle; η isthe viscosity of the medium; and FF is the form factor
calculated by the major, intermediate, and minor lengthsof the particle: FF = (lintermediate + lminor)/(2 × lmajor). Recently, Kozono et al. (2019) suggested that the relationship between size and velocity of ash particles with sizeof < 0.32 mm is explained by the model of Wilson and
Huang (1979) for the Vulcanian eruptions at Sakurajima
volcano. We used the velocity and major and minor
lengths of falling ash particles in the 2D images to calculate values of Re and Cd. The equivalent diameter D wasused for the characteristic length L and intermediate axis
in the calculation. The estimated value of FF ranged
from 0.25 to 0.75 with 0.56 in average. The estimatedvalues of Re and Cd ranged from 2 to 70 and 1 to 11, respectively (Fig. 5b). Both the Re and Cd values decreased
from HSC1057 to HSC1133 and HSC1200. The trend of
the Re and Cd covers those of the 2010 eruption at Eyjafjallajökull volcano (Taddeucci et al. 2011).
The density of airborne ash particles (ρs) was derived
using a modified equation from Dellino et al. (2005),
which balances the downward gravitational force (g) andthe drag force due to air resistance around a sphericalparticle (Cd):ρs ¼

3C d ρv2
þρ
4gL

ð3Þ

The calculated density ranged from 130 to 4400 kg
m−3, which spans the mean density of ash particles obtained using a helium pycnometer (= 2700 ± 50 kg m−3;
see Additional file 5: Text S1). The maximum calculated
density (4400 kg m−3) is lower than the highest possible
density of an ash particle, which is approximately 5200
kg m−3 derived from the inclusion of magnetite. The
minimum calculated density (130 kg m−3) is one order ofmagnitude lower than that of vesicular glass (minimumof 1500 kg m−3; Miwa et al. 2013). The calculated density
decreased from HSC1057 (mean = 2500 kg m−3, range =890–4400 kg m−3) to HSC1133 (mean = 1220 kg m−3,
range = 180–4270 kg m−3) and to HSC1200 (mean =1030 kg m−3, range = 130–3980 kg m−3) (Fig. 5c). Within
the footage for HSC1057, HSC1133, and HSC1200, thepercentage of particles with a density of < 1500 kg m−3
was 10 vol.%, 80 vol.%, and 80 vol.% of the total volume,
respectively.
The numbers of falling particles filmed by the HSC
imaging qualitatively decreases from HSC1057 to
HSC1133 and HSC1200. This seems to be consistent
with the time series of the ash falling rate (Fig. 4).
Grain size distributions

Table 2 summarizes the results of the GSD analyses of
the collected particles. The ash samples exhibited unimodal, bimodal, or trimodal GSDs and changed throughthe course of the ashfall (Fig. 6). Sampled grain sizes
ranged from − 1.5Φ to 8Φ (2.8 mm to 3.9 μm). For samples from 10:57 to 11:37, the GSDs showed bimodal
character in 10:57–10:59, unimodal distributions from10:59 to 11:04, and bimodal distributions from 11:04 to
11:37. For samples from 11:37 to 12:12, trimodal distributions were found in 11:37–11:47 and bimodal distributions in 11:47–12:12.
Md and Sd values of the bulk samples are summarizedin Table 2. Md ranged from − 0.01 to 3.06Φ, ranging
from very fine to very coarse ash. Sd ranged from 0.35 to
1.55Φ, ranging from well to poorly sorted. We also identified temporal variations in Md and Sd values for the
subpopulations (Fig. 7). For the first subpopulation(SP#1), Md generally decreased over time, although there
is a decreasing step of Md value in 11:37–11:42. Sdshowed very little variation throughout the ashfall. For
the second subpopulation (SP#2), Md gradually increased
from 10:57 to 11:37, showed a stepwise increase at 11:
37–11:42, and then gradually decreased until the end of
the ashfall. The fraction of grains that fell within thissecond subpopulation generally increased from 10:57 to
11:37 and then decreased from 11:37 to the end of theashfall. Sd for SP#2 gradually increased from 10:57 to 11:

(2020) 7:3

Miwa et al. Progress in Earth and Planetary Science

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Table 2 Summary of grain size analysis
Bulk

SP#1

SP#2

SP#3

Md phiσ phi

Md phiσ phi

Areal%

Md phiσ phi

Areal%

sj0326-01

-0.01

0.47

0.13

0.38

99.6

7.17

0.30

0.4

sj0326-02

0.19

0.35

0.30

0.32

99.5

7.26

0.27

0.5

sj0326-03

0.44

0.40

0.56

0.34

100.0

sj0326-04

0.65

0.38

0.76

0.34

100.0

sj0326-05

0.77

0.40

0.89

0.36

100.0

sj0326-06

0.92

0.41

1.02

0.37

99.6

7.14

0.27

0.4

sj0326-07

0.88

0.43

0.99

0.39

100.0

sj0326-08

1.04

0.44

1.15

0.39

99.3

6.33

0.39

0.7

sj0326-09

1.17

0.45

1.27

0.40

98.7

6.37

0.48

1.3

sj0326-10

1.24

0.45

1.38

0.35

99.5

6.34

0.35

0.5

sj0326-11

1.51

0.50

1.60

0.43

97.3

6.58

0.59

2.7

sj0326-12

2.04

0.96

2.09

0.45

92.3

5.87

0.83

7.7

sj0326-13

2.13

1.15

2.13

0.46

85.1

5.75

1.08

14.9

sj0326-14

3.06

1.55

2.79

0.39

65.0

5.93

0.88

35.0

sj0326-15

2.78

1.37

2.72

0.38

76.4

5.88

1.26

23.6

sj0326-16

1.14

1.47

0.90

0.44

62.3

2.56

0.73

sj0326-17

1.14

1.30

1.09

0.45

78.3

2.78

0.86

sj0326-18

1.50

1.18

1.51

0.48

86.9

3.14

sj0326-19

1.83

1.04

1.86

0.49

90.0

3.34

sj0326-20

2.22

1.06

2.24

0.47

88.3

sj0326-21

2.44

0.96

2.49

0.43

90.1

sj0326-22

2.64

0.93

2.68

0.39

88.4

37 and then decreased. The third subpopulation (SP#3)
started to appear only from 11:37 onwards, and its Mdvalue and fraction remained fairly constant from 11:37
until the end of the ashfall. Sd of SP#3 decreased slightly
throughout the ashfall. Sample collected in 11:37–11:42
marked a change in median grain size for all three subpopulations, with Md decreasing from that sample to theend of ashfall.
All GSDs obtained by high-speed imaging exhibited unimodal distributions (Fig. 8). We foundmodal grain sizes of − 0.77Φ, 1.52Φ, and 1.32Φ for
HSC1057, HSC1133, and HSC1200, respectively.
The GSDs from the samples that were simultaneously collected exhibited bimodal distributions,with this bimodality (compared with the unimodalityof airborne particles) implying that the size distribution of particles changed after colliding with theground surface.
Sedimentation process of ashfall during a vulcanianeruption

The results of the GSD analysis and high-speed
camera imaging demonstrate that the sedimentation
process during Vulcanian eruptions at Sakurajima

Md phiσ phi

Areal%

29.3

6.04

0.82

8.4

15.9

6.39

0.73

5.8

0.61

6.7

6.50

0.73

6.4

0.43

4.2

6.37

0.74

5.7

3.68

0.49

4.2

6.26

0.67

7.4

3.83

0.49

3.5

6.32

0.66

6.4

3.64

0.44

4.6

6.41

0.61

7.0

volcano is controlled mainly by size-dependent segregation of ash particles from discrete transient
eruption plumes. The median grain size of SP#1,
with corresponds to 70–99% of falling ash, decreased after each of the two eruptions (Fig. 7), indicating the size-depending segregation of ash
particles in which larger particles having higher terminal velocity fall earlier from the volcanic cloudthan smaller particles. This decrease in particle size
was also identified in the GSD obtained by highspeed camera imaging where the mode value of particle size is smaller in HSC1133 than that in
HSC1057 (Fig. 8). Trimodal GSDs found after 11:37
represent the arrival of falling ash particles from thesecond thermal cloud. We conclude that the ash
particles were separately supplied from two distinct
thermals from each eruption, as inferred from thecontinuous trend of SP#1 before and SP#2 after 11:
37 (Fig. 7a).
We examined the relationship between particle size
and its arrival time by using a simple model for the volcanic thermal cloud, which is approximated as a singlerelease source of ash particles at a constant height with
an effect of advection by horizontal wind (Fig. 9). In thisMiwa et al. Progress in Earth and Planetary Science(2020) 7:3

Page 11 of 16

Fig. 6 GSDs of the ash deposit obtained using laser diffraction. White circles represent data obtained by grain size analysis. Thick blue, orange,and green lines represent subpopulations #1, #2, and #3, respectively. The thin black line shows the synthetic GSD formed by summing the
subpopulations. The collection time is described in upper right corner in each graphcase, the horizontal (L) and vertical (H) locations of thefalling particle from the release source at time (t) are
given asL ¼ vx ðt−t 0 −Δt Þ

ð4Þ

H ¼ H 0 −vz ðt−t 0 −Δt Þð5Þ

where H0 is a height of the release source of falling
particles from the observation point, t0 is the onset timeof eruption, and Δt is the time required for the height of
the release source to reach the height H0. The vx and vz
are horizontal and vertical (falling) velocities of the particles, respectively. vx is expressed as function of H, which
is derived from the height profile of westerly componentof upper wind at 9:00 of 26 March 2016 in Kagoshimaregional meteorological station (Meteorological data set
published by JMA; http://www.data.jma.go.jp/obd/stats/
etrn/index.php) (Fig. 3), and approximated tovx ¼ 0:0045H−2:9191

ð6Þ

By combining the Eqs. (4), (5), and (6) with the relationship between the falling velocity (vz) and particle size
described as Eqs. (1), (2), and (3) (Wilson and Huang
1979), we calculated the particle size likely to fall at theobservation point at a particular time. The location ofobservation point is set to L = 3500 m and H = 0 m. The

values of H0 for the first and second eruptions are 3500
m and 2900 m, respectively, from the height difference
between volcanic cloud and the observation point. Δt is
180 s (Fig. 2), and the modeled particle density 2700 kg
m−3. This model does not take into account a complex
wind field in proximal area of volcano by orographic effects (e.g., Poulidis et al. 2017), because there is no dense
observation network for wind condition to capture sucha detailed wind field.
The calculation result mostly explains the trend of
temporal variation in the median grain size of SP#1
during both the first and second eruptions within thestandard deviations (Fig. 7). This finding indicatesthat a discrete, initially homogeneous batch of ash
from a discrete volcanic cloud is a plausible source
model for tephra sedimentation during a Vulcanianeruption. This consideration is consistent with a result of detailed observation of volcanic plume at
Sakurajima volcano by using high-speed (visible andthermal) camera, in which ring vortex at the head oftransient plume was interpreted as the main carrier oftephra (Tournigand et al. 2017).
It should be noted that our model does not accountfor the effects of multiple release sources in the eruption
cloud (e.g., Folch 2012). The variation of physical properties (such as grain size concentration of tephra) within
the eruptive column which determines the parameters ofMiwa et al. Progress in Earth and Planetary Science(2020) 7:3

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Fig. 7 Temporal variation in Md and Sd of the subpopulations. Blue, orange, and green lines show subpopulations #1, #2, and #3, respectively. a
Md of the subpopulations. Solid lines show the expected sizes of falling ash particles with densities of 2700 kg m−3 for FF (form factor) of 0.56.
Error bars indicates the standard deviation of particle size. b Sd of the subpopulations. c Fraction of each subpopulation in the GSDs. Verticaldashed gray lines represent onset times of the two Vulcanian eruptions. Left side gray shade indicates collection time of sj0326-08 (11:04–11:05)
in which the aggregation became effective to sedimentation. Right side gray shade indicates collection time of sj0326-16 (11:37–11:42) in which
the trimodal GSD started to appear

multi-release sources of tephra dispersal has not been
well constrained in the actual eruptions (e.g., Shimbori2016), and our field site was limited to one point (approximately 3.5 km from the vent), for which is not appropriate to apply such a model. This effect needs to beconsidered for building a more sophisticated model for
tephra transport in Vulcanian eruptions by using datafrom dense observation network for the falling ash.
The contribution of ash aggregates to ashfall dynamics
becomes significant during the course of ashfall following a Vulcanian eruption. We could observe the fallout
of ash aggregates which were broken at the impact on
the ground surface at least between 11:45 and 11:46(Additional file 4: Movie S4). The measured change inthe median grain size of SP#1 can be explained by thesize-dependent segregation of ash particles from the volcanic plume. Specifically, Md was finer than the modeled

individual particle size following the first eruption (Fig.
7). This suggests that the falling velocity of ash particles
increased as a result of aggregate formation. High-speed
camera imaging showed that volume fractions of fallingash particles with density of < 1500 kg m−3 were higher
in the ashfalls of 11:33–11:34 (HSC1133) and 12:00–12:
01 (HSC1200) than that of 10:57–10:58 (HSC1057) (Fig.
5). The low-density particles (< 1500 kg m−3) are interpreted as ash aggregates, since the likely lowest densityof an individual particle at Sakurajima volcano is approximately 1500 kg m−3 (for particles with approximately 40% vesicularity) (Miwa et al. 2013). This low
density is similar to that of an experimental or naturalash aggregate with abundant pores (James et al. 2003;
Taddeucci et al. 2011), thus supporting the interpretation. Bimodality in GSDs has been interpreted as evidence of ash aggregation, which increases the terminal

Miwa et al. Progress in Earth and Planetary Science(2020) 7:3

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Fig. 8 Comparison of GSDs obtained from high-speed camera imaging (top) and laser diffraction (bottom). The open column in the GSD
indicates falling particles with inferred densities of < 1500 kg m−3. The vertical dashed lines indicates lower limit of particle diameter for the highspeed camera imaging

velocity of fine ash particles (Brazier et al. 1983; Careyand Sigurdsson 1982). Within the measured bimodal
distributions, the finer SP#2 is considered to represent
the fine-aggregate component (Bagheri et al. 2016; Brazier et al. 1983; Schumacher 1994). The fraction of theoverall GSD constituted by SP#2 increased 8 min after
the start of ashfall produced by the first eruption (Fig.

7c). The fraction of SP#2 increased particularly in 11:
32–11:37, in which over 80 vol.% of falling ash particles
with size > 3φ had a density of < 1500 kg m−3 (Fig. 8).
This suggests that the aggregate content increased 8 min
and peaked 40 min after the start of the ashfall. Bagheri
et al. (2016) also reported an increase in ash aggregates
10 min after the commencement of ashfall following a

Fig. 9 Schematic diagram of the relationship between ash particle size and its arrival timeMiwa et al. Progress in Earth and Planetary Science(2020) 7:3

Vulcanian eruption at Sakurajima volcano. Thus, wesuggest that the increase in ash aggregation of falling ash
particles may occur ~ 10 min after the start of ashfall in
proximal area during normal Vulcanian eruptions emitting volcanic clouds with 1000–2000 m of height at
Sakurajima volcano. It is noted that particles of SP#2
and SP#3 never fell as individual particles within the observation time. The time required for the individual particles of SP#2 and SP#3 with size of 6–7 phi (8–16 μm)
to reach the field site from the vent can be calculated by
dividing the difference of altitude between the vent andfield site (900 m) with the falling velocity of such particle(around 0.001 m/s) (Wilson and Huang 1979). Therefore, the time required is calculated to approximately
250 h and is clearly ruled out from the result of our observation in which total time of observation was approximately 1.5 h. Moreover, we did not observe any
pyroclastic density current which could be the source offine ash particles during the observation of the eruption(Fig. 2).
Temporal variation in the rate of falling ash particles
during the course of a Vulcanian eruption is affected byash aggregation. The ash falling rate decreased rapidly
during the first 8 min and then increased (Fig. 4). This
recovery of the ash falling rate was accompanied by aslight increase in the volume fraction and Sd of SP#2
(Figs. 4 and 7) in 11:04–11:05 (8 min after the start ofashfall), which indicated the onset of ash aggregate formation. We suggest that the observed recovery of theash falling rate is similar to the process of secondary
thickening, which has been found in tephra deposits
generated by Plinian eruptions and attributed to the aggregation of fine ash particles (Carey and Sigurdsson1982). By combining a time series of ash falling rate andcharacteristics of GSDs, we have been able to demonstrate that secondary thickening can occur during ashfall
generated by Vulcanian eruptions, with the process being triggered by the settling of ash aggregates.

Conclusions
We quantified the sedimentation process of ash particles
during a Vulcanian eruption at Sakurajima volcano,Japan, using GSD analysis and high-speed camera imaging obtained during the eruption sequence at a site
3.5 km from the vent. We studied two Vulcanian eruptions on 26 March 2016, with the first eruption occurring at 10:44 and the second at 11:26 JST. Ashfall
continued from 10:57 to 12:12 at the field site. GSDs of
ash samples obtained by high-temporal resolution (1–5min) sampling exhibited unimodal, bimodal, and trimodal distributions. The grain size of the samples ranged
from 6 μm to 3 mm. The Md of the main particle size
subpopulation (SP#1), which accounts for 70–99% of thesampled ash volume, decreased during the course of the

Page 14 of 16

ashfall after both the first and second eruptions. This is
explained by size-dependent segregation of ash particles
from a single constant-height-release source with the effect of advection by horizontal wind. High-speed camera
imaging was performed at 10:57, 11:33, and 12:00 andyielded a particle size range of 0.12–2.2 mm and a falling
velocity range of 0.2–7.0 m/s. The fraction of falling ashwith a density of < 1500 kg m−3 is interpreted as ash aggregate and decreased over time. The finer subpopulation (SP#2) is interpreted as fine material that forms ash
aggregate and showed an increase in its volume fraction
throughout the course of the ashfall. These temporalchanges in the density of airborne ash particles and the
GSD of ash samples demonstrate that the sedimentation
process during a Vulcanian eruption at Sakurajima volcano can be explained by sedimentation of ash particles
from the transient volcanic cloud, in which ash aggregation increases over time. The ash falling rate decreased
rapidly during the first 8 min and then increased. This
recovery of the ash falling rate was accompanied by anincrease in the volume fraction and Sd of SP#2 for sample collected in 11:04–11:05 (8 min after the start of ashfall). We interpret this as showing that secondary
thickening can occur during an ashfall generated by a
Vulcanian eruption, with the process being triggered by
the settling of ash aggregates, which is similar with casein larger-scale Plinian eruptions. The sedimentation
process proposed in this study can be applied to modelsof tephra sedimentation from active volcanoes with frequent short-live eruptions.

Supplementary information
Supplementary information accompanies this paper at https://doi.org/10.
1186/s40645-019-0316-8.
Additional file 1: Movie S1. High-speed image of airborne ash particles
taken in HSC1057. The real length of the movie is 0.028 seconds.
Additional file 2: Movie S2. High-speed image of airborne ash particles
taken in HSC1133. The real length of the movie is 0.028 seconds.
Additional file 3: Movie S3. High-speed image of airborne ash particles
taken in HSC1200. The real length of the movie is 0.028 seconds.
Additional file 4: Movie S4. The movie was taken in the field sitearound 11:45-11:46. Diameter of circle patterns in the plastic tray was 1.4
cm.
Additional file 5: Text S1. Density measurement.

Acknowledgements
We thank R. Kazahaya, N. Geshi, and T. Kozono for advice on the fieldobservations. We are grateful to E. Sato for the series of images of theeruption taken by the JMA time-lapse camera. We acknowledge T. Kanamaru
for help with density measurements using the helium pycnometer. We
would appreciate two anonymous reviewers and C. J. N. Wilson for theircareful readings of the manuscript.
Authors’ contributions
TM designed this study, performed all the field and laboratory experiments,
analyzed the data of the high-speed camera imaging and grain size analysis,
and drafted the manuscript. YI performed the field observations and theMiwa et al. Progress in Earth and Planetary Science(2020) 7:3

grain size analysis and contributed to the drafting of the manuscript. MN
contributed to the study design, performed the field observations, and contributed to the drafting of the manuscript. FN performed the grain size analyses and contributed to the drafting of the manuscript. All the authors haveread and approved the final manuscript.
Funding
This study was supported by MEXT/JSPS KAKENHI Grant Number
JP26800261.
Availability of data and materials
The dataset used in this study are available at https://doi.org/10.5281/
zenodo.3379289.
Competing interests
The authors declare that they have no competing interests.
Author details
1
National Research Institute for Earth Science and Disaster Resilience,
Tennodai 3-1, Tsukuba 305-0006, Japan. 2Geological Survey of Japan, AIST,
AIST No.7, Higashi 1-1-1, Tsukuba 305-8567, Japan.
Received: 8 October 2019 Accepted: 13 December 2019

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