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<Disaster Reports>An Examination on the Announcement of Special Warnings Due to the Sakurajima Explosion
No full textAfter the vulcanian eruption on July 24, 2022 at Sakurajima volcano, a special warning (Level 5) was issued with an alert zone of 3 km from the Minamidake summit crater, and residents were evacuated from the southern villages of Sakurajima. Problems of issuing information, warning, and evacuation orders regarding this eruption are summarized. The flight distance of volcanic bombs estimated by JMA remains questionable. It is essential to have an accurate state of volcanic activity. It took 45 minutes from the onset of the eruption to issue the special warning, and 90 minutes to issue an evacuation order. It takes too much time to respond to the eruption. Rapid evacuation decision-making is necessary. The volcanic alert level is an index that indicates the extent of the area requiring alert. Level 5 means that the warning is required even in residential areas, but the keyword “Level 5” alone no longer indicates the extent of the alert zone. It is necessary to issue warning that pushes the range of the alert zone to the forefront
ON THE RELATION BETWEEN "HIGH-FREQUENCY ACOUSTIC WAVES" IN THE GROUNDWATER AND VOLCANIC ACTIVITY OF SAKURAJIMA VOLCANO
No full textA borehole with the depth of 405m was drilled in the lava dome, which is 2.8km northwestof the active crater of Minami-dake, Sakurajima Volcano. A hydrophone was installed inthe groundwater at the bottom of the borehole to observe acoustic waves associated with thevolcanic activity.The acoustic waves observed by the hydrophone are classified into two types by thewaveforms. One is a low-frequency acoustic wave of which dominant frequency is 8.8Hz.The acoustic waves of this type are inferred to be caused by seismic waves. The other isa high-frequency acoustic wave (HAW) of which dominant frequencies are both 25Hz and8.8Hz. No significant seismic events are observed at the time of the occurrence of HAW.Examining the relationship among the numbers of occurrence of HAW, seismic activityand the concentration of hydrogen gas monitored in Mochiki borehole-C which is 4.0kmsouthwest of the active crater, the following results were obtained.1) The increases in numbers of occurrence of HAW are observed after the significantincreases in the concentration of hydrogen gas.2) The increases in numbers of occurrence of HAW are observed at the almost same timewhen A-type earthquakes occurred beneath the active crater.3) The change of numbers of the occurrence of the HAW is similar to that of B-typeearthquakes. The increases in numbers of HAW are observed before B-type earthquakeswarms and eruptions occur.A borehole with the depth of 405m was drilled in the lava dome, which is 2.8km northwestof the active crater of Minami-dake, Sakurajima Volcano. A hydrophone was installed inthe groundwater at the bottom of the borehole to observe acoustic waves associated with thevolcanic activity.The acoustic waves observed by the hydrophone are classified into two types by thewaveforms. One is a low-frequency acoustic wave of which dominant frequency is 8.8Hz.The acoustic waves of this type are inferred to be caused by seismic waves. The other isa high-frequency acoustic wave (HAW) of which dominant frequencies are both 25Hz and8.8Hz. No significant seismic events are observed at the time of the occurrence of HAW.Examining the relationship among the numbers of occurrence of HAW, seismic activityand the concentration of hydrogen gas monitored in Mochiki borehole-C which is 4.0kmsouthwest of the active crater, the following results were obtained.1) The increases in numbers of occurrence of HAW are observed after the significantincreases in the concentration of hydrogen gas.2) The increases in numbers of occurrence of HAW are observed at the almost same timewhen A-type earthquakes occurred beneath the active crater.3) The change of numbers of the occurrence of the HAW is similar to that of B-typeearthquakes. The increases in numbers of HAW are observed before B-type earthquakeswarms and eruptions occur
DISTRIBUTION OF THE INITIAL MOTIONS OF VOLCANIC MICROEARTHQUAKES (B-TYPE)AT SAKURAJIMA VOLCANO
No full textVolcanic microearthquakes called "B-type" are closely related with explosive activity.Previous researchers devided B-type earthquakes into subclasses by the predominant frequen-cy. However, hypocenters and focal mechanism of the individual subclasses in B-typeearthquakes have not been determined because of the unclear onsets.In order to detect the initial motions clearly, seismometers were installed in the boreholesaround the active crater of Sakurajima Volcano. In this paper, B-type earthquakes whosedominant frequecies are 1-3 Hz and 5-8 Hz, are named as BL-type and BH-type earthquakes, respectively. The hypocenters and distribution of the initial motions of BL-type and BH-type earthquakes were examined. The results were summarised as follows.(1) Focal depths of BL-type earthquakes were determined at the depth of less than 2 kmbelow the sea level. BH-type earthquakes had deeper hypocenters than BL-type. Themaximum depth of BH-type was 2.5 km.(2) Most of BL-type earthquakes which occurred isolatedly and BH-type earthquakeshad all compressional arrivals. BL-type earthquakes which occurred in swarms had allcompressional or all dilatational arrivals, with the ratio of 61% to 35%. The initial motionsof the remaiders were mixed with compressions and dilatations. The distributions of initialmotions did not show that focal mechanisms of BL-type and BH-type earthquakes werequadrant types.Volcanic microearthquakes called "B-type" are closely related with explosive activity.Previous researchers devided B-type earthquakes into subclasses by the predominant frequen-cy. However, hypocenters and focal mechanism of the individual subclasses in B-typeearthquakes have not been determined because of the unclear onsets.In order to detect the initial motions clearly, seismometers were installed in the boreholesaround the active crater of Sakurajima Volcano. In this paper, B-type earthquakes whosedominant frequecies are 1-3 Hz and 5-8 Hz, are named as BL-type and BH-type earthquakes,respectively. The hypocenters and distribution of the initial motions of BL-type and BH-type earthquakes were examined. The results were summarised as follows.(1) Focal depths of BL-type earthquakes were determined at the depth of less than 2 kmbelow the sea level. BH-type earthquakes had deeper hypocenters than BL-type. Themaximum depth of BH-type was 2.5 km.(2) Most of BL-type earthquakes which occurred isolatedly and BH-type earthquakeshad all compressional arrivals. BL-type earthquakes which occurred in swarms had allcompressional or all dilatational arrivals, with the ratio of 61% to 35%. The initial motionsof the remaiders were mixed with compressions and dilatations. The distributions of initialmotions did not show that focal mechanisms of BL-type and BH-type earthquakes werequadrant types
<Special Contributions>From Volcano observation to Volcanic Disaster Risk Reduction
No full textThis paper describes the characteristics of volcanic eruptions obtained from observations at Sakurajima, Satsuma-Iwojima, Kuchinoerabujima, and Suwanosejima in the Ryukyu Islands, and Guntur and Merapi volcanoes in Indonesia, and how the characteristics of volcanic activity revealed by the observation can be linked to forecasting eruption and hazard assessment. Since 1955, Sakurajima has been experiencing Vulcanian eruptions that cause damage due to falling volcanic bombs and air-shocks. Tiltmeters and strainmeters installed in underground tunnels have enabled us to detect the uplift and inflation of the volcanic body as a precursor to a Vulcanian eruption. B-type earthquakes that precede a Vulcanian eruption are accompanied by Strombolian eruptions and degassing. This results in the formation of a lava dome and underlying gas pocket. Volatile content is an important parameter in forecasting eruption style. As on Sakurajima, the segregation of hypocenters of A-type and B-type earthquakes is identified at Satsuma-Iwojima and ground uplift preceding eruptions is detected at Suwanosejima. The precursors to the 2015 eruption at Kuchinoerabujima were stronger than those of the 2014 eruption. It was necessary to forecast the hazard factors and their extent from the observed phenomena, however the alert zone based on the observation could not extend prior to the 2015 eruption. The energy of seismic activity preceding the 2010 eruption at Merapi Volcano allows to forecast the extent of pyroclastic flow that occurred after the seismicity. A support system for decision making was developed in Indonesia to utilize the forecasting for warning announcements. It is necessary to evaluate the types of hazards and their extent from observation data. Hazard assessment is important for Sakurajima, where a large-scale eruption is expected to occur in the future based on the amount of ground uplift in the Aira caldera, and should be reflected to alert zone of Level 5 (evacuation) warnings issued by the JMA
Prediction of Volume of Volcanic Ash Ejected from Showa Crater of Sakurajima Volcano, Japan
No full text桜島火山の昭和火口から放出される火山灰放出量を観測坑道内に設置された伸縮計の爆発直後のひずみ変化を用いて予測することを試みた。爆発的噴火直後には火口直下浅部の圧力源の収縮に伴うひずみが観測される。収縮体積量と放出火山灰量はおおよそ比例することが昭和火口噴火においても確かめられたので，マグマ溜まり-火道のマグマ供給系を理想気体が占めているとするモデル(Nishimura，1998)を用いて初期ひずみ変化から最終ひずみを予測した。収縮継続時間の1/6のデータを用いて，10%の誤差で予測ができた。Amount of volcanic ash ejected by explosive eruption at Showa crater of Sakurajima volcano is estimated by using initial strain record immediately after the beginning of the explosion. Extensometers record extension strain in radial component from the crater and contraction strain in tangential one induced by deflation of a shallow part beneath the crater. Deflation volume of the pressure source is proportional to weight of volcanic ash ejected by the eruption. Final strain change is predicted by short-term strain change after the beginning of the explosion by fitting a model of magma reservoir-conduit (Nishimura, 1998) to the strain record. The magma reservoir is filled with perfect gas, which is ejected through a conduit after failure of a cap-rock at the top of the conduit. The final strain is predicted with an error of 10% by using 1/6 data of the whole contraction strain record.桜島火山の昭和火口から放出される火山灰放出量を観測坑道内に設置された伸縮計の爆発直後のひずみ変化を用いて予測することを試みた。爆発的噴火直後には火口直下浅部の圧力源の収縮に伴うひずみが観測される。収縮体積量と放出火山灰量はおおよそ比例することが昭和火口噴火においても確かめられたので，マグマ溜まり-火道のマグマ供給系を理想気体が占めているとするモデル(Nishimura，1998)を用いて初期ひずみ変化から最終ひずみを予測した。収縮継続時間の1/6のデータを用いて，10%の誤差で予測ができた。Amount of volcanic ash ejected by explosive eruption at Showa crater of Sakurajima volcano is estimated by using initial strain record immediately after the beginning of the explosion. Extensometers record extension strain in radial component from the crater and contraction strain in tangential one induced by deflation of a shallow part beneath the crater. Deflation volume of the pressure source is proportional to weight of volcanic ash ejected by the eruption. Final strain change is predicted by short-term strain change after the beginning of the explosion by fitting a model of magma reservoir-conduit (Nishimura, 1998) to the strain record. The magma reservoir is filled with perfect gas, which is ejected through a conduit after failure of a cap-rock at the top of the conduit. The final strain is predicted with an error of 10% by using 1/6 data of the whole contraction strain record
ON THE RANGE OF BLOCK AND LAPILLI EJECTED BY THE VOLCANIC EXPLOSIONS
No full textEffect of some parameters on the range of volcanic blocks and lapilli is examined. They aretopography, diameter and maximum initial velocity of blocks, inclination of explosion axis andvelocity of wind for the former, and velocity of wind and column height for the latter.Some examples of calculated results under certain parameters are as follows. The range ofblocks ejected south to the active crater increases by 3, 6 and 9% due to the fair wind of whichvelocity is 10, 20 and 30%, respectively. Also, the range does by 34, 60 and 69% (maximum) dueto the inclination of explosion axis of which angle is 15, 30 and 51 deg, respectively. Air fall oflapilli could attain a range of 3.5, 6.9 and 10.4 km from the active crater when the column height is3000 m and the velocity of wind is 10, 20 and 30 m/s, respectively. When the column height is4000 m, the range of lapilli above mentioned increase by about 30%.Effect of some parameters on the range of volcanic blocks and lapilli is examined. They aretopography, diameter and maximum initial velocity of blocks, inclination of explosion axis andvelocity of wind for the former, and velocity of wind and column height for the latter.Some examples of calculated results under certain parameters are as follows. The range ofblocks ejected south to the active crater increases by 3, 6 and 9% due to the fair wind of whichvelocity is 10, 20 and 30%, respectively. Also, the range does by 34, 60 and 69% (maximum) dueto the inclination of explosion axis of which angle is 15, 30 and 51 deg, respectively. Air fall oflapilli could attain a range of 3.5, 6.9 and 10.4 km from the active crater when the column height is3000 m and the velocity of wind is 10, 20 and 30 m/s, respectively. When the column height is4000 m, the range of lapilli above mentioned increase by about 30%
Analysis of Sakurajima Volcanic Ash Particles Measured with Optical Disdrometers
Get PDFIn the present study, we analyzed the particle size distribution (PSD) of falling volcanic ash particles collected for a total of 205 eruptions in 2018-2020 at Sakurajima volcano in Kagoshima Prefecture, Japan. PSD data were measured with the DPRI optical disdrometer network. Assuming the gamma PSD model, we examined the relationships between each of the gamma PSD parameters calculated by the complete moment method. It was shown that there were good correlations between each of the gamma PSD parameters, which might be one of the characteristics of falling volcanic ash particles. We confirmed from the normalized gamma PSD analysis that the normalized intercept parameter and mass-weighted mean diameter are suitable for estimating the ash fall rate. The results of the present study provide essential information for studying microphysical processes in volcanic ash clouds and improving ash transport and sedimentation models. We also derived theoretical power law relationships between pairs of integrated PSD parameters, which can be applied to weather radar monitoring of ash fall distributions
A Method for Monitoring of Discharge Volume of Volcanic Ash by Using Volcanic Tremor
Get PDFA method for estimation of amount of volcanic ash ejected by explosive eruption at Showa crater of Sakurajima volcano is proposed. Volcanic tremor is observed associated with continuous ejection of volcanic ash. Monthly sum of seismic energy released by volcanic tremor is correlated with monthly amount of volcanic ash ejected from the crater. Maximum power with better correlation is found in the frequency range of 2-3 Hz of the volcanic tremor. Considering monthly number of explosive eruptions which eject volcanic ash instantaneously, the monthly weight of volcanic ash can be estimated from seismic energy of volcanic tremor.A method for estimation of amount of volcanic ash ejected by explosive eruption at Showa crater of Sakurajima volcano is proposed. Volcanic tremor is observed associated with continuous ejection of volcanic ash. Monthly sum of seismic energy released by volcanic tremor is correlated with monthly amount of volcanic ash ejected from the crater. Maximum power with better correlation is found in the frequency range of 2-3 Hz of the volcanic tremor. Considering monthly number of explosive eruptions which eject volcanic ash instantaneously, the monthly weight of volcanic ash can be estimated from seismic energy of volcanic tremor
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