10:45 AM - 12:15 PM
[SVC31-P16] Pressure Source Estimation by Level Surveying at Sakurajima Volcano
Keywords:Sakurajima, Levelling survey, Finite sphere model
Sakurajima, located in Kagoshima Prefecture, was formed on the southern rim of the Aira caldera at the back of Kinko Bay. It is an andesitic-dacitic post-caldera volcano consisting of two stratovolcanoes, Kitadake and Minamidake. Four major eruptions have been recorded on Sakurajima since the dawn of history: the Bunmei Eruption, the An'ei Eruption, the Taisho Eruption, and the Showa Eruption, all of which caused serious damage. The pressure source beneath the Aira caldera, Sakurajima's main magma chamber, continues to supply magma to date, and active eruptions have been observed at the Minamidake volcano. Therefore, various observations of ground deformation associated with volcanic activities have been conducted at Sakurajima to understand the eruption mechanism. Among these, leveling is an observation method that enables very precise measurement of the vertical movement of the crust. In the repeated leveling observations on Sakurajima, the relative heights between each level point and S.17, a level point on the west coast of Sakurajima, were used as survey data. Calculation of the uplift and downlift of the ground from 2016 to 2021 shows uplift at the benchmarks on the north coast of Sakurajima and subsidence at the benchmarks in the central and south coast of Sakurajima. (Yamamoto et al. 2021)
The "Mogi model" is one of the most popular models for volcanic crustal deformation. The model assumes that the pressure source radius is sufficiently small relative to the depth of the pressure source center. Thus, there is a possibility that errors might occur in the analytical solution for a shallow pressure source under Sakurajima. On the other hand, McTigue (1987) proposed a "finite magma body" that adds a higher-order term to the Mogi model solution by considering the interaction between the pressure source and the ground surface due to stress, so that an accurate analytical solution can be established even when the ratio between the radius and depth of the pressure source is large.
In this study, we compared the root-mean-square error between the theoretical and observed values of the shallow pressure source at Sakurajima using a finite sphere model and using a Mogi model. As a result, it is clarified that the analytical solution with the finite sphere model can obtain the displacement of the ground surface more accurately.
Next, we estimated the location and volume change of pressure sources in the shallow area beneath Sakurajima by applying a finite sphere model and using data on vertical ground motion within the island from 2016 to 2021. We considered two pressure sources beneath Sakurajima in addition to the main magma chamber beneath the Aira caldera.The pressure source under the Aira caldera (SourceA) was fixed to the position of Miki et al. (2021), and only the volume change was considered as an unknown parameter, while the two pressure sources under Sakurajima were analyzed with both the position and volume change as unknown factors. As a result of the analysis, an expanding pressure source (SourceK) was estimated at 3.4 km beneath the northwestern part of Sakurajima, and a contracting pressure source (SourceM) was estimated at 2.5 km beneath near the Minamidake. The volume change of each pressure source is 10.9 x 106 m3 for SourceA, and 0.8 x 106m3, -2.4 x 106m3 for SourceK and SourceM, respectively.This result is consistent with the fact that the epicenter of Sakurajima is mainly distributed at a depth of 0 to 3 km just below Minamidake. (JMA, 2021) We compared the analytical solution with the actual crustal vertical movement for each level channel. The results for the Kitadake route were generally consistent. However, the displacement of the benchmarks on the north side of Sakurajima and the level points on Mt.Haruta. The benchmarks on the north coast of Sakurajima are greatly affected by the pressure source beneath the Aira caldera, and therefore, the location of SourceA should be analyzed as an unknown parameter.
In the future, we will conduct analysis using a wider range of data, and at the same time, we will conduct a complex analysis using other observation data, such as GNSS observations and tiltmeters, to improve the accuracy of the analysis.
The "Mogi model" is one of the most popular models for volcanic crustal deformation. The model assumes that the pressure source radius is sufficiently small relative to the depth of the pressure source center. Thus, there is a possibility that errors might occur in the analytical solution for a shallow pressure source under Sakurajima. On the other hand, McTigue (1987) proposed a "finite magma body" that adds a higher-order term to the Mogi model solution by considering the interaction between the pressure source and the ground surface due to stress, so that an accurate analytical solution can be established even when the ratio between the radius and depth of the pressure source is large.
In this study, we compared the root-mean-square error between the theoretical and observed values of the shallow pressure source at Sakurajima using a finite sphere model and using a Mogi model. As a result, it is clarified that the analytical solution with the finite sphere model can obtain the displacement of the ground surface more accurately.
Next, we estimated the location and volume change of pressure sources in the shallow area beneath Sakurajima by applying a finite sphere model and using data on vertical ground motion within the island from 2016 to 2021. We considered two pressure sources beneath Sakurajima in addition to the main magma chamber beneath the Aira caldera.The pressure source under the Aira caldera (SourceA) was fixed to the position of Miki et al. (2021), and only the volume change was considered as an unknown parameter, while the two pressure sources under Sakurajima were analyzed with both the position and volume change as unknown factors. As a result of the analysis, an expanding pressure source (SourceK) was estimated at 3.4 km beneath the northwestern part of Sakurajima, and a contracting pressure source (SourceM) was estimated at 2.5 km beneath near the Minamidake. The volume change of each pressure source is 10.9 x 106 m3 for SourceA, and 0.8 x 106m3, -2.4 x 106m3 for SourceK and SourceM, respectively.This result is consistent with the fact that the epicenter of Sakurajima is mainly distributed at a depth of 0 to 3 km just below Minamidake. (JMA, 2021) We compared the analytical solution with the actual crustal vertical movement for each level channel. The results for the Kitadake route were generally consistent. However, the displacement of the benchmarks on the north side of Sakurajima and the level points on Mt.Haruta. The benchmarks on the north coast of Sakurajima are greatly affected by the pressure source beneath the Aira caldera, and therefore, the location of SourceA should be analyzed as an unknown parameter.
In the future, we will conduct analysis using a wider range of data, and at the same time, we will conduct a complex analysis using other observation data, such as GNSS observations and tiltmeters, to improve the accuracy of the analysis.