5:15 PM - 7:15 PM
[SGD02-P05] Numerical calculation of ground gravity change due to the 2015 inflation event in Sakurajima: the heterogeneity effect
Keywords:gravity changes, finite element method, Sakurajima Volcano, dike, heterogeneous structure, crustal deformation
Geodetic observations such as crustal deformation and ground gravity observations are effective in understanding internal physical processes associated with seismic and volcanic activities. When modeling those solid-earth phenomena using observed geodetic data, analytical solutions for the dislocation source (Okada, 1992; Okubo, 1992) are often used. However, these analytical solutions were derived by assuming a half-infinite homogeneous elastic medium, so estimated source parameters may be biased by the analytical solutions if the effects of heterogeneity, topography or the Earth’s curvature cannot be ignored. Finite element method (FEM) can solve the above problem because it can calculate crustal deformation and gravity changes by considering complexity such as topography and inhomogeneous underground structure. There are several methods for calculating gravity changes from the deformation field calculated by FEM, but the method proposed by Kohama et al. (JpGU, 2024) is particularly effective for calculating ground gravity changes because it can accurately calculate gravity changes due to the displacement of observation points, which the other methods simplify.
Kohama and Kazama (Geod. Soc. Japan ,2024) applied the method proposed by Kohama et al. (JpGU, 2024) to the dike intrusion event on August 15, 2015 in Sakurajima Volcano, calculated ground gravity changes due to the dike intrusion considering its topography and compared the calculated results with the observed data. As a result, they discovered that the topography of Sakurajima significantly affects the calculation of ground gravity changes and estimation of the intruded mass density. In addition, it is known that heterogeneity of the medium also has a significant effect on the calculation of gravity changes. In order to estimate the intruded mass density accurately, it is important to calculate ground gravity changes using a model that considers not only the topography but also the heterogeneous structure of Sakurajima Volcano.
Therefore, we created a finite element model with a heterogeneous structure based on the P-wave velocity structure of Sakurajima Volcano (Miyamachi et al., 2013), calculated crustal deformation and ground gravity changes, and compared the results with those obtained from a homogeneous model with its Poisson ratio of 0.25 and density of 2670 kg/m3. Consequently, we found that the heterogeneous structure affects the calculation of ground gravity changes mainly because it alters the surface displacements. The maximum surface displacement calculated by the heterogeneous model is larger than the one calculated by the homogeneous model. This is because, the P-wave velocity of the heterogeneous medium above the dike is slower compared to that surrounding the dike. On the other hand, there are some areas where the amplitude of surface displacements is smaller, and they exhibits complex behavior. We will next separate the effect of topography and that of medium heterogeneity, and provide a physical interpretation of the surface displacements. Moreover, we will evaluate the validity of the assumed heterogeneous structure. by comparing the calculated crustal deformation with the observed data.
Kohama and Kazama (Geod. Soc. Japan ,2024) applied the method proposed by Kohama et al. (JpGU, 2024) to the dike intrusion event on August 15, 2015 in Sakurajima Volcano, calculated ground gravity changes due to the dike intrusion considering its topography and compared the calculated results with the observed data. As a result, they discovered that the topography of Sakurajima significantly affects the calculation of ground gravity changes and estimation of the intruded mass density. In addition, it is known that heterogeneity of the medium also has a significant effect on the calculation of gravity changes. In order to estimate the intruded mass density accurately, it is important to calculate ground gravity changes using a model that considers not only the topography but also the heterogeneous structure of Sakurajima Volcano.
Therefore, we created a finite element model with a heterogeneous structure based on the P-wave velocity structure of Sakurajima Volcano (Miyamachi et al., 2013), calculated crustal deformation and ground gravity changes, and compared the results with those obtained from a homogeneous model with its Poisson ratio of 0.25 and density of 2670 kg/m3. Consequently, we found that the heterogeneous structure affects the calculation of ground gravity changes mainly because it alters the surface displacements. The maximum surface displacement calculated by the heterogeneous model is larger than the one calculated by the homogeneous model. This is because, the P-wave velocity of the heterogeneous medium above the dike is slower compared to that surrounding the dike. On the other hand, there are some areas where the amplitude of surface displacements is smaller, and they exhibits complex behavior. We will next separate the effect of topography and that of medium heterogeneity, and provide a physical interpretation of the surface displacements. Moreover, we will evaluate the validity of the assumed heterogeneous structure. by comparing the calculated crustal deformation with the observed data.