Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

https://doi.org/10.20965/jdr.2019.p0798 Iguchi, M. et al.

Paper: Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

Masato Iguchi∗1,†, Haruhisa Nakamichi∗1, Hiroshi Tanaka∗2, Yusaku Ohta∗3, Atsushi Shimizu∗4, and Daisuke Miki∗1

∗1Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University 1722-19 Sakurajima-Yokoyama-cho, Kagoshima 891-1419, Japan †Corresponding author, E-mail: [email protected] ∗2University of Tsukuba, Ibaraki, Japan ∗3Tohoku University, Miyagi, Japan ∗4National Institute for Environmental Studies, Ibaraki, Japan [Received February 8, 2019; accepted May 9, 2019]

The Sakurajima volcano is characterized by frequent 1. Introduction vulcanian eruptions at the Minamidake or Showa crater in the summit area. We installed an integrated At the Sakurajima volcano in Japan, vulcanian erup- monitoring system for the detection of volcanic ash tions frequently occur at Minamidake crater or Showa (composed of remote sensing sensors XMP radars, li- crater 500 m east of Minamidake. This activity, which dar, and GNSS with different wave lengths) and 13 op- has occurred continuously at the Minamidake crater since tical disdrometers on the ground covering all direc- 1955, features frequent vulcanian and strombolian eruptions from the crater to measure drop size distribu- tions and the continuous emission of ash. The crater was tion and falling velocity. Campaign sampling of vol- relatively active from 1972 to 1992, with 4,919 vulcanian canic ash supports the conversion of particle counts eruptions and the emission of 240 M tons of volcanic ash. measured by the disdrometer to the weight of volcanic After 1993, this activity gradually decreased and the an- ash. Seismometers and tilt/strain sensors were used nual number of vulcanian eruptions fell to below 10 times to estimate the discharge rate of volcanic ash from per year between 2003 and 2017. Eruptive activity re- the vents. XMP radar can detect volcanic ash clouds sumed in phreato-magmatic style on June 4, 2006, at even under visual difficulty because of weather such the Showa crater, becoming magmatic in February 2008 as fog or clouds. A vulcanian eruption on Novem- with frequent associated vulcanian eruptions. The erup- ber 13 was the largest event at the Sakurajima vol- tive activity of the crater continued until October 2017. cano in 2017; however, the volcanic plume was not We counted 5,921 vulcanian eruptions at the crater dur- visible due to clouds covering the summit. Radar re- ing the period from February 2008 to October 2017, vealed that the volcanic plume reached an elevation and 44 M tons of volcanic ash was emitted. In Novem- of 4.2–6.2 km. Post-fit phase residuals (PPR) from the ber 2017, eruptive activity returned to Minamidake crater. GNSS analysis increased suddenly after the eruption, The heavy and long-term discharge of volcanic ash im- and large-PPR paths from the satellites to the ground- pacted the area around the Sakurajima volcano in vari- based receivers intersected each other at an elevation ous ways. The most severe impact was a reduction in the of 4.2 km. The height of the volcanic plume was also output from agriculture. The government of Kagoshima estimated from the discharge rate of volcanic ash to prefecture estimate the amount of agricultural damage be 4.5 km, which is empirically related to seismic en- due to the Sakurajima eruption at 236 billion yen over ergy and the deflation volume obtained via ground de- the 39 years since 1978, when statistics for the damage formation monitoring. Using the PUFF model, the began to be calculated. The second impact concerns traf- weight of the ash-fall deposit was accurately forecast fic, including airlines, roads, and railways. The wind- in the main direction of transport of the volcanic ash, shields of airplanes were occasionally damaged by vol- which was verified by disdrometers. For further ad- canic lapilli and ash ejected by eruptions that occurred at vances in forecasting of the ash-fall deposit, we must the Minamidake crater in the 1970s [1]. Since then the consider high-resolution wind field, shape of volcanic airspace around Sakurajima was set as a no-fly area, and plume as the initial value, and the particle number dis- the occurrence of this type of damage ceased. Awareness tribution along the volcanic plume. of the air problems that are often caused by volcanic ash has increased since an eruption at Galunggung in 1982, followed by eruptions at Redoubt in 1989, Pinatubo in Keywords: volcanic ash, XMP radar, lidar, vulcanian 1991, and Eyjafjallajökull in 2010. Because of the ef- eruption, Sakurajima volcano fects these events have had on air travel, the possibility

798 Journal of Disaster Research Vol.14 No.5, 2019

© Fuji Technology Press Ltd. Creative Commons CC BY-ND: This is an Open Access article distributed under the terms of the Creative Commons Attribution-NoDerivatives 4.0 International License (http://creativecommons.org/licenses/by-nd/4.0/). Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

of cancelling flights has increased. Roads have on occa- craters cannot be seen visually, and are compared with sion been closed due to heavy ash-fall from Sakurajima, plume height which is estimated from the discharge rate for example, during eruptions on August 24, 1995 and on based on seismic and ground deformation observations. July 24, 2012. Large amounts of ash-fall have also been The actual height of the plume is then confirmed by com- known to cause train derailments and create obstructions parison with the aerial amount of ash produced by simu- to train operations. The third issue is the impact on the lation. health of residents living around Sakurajima. After ana- lyzing the fine volcanic ash particles, Hillman et al. [2] concluded that the potential health hazard of the ash is 2. Remote Sensing for the Detection of Volcanic low, but high exposure and respiratory conditions should Ash still be monitored given the high frequency and duration of such exposure. For this study, we installed an integrated observation Considering this situation, we propose a study for the system to detect volcanic plume at the Sakurajima vol- “Development of a real-time volcanic ash hazard assess- cano. The system is composed of lidar, XMP radar, and ment method” for the Integrated Program for Next Gen- a set of GNSS. These instruments radiate electromagnetic eration Volcano Research and Human Resource Develop- waves with different wavelengths, lidar emits light with ment under the Ministry of Education, Culture, Sports, a wavelength of 532 nm, and XMP radar and GNSS ra- Science and Technology (MEXT). This proposed study is diate waves in the X-band (wavelength: 2.5–3.7 cm) and composed of 1) the detection of volcanic ash using inte- the L-band (wavelength: 20–60 cm), respectively. By us- grated remote sensing immediately after the onset of erup- ing multiple wave-lengths, the integrated system can be tions, 2) a reduction in the time taken to forecast ash-fall, applied at various scales of eruption, which produce dif- based on seismic and ground deformation data, 3) precise ferent spatial concentrations of volcanic ash particles in forecasts of ash-fall deposits based on spatially high reso- the atmosphere. lution wind fields, 4) the technical development of an on- line system for forecasting, and 5) stochastic forecasting of ashfall by statistical processing of precursory ground 2.1. Lidar deformation prior to volcanic eruptions. We studied top- Lidar is usually used for the observation of aerosols in ics 1–3 for the first 4 years under the 10-year project. To the atmosphere [11]. Lidar can detect volcanic ash parti- reduce the time taken before forecasts of ash-fall, we used cles at a distance. For example, volcanic ash particles that the PUFF model [3], considering the emission rate of vol- erupted from Augustine volcano were detected by a lidar canic ash [4]. Grid point values (GPV) for wind direction system at Fairbanks, 700 km away from the volcano [12]. and velocity forecast by the Japan Meteorological Agency For our observation of the Sakurajima volcano, we use a (JMA) were used in the model. At the Sakurajima vol- polarization and dual-wavelength lidar manufactured by cano, the emission rate of volcanic ash is well formulated FIT Leadintex and set the radiative direction of light from by using a linear combination of the seismic amplitudes of the lidar to the area immediately above the crater to detect explosion earthquakes and volcanic tremors and the vol- ash particles independently from wind direction. Lidars ume of deflation due to ground deformation source [5]. were located on the west and east flanks of the volcano For topic 3, high-resolution data for wind fields are re- (Fig. 1). The lidar radiates linearly polarized light pulses quired for a precise forecast of volcanic ash volume, be- at wavelengths of 532 and 1064 nm. Both the parallel and cause the wind field near volcanoes is affected by topogra- perpendicular components of backscatter light at 532 nm phy, and downward flow is dominant leeward of the sum- and the total backscatter at 1064 nm are detected. The mit [6]. The Weather Research and Forecasting (WRF) light pulse is radiated 40 times in the first 2 seconds. Af- model [7] is used to downscale the JMA forecast data and ter 8 s, 40 pulses are radiated again. The sequence is re- the high-resolution meteorological data are then used for peated with a time interval of 10 s. the FALL3D model [8] to forecast the ashfall deposits [9]. The emission rate of volcanic ash can be estimated by the observation of seismic and ground deformation [5]; 2.2. XMP Radar however, the growth of a volcanic plume is affected by XMP radars are usually utilized to understand the wind [10] and other factors. Therefore, it is necessary distribution of rain drops for meteorological purposes. to understand the formation of volcanic plumes for more We installed six sets of XMP radars (WR2100, Fu- precise forecasting of ash-fall deposits. In this study, runo) for the detection of volcanic ash clouds from firstly we propose an integrated detection system for vol- the Kirishima Volcano-Complex, Sakurajima, Satsuma- canic ash using various different types of remote sensing; Iwojima, Kuchinoerabujima, and Suwanosejima in 2017. X-band multi-parameter (XMP) radar, light detection and All of these volcanoes have erupted during this cen- ranging (lidar), the Global Navigation Satellite System tury: the Shinmoedake volcano in the Kirishima volcano- (GNSS), and ground-based measurements of the ashfall complex erupted in 2008, 2010, 2011 (VEI 3), and 2017- using disdrometers. We then try to determine how useful 2018; Satsuma-Iwojima erupted on June 5, 2013; Kuchi- the system is. Secondly, the height of volcanic plumes noerabujima has been active since August 2014 with are estimated by remote sensing at altitudes above which the first eruption on August 3, 2014 and the second on

Journal of Disaster Research Vol.14 No.5, 2019 799 Iguchi, M. et al.

R ( N high above the crater to the west, whereas a decrease in the SNR is detected in the path passing just above the crater [16]. The anomalous PPR increase and SNR de- )87 crease might reﬂect the different characteristics of wa- .20 ter vapor and volcanic ash. Therefore, at the Sakurajima Sakurajima volcano, a dense network of 25 continuously-recording /LGDU .927 ;03 UDGDU GNSS stations within 7 km from the crater (Fig. 3)and +927 a wider network around Aira caldera region [17] were in- K 692 /LGDU stalled to understand the temporal and spatial distribution

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$927 3. Monitoring of Ash-Fall Deposit $5, 3.1. Ash-Fall Monitoring Within a 50 km Range 3km Manual sampling of ash-fall deposits has been car- ried out by the Kagoshima Prefectural Government since Fig. 1. Location of the stations used in this study. Dots in- 1978, and the mass of ash deposits measured at monthly dicate locations of disdrometers. Squares are underground increments from 62 sampling points are therefore avail- tunnels where water-tube tiltmeters and extensometers are able. From the weight of the volcanic ash samples, installed. The AVOT site includes seismometers and infra- the amount of volcanic ash discharged monthly is esti- sonic microphones. Lidars are installed at stations SVO and mated [5], ranging from 0.01 M tons to 5.80 M tons over KUR. XMP radar is added to SVO. The dashed line indi- the 38 years (Fig. 4). Eruptions frequently occurred until cates sector RHI scan direction of the radar. Symbols “M,” 1992. The monthly discharge weight exceeded 5 M tons “K,” and “S” represent Minamidake, Kitadake, and Showa in July and August 1985 and November 1991. Eruptive craters. activity decreased in 1993. The monthly discharge weight of volcanic ash did not exceed 0.1 M tons during the period from 2001 to 2008. However, the discharge weight May 29, 2015 when the volcanic ash cloud reached an el- increased in 2009 because vulcanian eruptions were fre- evation of 10 km above the summit crater (VEI 2). The quent at Showa crater, at which eruptive activity resumed third eruptive sequence began on October 21, 2018 and in 2006. The monthly discharge weight remained at a ended on February 2, 2019. The eruption of Suwanose- level of 1 M tons until June 2015, reflecting an increase in jima is frequent. The locations of the radars are shown in the number of eruptions. Fig. 2. At Sakurajima, two XMP radars (WR2100, Fu- However, this measurement provides the total monthly runo) are installed at distances of 18 km and 5.6 km from weight of ash-fall deposits. In order to obtain the weight Minamidake crater (Figs. 2a and b). Details of the speci- of ash deposits from an individual vulcanian eruption, an fication of the XMP radar is described in [13]. The radar automatic instrument for ash-fall with a higher measure- is a dual polarimetry type in the vertical and horizontal, ment rate is required. For the monitoring of ash-fall de- from which waves with a frequency of 9470 MHz are si- posits at a higher rate, the Ministry of Land, Infrastruc- multaneously transmitted and received within a maximum ture and Transport (MLIT) has installed 22 automated range of 30 km. The radar outputs multi-parameter re- tephrometers [18] on the flanks of the Sakurajima vol- sults. For this study only reflectivity was used. cano, from which the weight of volcanic ash-fall deposits every 10 minutes can be provided. However, the precision of this instrument is only ±1 mm and it is therefore in- 2.3. GNSS sufficiently sensitive for obtaining the amount of ash-fall GNSS, which is normally used for positioning, can be deposit from individual eruptions. applied to the detection of volcanic plumes using two methods. One is using post-fit phase residuals (PPR) of GNSS signals between ground stations and GNSS satel- 3.2. Disdrometer lites after the application of basic GNSS data processing A disdrometer is generally used for the observation of for daily or sub-daily coordinate estimation [14]. The the size and falling velocity of meteorological particles. other uses the signal-to-noise ratio (SNR) of the GNSS For this study we use the disdrometer Parsivel2 (OTT Hy- data, which can be recorded by each GNSS receiver with- droMet GmbH). The disdrometer measures the particle out any data processing [15]. This method was applied size and falling velocity using a laser sensor that produces to eruptions at the Okmok and Redoubt volcanoes, where a horizontal strip of light with a wavelength of 780 nm. decreases in the SNR were associated with the eruptions. Particles passing through the laser beam block off a por- Both the PPR and SNR methods are applied to an eruption of the laser beam corresponding to their diameter, tion at Sakurajima on July 24, 2012, and an anomalous in- and the output voltage from the sensor is therefore re- crease of PPR paths are detected at approximately 3000 m duced by an amount that depends on the particle size.

800 Journal of Disaster Research Vol.14 No.5, 2019 Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

Fig. 2. Locations of XMP radars. Triangles show locations of the active craters. a: The South Kyushu area including the Sakurajima and Kirishima Volcano Complex. b: Sakurajima. The Minamidake crater of Sakurajima is covered by two radars. c: Satsuma-Iwojima. XMP radar is installed at Takeshima, 10 km east of the summit of Satsuma-Iwojima. d: Kuchinoerabujima and e: Suwanosejima volcanoes.

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Journal of Disaster Research Vol.14 No.5, 2019 801 Iguchi, M. et al.

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Fig. 5. Temporary changes in seismic and infrasonic activity, and ground deformation associated with the eruptions The falling velocity is measured from the duration of the on November 13–14, 2017. Upper: seismic amplitude at the reduction in output voltage due to shading by the parti- AVOT station. Middle: infrasonic amplitude at the same sta- cles. The disdrometer detects particles with sizes ranging tion. Lower: strain changes in radial (EX-R) and tangential from 0.2 mm to 25 mm and velocity ranging from 0.2 m/s (EX-T) components at HVOT station. The radial compo- to 20 m/s, and it counts particle numbers for 32 classes nent is directed towards Minamidake crater and the tangen- of size and 32 classes of velocity. This is sufficiently tial components are perpendicular to the radial. sensitive to detect volcanic ash particles ejected on the scale of a vulcanian eruption at the Sakurajima volcano. Kozono et al. [19] were the first to apply the disdrome- in tilt and strain, assuming a small spherical source [20]. ter to the measurement of ash-fall at the south flank of This eruption is the largest among the eruptive events at the Sakurajima volcano and obtain an empirical diameter- the Sakurajima in 2017. velocity relationship and particle size distribution for ash- A time series of seismic amplitude, infrasonic ampli- fall. 2 tude, and strain changes at AVOT are illustrated in Fig. 5. An array of 13 Parsivel distrometers was newly estab- A sudden increase in seismic and infrasonic amplitudes lished at the Sakurajima volcano, covering all directions is caused by an explosion earthquake and the air-shock from the Minamidake and Showa craters at the summit. generated by the vulcanian eruption. The seismic and in- The location of the disdrometers is shown in Fig. 1. frasonic amplitudes reached 120 µm/s and 68 Pa, respectively, and decreased gradually after the impulsive peaks; however, the amplitudes increased again from approxi- 4. Integrated Detection and Forecasting Vol- mately 22:30 and reached peaks at 23:10–23:20. Seismic canic Ash and infrasonic waveforms around the peaks of the activity are shown in Fig. 6. Volcanic tremors are recorded with 4.1. Chronology of the Eruption on November 13, the seismograms, and infrasonic records show repeating 2017 shocks with amplitudes smaller than 15 Pa and time in- tervals of shorter than 10 s. The repeat of the abrupt A vulcanian eruption occurred at Minamidake crater shock in the infrasonic records is similar to that in the in- at 22:07 on November 13, 2017. Prior to the erup- frasonic records associated with strombolian eruptions in tion, inflation at the surface of the volcano began at ap- 1988 [21]. It is therefore inferred that strombolian erup- proximately 20:00 on November 7, as detected by tilt- tions also occurred in this case. The seismicity with an meters and extensometers in the underground tunnels of amplitude > 5 µm/s, infrasonic activity > 1 Pa, and con- the AVOT, HVOT, and KVOT stations (Fig. 1), and ceased traction strain continued until 03:00 on November 14. It is on November 11. After the onset of the eruption, the in- inferred that the eruptive activity stopped at around 03:00. flation stopped, and the area starting deflating. The con- traction strain of the ground associated with the eruption amounted to 0.25 × 10−6,0.17 × 10−6 on the radial and 4.2. Integrated Detection of Volcanic Ash tangential components of the extensometers at HVOT, re- Ash-fall related with the eruption was monitored by spectively (Fig. 5). The deflation volume from the pres- disdrometers at KOM and FUT stations (Fig. 1)asshown sure source is estimated at 2 × 105 m3 from the changes in Fig. 7. The disdrometers automatically output the in-

802 Journal of Disaster Research Vol.14 No.5, 2019 Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

PPV 6HLVPRJUDP $927 KOM and FUT are caused by distance from the crater: KOM station is at a distance of 4.8 km and FUT station is 5.2 km away. The ash-fall corresponds to the vulcanian eruption starting at 22:07. ash-fall continued for a longer time at FUT station. The PIP increased again at 22:43 and reached a second peak (7.7 mm/h) at 22:54. The ash-fall V corresponds with the strombolian eruption that started at 3D ,QIUDVRQLF $927 approximately 22:30. However, no ash-fall was detected at KOM station that could be related to the strombolian eruption. The KOM and FUT stations are located to the northeast and north of the crater, respectively. Considering the wind proﬁle (Fig. 8), KOM and FUT are located leeward V of the winds at elevations of 1700 m and 1200 m, respectively. The volcanic plume from the vulcanian eruption Fig. 6. Seismograms and infrasonic records associated with that started at 22:07 exceeded an elevation of 1700 m the strombolian eruptions on November 13, 2017. The index for 10 minutes. The following volcanic plume from the on the left represents time. strombolian eruption starting at approximately 22:30 did not exceed an elevation of 1200 m. Fig. 9 shows the

particle-size distribution of the falling ash obtained by the Vulcanian Parsivel2 at these stations. In the ﬁrst ash-fall (22:19–

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KOM station, whereas smaller-size particles are dominant .20 at FUT station. The second ash-fall (22:43–23:59) from 3,3 .85 the strombolian eruption was detected only at FUT station, and is dominated by finer particles (0.5 mm). Unfortunately, the top height of the volcanic plume was 3,3.85 PPK 3,3.20 PPK not reported by the JMA because the volcanic plume en- )87 tered into clouds that remained at an elevation of 300 m 5DLQIDOO above the crater. However, XMP radar can detect a volcanic plume even if it is invisible by other methods. 3,3 $5, Fig. 10 shows the intensity of the reflectivity obtained 3,3$5, PPK 3,3)87 PPK from a sector Range Height Indicator (RHI) scan of a cross-section from SVO station in the direction of the Mi- K ◦ 1RY1RY namidake crater (95 from north, Fig. 1). High reflectivity is detected at an elevation of 3.5 km asl, approxi- Fig. 7. Time series of the pseudo intensity of precipitation mately 2 km from the crater (Fig. 10a). This corresponds (PIP) measured by disdrometers at KOM (middle panel) and to the eastward transport of volcanic plume material that FUT (lower panel) stations. Temporary changes in seismic was ejected by the vulcanian eruption at 22:07. How- amplitude and strain are plotted on the upper panel. The ever, no clear changes in reflectivity were detected dur- grey bars indicate PIP at disdrometer stations KUR and ARI. ing the strombolian eruption with a plume height of lower Since the PIP at the four stations increased simultaneously Fig. 10b after 23:30, it is inferred that the PIP after 23:30 was affected than 1200 m asl after 22:30 ( ). A plume at such by rain fall. low heights was not detected by the XMP radar. A reflectivity image by a horizontal sequence scan is obtained from the XMP radar at the KSH station (Fig. 2a). As shown in Fig. 11, an echo of the volcanic plume is detected at the northeast part of Sakurajima from hori- tensity of precipitation in mm/h. In this study this value is ◦ applied to ash-fall and is therefore named pseudo intensity zontal scans with elevations lower than 10 (4.2 km asl at the location); however, no echo was detected at an el- of precipitation (PIP). Falling ash particles were detected ◦ at KOM station from 22:19, 12 minutes after the onset of evation of 15 (6.2 km asl). This indicates that the top the eruption. The PIP reached a peak value of 33 mm/h of volcanic plume reached an elevation between 4.2 km at 22:25. At FUR station, falling ash particles were de- and 6.2 km asl. During the strombolian eruptions, no echo tected from 22:22, 15 minutes after the onset of the erup- was detected at such high elevations. tion and the PIP reached a peak of 12 mm/h at 22:30. Ash- Temporal and spatial changes to postfit phase residu- fall stopped at 22:29 at KOM station and at 22:35 at FUT als (PPR) in GNSS data [16]. The GNSS data processing station. The time difference of 3–6 minutes for the on- package GIPSY/OASIS II version 6.4 was used to process set, peak, and termination of ash-fall between the stations the GNSS data. This study used only Global Positioning

Journal of Disaster Research Vol.14 No.5, 2019 803 Iguchi, M. et al.

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Fig. 8. Wind proﬁle measured at the Kagoshima Local Mete- Fig. 9. Particle size distribution of volcanic ash at KOM and orological Observatory of the JMA at 21:00 on November 13, FUT stations. The plots from FUT station are divided into two 2017. The observatory is located 10.7 km southwest of Mi- time-windows, corresponding to the vulcanian (22:19–22:42) namidake crater. Squares and open circles indicate the velocity and strombolian (22:43–23:35) eruptions. No particles are de- and direction of wind, respectively. Downward arrows indi- tected at KOM station in the second time window. cate the direction of Minamidake crater from stations FUT and KOM.

Fig. 10. Reflectivity intensity of the cross-section from the XMP radar site SVO to Minamidake crater. a: Reflectivity images after the onset of the vulcanian eruption (middle and lower). The top figure is of the reflectivity image before the eruption. b: Reflectivity images during the strombolian eruption. These are almost identical to the top image of Fig. 10a. No clear change in the images was detected.

System (GPS) satellites. Static precise point positioning were relatively small compared to the noise level. For (PPP) was applied as the processing strategy. Finally, we the subtraction, we take the sidereal time shift into con- calculated the PPR of the ionosphere-free linear combina- sideration (approx. 4 min.). Figs. 12a and b show the tion (LC) between each station and the GNSS satellites. time series of the PPR from the GPS SVN (Space Vehi- To eliminate the multipath noise, we removed any data cle Number) 41 satellite (hereafter, we call as GPS 41) from the day before the PPR time series from the day of to SNYM station, and from GPS 52 satellite to KURG the volcanic event because the obtained PPR anomalies station. The PPR along the ray path from GPS 41 to

804 Journal of Disaster Research Vol.14 No.5, 2019 Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

4.3. Forecasting Ash-Fall Deposits

It is possible to estimate the discharge rate of volcanic ash using co-eruptive seismic amplitude and ground deformation [5], formulated using a linear combination of seismic amplitudes in a specific frequency band (2–3 Hz) and the volume change of a pressure source. Tanaka and Iguchi [4] established an empirical formula to obtain the height of a volcanic plume from the discharge rate. By using these, it is estimated that the volcanic plume reached an elevation of 4.5 km asl. This value matched the plume height obtained by radar observation and the PPR of GNSS. Based on the PUFF model [3], Tanaka and Iguchi [4] simulated the transport and ash-fall deposit using the discharge rate of volcanic ash and the plume height of the vulcanian eruption on November 13, 2017 (Fig. 13). Wind field for this simulation is based on the GPV of wind field as forecast by the JMA. An empirical relationship between the actual deposit (W)ing/m2 with the pseudo intensity of precipitation (PIP, Ip) from the disdrometer is obtained using:

W = 24Ip. Therefore, the amount of ash-fall deposit throughout the 2 Fig. 11. Reflectivity intensity by horizontal sequence scan at eruptive event is estimated at 2.4 kg/m from the summa- 10◦ (top) and 15◦ (bottom) elevation angles. Dashed circles tion of the PIP of the disdrometer at KOM station. As the 2 indicate the area of ash-fall. The dots represents the location aerial amount forecast by the PUFF model is 3.2 kg/m of the XMP radar at KHS. The triangle is Minamidake crater. at the KOM station, the amount forecast is only overes- timated by 33%. The aerial amount of ash-fall deposit forecast coincides well with that observed in the main direction of the movement of the ash. On the other hand, the aerial amount forecasted does not coincide with the SNYM station increased 5 minutes (22:12) after the on- actual measurement at FUT station. The aerial amount set of the vulcanian eruption at 22:07 and reached a peak forecasted is only 0.03 kg/m2, but the actual density of of 3 cm. The PPR along the ray path from GPS 52 to the ash-fall deposit amounted to 2.6 kg/m2.Conversely, KURG station reached a maximum 8 minutes (22:15) af- the forecasted value of 1.0 kg/m2 is significantly greater ter the eruption. The shape of the time series of the paths than the observed value at KUR station where no ash-fall (SNYM-GPS41 and KURG-GPS52) were similar to each was detected by the disdrometer. other. This suggests that the both of the PPR anomalies may have captured the same phenomenon produced by the eruption. The PPR from GPS 52 (Fig. 12c) and 71 5. Discussion (Fig. 12d) to KSHL station increased similarly 6 minutes and 3 minutes after the onset of the eruption and the In this study, we forecast the aerial amount of ash-fall PPR anomalies reached 2 cm. The anomalous PPRs were deposits using the PUFF model, and the weight of the only detected by GNSS stations at the northeast and east- forecast ash-fall deposit coincides with the actual weight ern parts of Sakurajima (Fig. 3) and are caused by the of the deposits in the main direction of the volcanic ash transport of the volcanic plume in a northeasterly direc- movement. The biggest advantage of using the PUFF tion. The ray paths with the anomalous PPR increases model is the immediate prediction after an eruption. It is intersect each other, and the highest intersection is found necessary to use the height of plume as an initial value in at an elevation near 4.2 km asl (between SNYM-GPS41 this model, but prediction of the aerial amount of ash-fall and KURG-GPS52). As we mentioned previously, the can be made faster by calculating the emission rate and the shapes of the PPR time series were similar. Thus, the height of plume using the seismic amplitude and ground highest intersection point should reflect the same phe- deformation, which can be observed simultaneously with nomenon caused by the plume. Based on the echo de- the occurrence of an eruption. tection by the XMP radars and the PPR by GNSS, it is There are some problems to be solved in order to im- inferred that the volcanic plume reached a higher eleva- prove the forecasting accuracy. The first is the require- tion than 4 km asl. Anomalous PPR was also detected in ment for higher accuracy of wind data near the volcano. the peak of the strombolian type eruption at around 23:00. In the present model, since the GPV data from the JMA’s

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Fig. 12. Time series of the post-ﬁt phase residual (PPR) anomalies during the period from 0.2 hour before to 0.8 hour after the onset of the vulcanian eruption at 22:07 on November 13, 2017. Black and gray lines denote the PPR time series from the day of the eruption and the day before the eruption, respectively. To accurately compare each time series, the time series from the day before the eruption (gray time series) were shifted by four minutes to consider sidereal time. The blue time series denotes the subtraction result between the two time series. a: Time series of PPR anomalies from SNYM station, relative to the GPS SVN 41 satellite. b: KURG station relative to the GPS SVN 52 satellite. c, d: KSHL station relative to the GPS SVN 52 and GPS SVN 71 satellite. See Fig. 3 for the locations of the stations.

numerical prediction model is used, the horizontal res- maximum height. Tanaka et al. [22] calculated the trans- olution is insufficient. As for altitude, at 500 hPa or port of the volcanic plume ejected by the 2014 eruption more, which is assumed at a normal elevation of volcanic from the Kelud volcanoes in East Java, Indonesia, assum- plume ejected by vulcanian eruptions of Sakurajima, there ing that the width of a volcanic plume increases with ele- is only coarse resolution corresponding to the mandatory vation above the crater. As shown in this study, even if the pressure levels. There is a significant difference between volcanic plumes are hidden by clouds, we can still ascer- the observed and forecast values at FUT station. The di- tain the height and shape of the plume. It is expected that rection of Minamidake crater from FUT station is 180◦ better results can be obtained for the accurate forecasting from the north, and the elevation corresponding to the of ash-fall deposits by using the spatial distribution of the wind direction at the station is 1000 m to 1500 m asl. volcanic plume obtained by radar as the initial value in the (900 hPa to 850 hPa). This low altitude information is simulation. not sufficiently reflected in the wind model. It is there- The third is the particle number distribution along the fore necessary to generate high definition wind data using elevation of the eruption column. In the simulation for WRF-Chem [7]. Poulidis et al. [6] showed that downward the eruption on November 13, 2017, the amount of ash- winds predominate on the leeward side beyond the sum- fall forecast for the eastern side of Minamidake is exces- mit. In addition, Poulidis et al. [9] obtained good results sive. For example, at KUR station, the amount forecasted by using wind data with a resolution of 90 m for forecast- is 1.0 kg/m2, but in reality no volcanic ash-falls were de- ing ash-fall deposits from an eruption on June 16, 2018. tected. Westerly winds are necessary for the volcanic ash The second is the shape of the volcanic plume and its from Minamidake crater to reach KUR station. Since the

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Fig. 13. The forecast ash-fall deposits using the PUFF model for the eruption on November 13, 2017. a: time series of the emission rate estimated from linear combination of seismic amplitude and volume change of the pressure source. The value is converted to 5 min interval (ton/5 min). b: the corresponding plume height (m) based on the empirical relationship between emission rate and plume height. The emission rate and plume height are used as the input for the PUFF model. The simulation started at 22:00 JST (13:00 UTC) and ended at 04:00 JST (19:00 UTC). c: the estimated amount of ash-fall deposit (g/m2) in log-scale. The contours are calculated using 100 m grid meshes. Figs. 13a and b refer to Fig. 8 of [4], and Fig. 13c to Fig. 11b of [4].

westerly wind is measured at 700 hPa or less (3106 m asl Acknowledgements or higher) as shown in Fig. 8, it can be pointed out that The study is conducted under the Integrated Program for Next the initial value for particle numbers at high elevations Generation Volcano Research and Human Resource Development was excessive. supported by MEXT. Osumi Office of River and National High- way, Kyushu Regional Development Bureau, MLIT provides the seismic and geodetic data at the station AVOT. 6. Conclusion References: An integrated monitoring system for volcanic ash is in- [1] S. Onodera, M. Iguchi, and K. Ishihara, “Prevention and mitigation stalled at Sakurajima. The volcanic plume is remotely de- of aircraft accidents caused by volcanic eruption,” Annuals Disas- ter Prevention Research Institute, No.40 B-1, pp. 73-81, 1997 (in tected by LIDAR, XMP radar, and GNSS and ash-fall par- Japanese with English abstract). ticles are counted automatically by 13 disdrometers for an [2] S. E. Hillman, C. J. Horwell, A. L. Densmore, D. E. Damby, B. Fu- in-situ observation of the ash-fall. The vulcanian eruption bini, Y. Ishimine, and M. Tomatis, “Sakurajima volcano: a physico- chemical study of the health consequences of long-term exposure on November 13 is the largest event in 2017 at the vol- to volcanic ash,” Bull. of Volcanol., Vol.74, Issue 4, pp. 913-930, cano. The XMP radar and GNSS provide information on doi:10.1007/s00445-012-0575-3, 2012. the volcanic plume hidden by clouds. The weight of the [3] H. L. Tanaka and K. Yamamoto, “Numerical simulation of volcanic plume dispersal from Usu volcano in Japan on 31 March 2000 using ash-fall deposit is accurately forecast in the main direction PUFF model,” Earth, Planets and Space, Vol.54, Issue 7, pp. 743- of volcanic ash transport by the PUFF model, considering 752, 2002. the discharge rate of volcanic ash and the height of the [4] H. L. Tanaka and M. Iguchi, “Numerical simulations of volcanic ash plume dispersal for Sakura-Jima using real-time emission rate esti- plume, which are estimated empirically from seismic en- mation,” J. Disaster Res., Vol.14, No.1, pp. 160-172, doi:10.20965/ ergy, and the deflation volume from ground deformation jdr.2019.p0160, 2019. [5] M. Iguchi, “Method for real-time evaluation of discharge rate of vol- monitoring. For further advances in forecasting ash-fall canic ash – Case study on intermittent eruptions at the Sakurajima deposits, it is necessary to consider more accurate mea- volcano, Japan –,” J. Disaster Res., Vol.11, No.1, pp. 4-14, 2016. surement of wind fields, the shape of a volcanic plume, as [6] A. P. Poulidis, T. Takemi, M. Iguchi, and I. A. Renfrew, “Oro- graphic effects on the transport and deposition of volcanic ash: well as the initial value and particle number distribution A case study of Mount Sakurajima, Japan,” J. Geophys. Res.: along the volcanic plume. Atmospheres, Vol.122, Issue 17, pp. 9332-9350, doi:10.1002/ 2017JD026595, 2017. [7] W. C. Skamarock, J. B. Klemp, J. Dudhia, D. O. Gill, D. M. Barker, M. G. Duda, X.-Y. Huang, W. Wang, and J. G. Powers, “A Descrip-

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tion of the Advanced Research WRF Version 3,” NCAR Technical Note, NCAR/TN-4751+STR, 2008. Name: [8] A. Folch, A. Costa, and G. Macedonio, “FALL3D: A computational Masato Iguchi model for transport and deposition of volcanic ash,” Computers & Geosciences, Vol.35, Issue 6, pp. 1334-1342, doi:10.1016/j.cageo. 2008.08.008, 2009. Affiliation: [9] A. P. Poulidis, T. Takemi, and M. Iguchi, “Experimental high- Disaster Prevention Research Institute (DPRI), resolution forecasting of volcanic ash hazard at Sakurajima, Japan,” Kyoto University J. Disaster Res., Vol.14, No.5, 2019. [10] C. Bonadonna, M. Pistolesi, R. Cioni, W. Degruyter, M. Elissondo, and V. Baumann, “Dynamics of wind-affected volcanic plumes: The example of the 2011 Cordón Caulle eruption, Chile,” J. Geo- phys. Res.: Solid Earth, Vol.120, Issue 4, pp. 2242-2261, doi: 10.1002/2014JB011478, 2015. Address: [11] A. Shimizu, N. Sugimoto, I. Matsui, K. Arao, I. Uno, T. Murayama, 1722-19 Sakurajima-Yokoyama-cho, Kagoshima 891-1419, Japan N. Kagawa, K. Aoki, A. Uchiyama, and A. Yamazaki, “Continuous Brief Career: observations of Asian dust and other aerosols by polarization lidars in China and Japan during ACE-Asia,” J. Geophysical Research, 1981- Research Associate, DPRI Vol.109, Issue D19, D19S17, doi:10.1029/2002JD003253, 2004. 1995- Associate Professor, DPRI [12] K. Sassen, J. Zhu, P. Webley, K. Dean, and P. Cobb, “Volcanic ash 2012- Professor, DPRI plume identification using polarization lidar: Augustine eruption, Selected Publications: Alaska,” Geophys. Res. Let., Vol.34, Issue 8, L08803, doi:10.1029/ • M. Iguchi et al., “Contribution of monitoring data to decision making for 2006GL027237, 2007. evacuation from the 2014 and 2015 eruptions of Kuchinoerabujima [13] S. Oishi, M. Iida, M. Muranishi, M. Ogawa, R. I. Hapsari, and M. Volcano,” J. of Natural Disaster Science, Vol.38, No.1, pp. 31-47, 2017. Iguchi, “Mechanism of volcanic tephra falling detected by X-band • M. Iguchi, “Volcanic activity of Sakurajima monitored using Global multi-parameter radar,” J. Disaster Res., Vol.11, No.1, pp. 43-52, Navigation Satellite System,” J. Disaster Res., Vol.13, No.3, pp. 518-525, 2016. 2018. [14] R. Grapenthin, J. T. Freymueller, and A. M. Kaufman, “Geode- • M. Iguchi et al., “Forecast of the pyroclastic volume by precursory tic observations during the 2009 eruption of Redoubt Volcano, Alaska,” J. Volcanol. Geotherm. Research, Vol.259, pp. 115-132, seismicity of Merapi volcano,” J. Disaster Res., Vol.14, No.1, pp. 51-60, doi:10.1016/j.jvolgeores.2012.04.021, 2013. 2019. [15] K. M. Larson, “A new way to detect volcanic plumes,” Geophys. Academic Societies & Scientific Organizations: Res. Let., Vol.40, Issue 11, pp. 2657-2660, doi:10.1002/grl.50556, • Volcanological Society of Japan (VSJ) 2013. • American Geophysical Union (AGU) [16] Y. Ohta and M. Iguchi, “Advective diffusion of volcanic plume cap- • International Association of Volcanology and Chemistry of the Earth’s tured by dense GNSS network around Sakurajima volcano: a case Interior (IAVCEI) study of the vulcanian eruption on July 24, 2012,” Earth, Planets and Space, Vol.67, Article No.157, doi:10.1186/s40623-015-0324-x, 2015. [17] M. Iguchi, “Volcanic activity of Sakurajima monitored using Name: Global Navigation Satellite System,” J. Disaster Res., Vol.13, No.3, Haruhisa Nakamichi pp. 518-525, doi:10.20965/jdr.2018.p0518, 2018. [18] Y. Tajima, D. Ohara, K. Fukuda, and S. Shimomura, “Development of automatic tephrometer for monitoring of volcano,” Nippon Koei Affiliation: Technical Forum, No.23, pp. 39-46, 2015 (in Japanese with English Associate Professor, Disaster Prevention Re- abstract). search Institute (DPRI), Kyoto University [19] T. Kozono, T. Miwa, M. Maki, T. Maesaka, D. Miki, and M. Iguchi, “PARSIVEL Tephra-fall observations at Sakurajima volcano,” An- nuals Disaster Prevention Research Institute, No.58 B, pp. 86-90, 2015 (in Japanese with English abstract). [20] K. Ishihara, “Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakura- Address: jima volcano, Japan,” M. P. Ryan (Ed.), “Magma Transport and 1722-19 Sakurajima-Yokoyama-cho, Kagoshima 891-1419, Japan Storage,” John Wiley & Sons, pp. 335-356, 1990. Brief Career: [21] M. Iguchi and K. Ishihara, “Comparison of earthquakes and air- 2003-2005 Research Fellow of the Japan Society for the Promotion of shocks accompanied with explosive eruptions at Sakurajima and Suwanosejima volcanoes,” Annuals Disaster Prevention Research Science (JSPS), National Research Institute for Earth Science and Disaster Institute, No.33 B-1, pp. 1-12, 1990 (in Japanese with English ab- Prevention stract). 2004-2005 Visiting Research Fellow, United States Geological Survey [22] H. L. Tanaka, M. Iguchi, and S. Nakata, “Numerical simulations of 2005-2013 Assistant Professor, Graduate School of Environmental volcanic ash plume dispersal from Kelud volcano in Indonesia on Studies, Nagoya University February 13, 2014,” J. Disaster Res., Vol.11, No.1, pp. 31-42, 2016. 2013- Associate Professor, DPRI Selected Publications: • H. Nakamichi et al., “Quantification of seismic and acoustic waves to characterize the 2014 and 2015 eruptions of Kuchinoerabujima Volcano, Japan,” J. of Natural Disaster Science, Vol.38, No.1, pp. 65-83, 2017. • H. Nakamichi et al., “Differences of precursory seismic energy release for the 2007 effusive dome-forming and 2014 Plinian eruptions at Kelud volcano, Indonesia,” J. of Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2017.08.004, 2017. • H. Nakamichi et al., “A Newly Installed Seismic and Geodetic Observational System at Five Indonesian Volcanoes as Part of the SATREPS Project,” J. Disaster Res., Vol.14, No.1, pp. 6-17, doi:10.20965/jdr.2019.p0006, 2019. Academic Societies & Scientific Organizations: • Volcanological Society of Japan (VSJ) • Japan Geoscience Union (JpGU) • International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) • American Geophysical Union (AGU)

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Name: Name: Hiroshi L. Tanaka Atsushi Shimizu

Afﬁliation: Afﬁliation: Division of Global Environmental Science. Cen- Senior Researcher, Center for Regional Environ- ter for Computational Sciences (CCS), Univer- mental Research, National Institute for Environ- sity of Tsukuba mental Studies (NIES)

Address: Address: 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan Brief Career: Brief Career: 1981-1988 Senior Research Specialist, Atmospheric Science, University of 1994-1999 Graduate school of Science, Kyoto University Missouri 1999- NIES 1988-1991 Assistant Professor, Geophysical Institute, University of Alaska Selected Publications: Fairbanks • A. Shimizu, N. Sugimoto, T. Nishizawa, Y. Jin, and D. Batdorj, 1991-2001 Assistant Professor, CCS “Variations of Dust Extinction Coefficient Estimated by Lidar 2001-2005 Associate Professor, CCS Observations over Japan, 2007-2016,” SOLA, Vol.13, pp. 205-208, 2017. 2005- Professor, CCS • A. Shimizu et al., “Continuous observations of Asian dust and other Selected Publications: aerosols by polarization lidars in China and Japan during ACE-Asia,” J. of • H. L. Tanaka and K. Yamamoto, “Numerical simulations of volcanic Geophysical Research, Vol.109, Issue D19, D19S17, 2004. plume dispersal from Usu volcano in Japan on 31 March 2000 using PUFF Academic Societies & Scientific Organizations: model,” Earth, Planets and Space, Vol.54, Issue 7, pp. 743-752, 2002. • Meteorological Society of Japan (MSJ) • H. L. Tanaka, M. Iguchi, and S. Nakada, “Numerical Simulations of • Japan Association of Aerosol Science and Technology (JAAST) Volcanic Ash Plume Dispersal from Kelud Volcano in Indonesia on • Laser Radar Society of Japan (LRSJ) February 13, 2014,” J. Disaster Res. Vol.11, No.1, pp. 31-42, 2016. • H. L. Tanaka and M. Iguchi, “Numerical Simulation of Volcanic Ash Plume Dispersal from Kuchinoerabujima,” J. of Natural Disaster Science, Vol.37, Issue 2, pp. 79-90, 2016. Name: • H. L. Tanaka and M. Iguchi, “Numerical Simulations of Volcanic Ash Daisuke Miki Plume Dispersal for Sakura-Jima Using Real-Time Emission Rate Estimation,” J. Disaster Res., Vol.14, No.1, pp. 160-172, 2019. Affiliation: Academic Societies & Scientific Organizations: Disaster Prevention Research Institute (DPRI), • Meteorological Society of Japan (MSJ) Kyoto University • American Meteorological Society (AMS) • Japan Geoscience Union (JpGU)

Name: Address: Yusaku Ohta 1722-19 Sakurajima-Yokoyama-cho, Kagoshima 891-1419, Japan Brief Career: Affiliation: 1992 Research Associate, DPRI Associate Professor, Research Center for Pre- 2007 Assistant Professor, DPRI diction of Earthquakes and Volcanic Eruptions, Academic Societies & Scientific Organizations: Graduate School of Science, Tohoku University • Volcanological Society of Japan (VSJ) • Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS) • American Geophysical Union (AGU) Address: 6-6 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan Brief Career: 2007 Doctor of Science, Nagoya University 2007 Assistant Professor, Graduate School of Science, Tohoku University 2014- Associate Professor, Graduate School of Science, Tohoku University Selected Publications: • Y. Ohta et al., “Quasi real-time fault model estimation for near-field tsunami forecasting based on RTK-GPS analysis: Application to the 2011 Tohoku-Oki Earthquake (Mw 9.0),” J. of Geophysical Research: Solid Earth, Vol.117, Issue B2, B02311, doi:10.1029/2011JB008750, 2012. • Y. Ohta et al., “Geodetic constraints on afterslip characteristics following the March 9, 2011, Sanriku-oki earthquake, Japan,” Geophysical Research Letters, Vol.39, Issue 16, L16304, doi:10.1029/2012GL052430, 2012. • Y. Ohta and M. Iguchi, “Advective diffusion of volcanic plume captured by dense GNSS network around Sakurajima volcano: a case study of the vulcanian eruption on July 24, 2012,” Earth, Planets and Space, Vol.67, Issue 1, Article No.157, doi:10.1186/s40623-015-0324-x, 2015. Academic Societies & Scientific Organizations: • Geodetic Society of Japan (GSJ) • Seismological Society of Japan (SSJ) • American Geophysical Union (AGU)

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