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[SVC26-01] Depth of source of volcanic ash ejected in the vulcanian eruptions at Shinmoe-dake and the image of eruption site below the summit crater
Keywords:vulcanian eruption, depth of source of volcanic ash, tilt change with volcanic eruptions, image of eruption site below the summit crater
The latest eruptive activity of Shinmoe-dake started on August 22, 2008 after 16 years dormancy. Subsequently, it erupted on March 30, April 17, May 27, June 27 and 28, and July 5 and 10, 2010. In 2011, the eruption started on January 19 and was followed by sub-Plinian eruption on 27 January. Eruptive activity gradually ceased since February 2 and moved to Vulcanian activities. It erupted in October 2017 and in March 2018. Eruptions continued until June 27. Oikawa and Tajima (2019a, 2019b) discussed the relation of ashfall volumes and durations of vulcanian eruptions that is characterized by the power law: ashfall volume is proportional to durations to the second power. Furthermore, they showed that the amount of eruption volume is proportional to the square of the eruption duration assuming a simple model. The important assumption is that the flow in the conduit is a quasi-steady turbulent flow, and that the shape of the conduit and the physical properties of the eruptive magma do not change. In this study, we estimate the length of the conduit, which is an important parameter of the model, that is, the depth of the source of volcanic ash ejected in the volcanic eruptions and consider the image of eruption site below the summit crater.
Assuming that the source of volcanic ash is located at a depth H just below the crater, and the volume change due to the eruption is ΔV, the tilt change uzr in the direction at a horizontal distance r from the crater is expressed as uzr=-(9HrΔV)/(4π(r2+H2)5/2) using the Mogi model. In this study, among the eruption events for which the amount of volcanic ash emitted was estimated by Oikawa and Tajima (2019a, 2019b, 2020), eruptions in which tilt changes associated with the eruption were observed are used for analysis. The date and time of each eruption and the amount of tilt change observed at KITK, 3.2km from the summit crater of Shinmoe-dake are listed below.
1. 2011/Mar./13, 13x10-8 rad
2. 2011/Apr./3, 11x10-8 rad
3. 2011/Apr./9, 4.9x10-8 rad
4. 2011/Apr./18, 12x10-8 rad
5. 2018/Apr./5, 8.3x10-8 rad
6. 2018/May/11, 7.5x10-8 rad
As examples of tilt changes associated with eruptions, records of tilt changes corresponding to 1 and 6 are shown in Fig.1a and Fig.1b, respectively. Fig.2 shows the results of determining the depth of the pressure source using these amounts of tilt change and the volcanic ash ejection volume reported by Tajima et al. (2019) and Nishiki et al. (2013) as ΔV. The horizontal axis shows the event number, and the vertical axis shows the depth. The eruption fluid chamber exists in the shallow area just bolow the crater, and considering the errors, the depth can be assumed to be a constant value of about 200 m to 400 m. Oikawa and Tajima (2020) shows that there is a relationship between the amount of volcanic ash M and the eruption duration T as M = CT2. The constant C is determined by the conduit radius a, the roughness scale of the conduit wall, the radius R and the bulk modulus K of the eruption fluid chamber, the rigidityμof the surrounding rock, the conduit length H, and the constant b (b = 0.417) determined by turbulence experiments. In the case of Shinmoe-dake, it is estimated that C=1, and by substituting appropriate values as constants (for example, a/d=10, μ=k=1010 Pa, ρ=1000kg/m3), a, R and H are related to a5/(HR3)=1.4x10-7. Substituting H as approximately 300 m, we get a5/(R3) =4.2x10-5. As a result, when the conduit radius a is 1 to 3 m, the radius R of the eruption fluid chamber is 30 to 200 m. From the above, we can have the image of an eruption site that there is an area with a radius of about 100 m accumulating eruption fluid at a depth of about 300 m directly below the summit crater, and in the event of an explosive eruption, volcanic ash will be released through a conduit with a radius of several meters.
At Shinmoe-dake volcano, lava has filled the old crater during eruptive activity including sub-Plinian eruption that started in January 2011, and the eruption fluid chamber shown in this study was at the floor of the old crater or just below it. Therefore, in the case of the explosive eruptions targeted in this study, the crater was buried by the lava flow, and an area was created at the bottom of the old crater just below the crater where eruptive material could easily accumulate, and repeated explosive eruptions occurred from the area.
Acknowledgments: We thank the Japan Meteorological Agency for providing tilt data. We also thank Yuta Maeda for providing information on ground deformation associated with explosive eruptions occurred at Shinmoe-dake volcano.
Assuming that the source of volcanic ash is located at a depth H just below the crater, and the volume change due to the eruption is ΔV, the tilt change uzr in the direction at a horizontal distance r from the crater is expressed as uzr=-(9HrΔV)/(4π(r2+H2)5/2) using the Mogi model. In this study, among the eruption events for which the amount of volcanic ash emitted was estimated by Oikawa and Tajima (2019a, 2019b, 2020), eruptions in which tilt changes associated with the eruption were observed are used for analysis. The date and time of each eruption and the amount of tilt change observed at KITK, 3.2km from the summit crater of Shinmoe-dake are listed below.
1. 2011/Mar./13, 13x10-8 rad
2. 2011/Apr./3, 11x10-8 rad
3. 2011/Apr./9, 4.9x10-8 rad
4. 2011/Apr./18, 12x10-8 rad
5. 2018/Apr./5, 8.3x10-8 rad
6. 2018/May/11, 7.5x10-8 rad
As examples of tilt changes associated with eruptions, records of tilt changes corresponding to 1 and 6 are shown in Fig.1a and Fig.1b, respectively. Fig.2 shows the results of determining the depth of the pressure source using these amounts of tilt change and the volcanic ash ejection volume reported by Tajima et al. (2019) and Nishiki et al. (2013) as ΔV. The horizontal axis shows the event number, and the vertical axis shows the depth. The eruption fluid chamber exists in the shallow area just bolow the crater, and considering the errors, the depth can be assumed to be a constant value of about 200 m to 400 m. Oikawa and Tajima (2020) shows that there is a relationship between the amount of volcanic ash M and the eruption duration T as M = CT2. The constant C is determined by the conduit radius a, the roughness scale of the conduit wall, the radius R and the bulk modulus K of the eruption fluid chamber, the rigidityμof the surrounding rock, the conduit length H, and the constant b (b = 0.417) determined by turbulence experiments. In the case of Shinmoe-dake, it is estimated that C=1, and by substituting appropriate values as constants (for example, a/d=10, μ=k=1010 Pa, ρ=1000kg/m3), a, R and H are related to a5/(HR3)=1.4x10-7. Substituting H as approximately 300 m, we get a5/(R3) =4.2x10-5. As a result, when the conduit radius a is 1 to 3 m, the radius R of the eruption fluid chamber is 30 to 200 m. From the above, we can have the image of an eruption site that there is an area with a radius of about 100 m accumulating eruption fluid at a depth of about 300 m directly below the summit crater, and in the event of an explosive eruption, volcanic ash will be released through a conduit with a radius of several meters.
At Shinmoe-dake volcano, lava has filled the old crater during eruptive activity including sub-Plinian eruption that started in January 2011, and the eruption fluid chamber shown in this study was at the floor of the old crater or just below it. Therefore, in the case of the explosive eruptions targeted in this study, the crater was buried by the lava flow, and an area was created at the bottom of the old crater just below the crater where eruptive material could easily accumulate, and repeated explosive eruptions occurred from the area.
Acknowledgments: We thank the Japan Meteorological Agency for providing tilt data. We also thank Yuta Maeda for providing information on ground deformation associated with explosive eruptions occurred at Shinmoe-dake volcano.