11:00 AM - 1:00 PM
[SVC31-P10] Development of a volcano deformation calculation system considering volcanic topography - Examining the effects of seafloor topography -
Keywords:volcano deformation, BEM, Izu-Oshima
Volcanic deformation data observed by GNSS and/or tilt meter were important for evaluating volcanic activity. The Japan Meteorological Agency (JMA) have been installing volcanic deformation observation stations around active volcanoes, and these continuous observed data were utilized for monitoring of volcanic activities. Because these observation stations were located on the steep topography of volcano, the observed data were affected by the topography. It was important to consider the effect of topography in the analysis of volcanic deformation data. We have developed a numerical calculation program for volcanic deformation that takes into account the topography of volcano by using the boundary element method (BEM), and have investigated the effect of the topography on the analysis results such as the location of the pressure source. As a result, it was found that when the depth of the pressure source is about the same as the elevation of the observation station, the analysis assuming an elastic half space was overestimates or underestimates the depth and the magnitude of the volume change of pressure source (Kawaguchi, 2021, JpGU). In the previous study, we have used only digital elevation models (DEMs) of the land area to make mesh models of volcanic topography for numerical calculations, and the seafloor topography around island volcanoes was not taken into account. In this study, we improved the program to make a mesh model of the volcanic topography by combining the seafloor topography, and investigated the effect of seafloor topography on the numerical calculation results. In this presentation, we report on the effects of the seafloor topography on calculation results of volcanic deformation for Izu-Oshima Island, where the Meteorological Research Institute installed many observation stations.
The mesh model of the volcanic topography and the numerical calculation of volcanic deformation with the topography were based on the volcanic deformation calculation program with the topography using the boundary element method. The mesh model was made from the 10-m mesh DEM of the Geospatial Information Authority of Japan (GSI) for the land area, and 500-m mesh bathymetry data of the Japan Oceanographic Data Center (JODC) for the sea area. We made a mesh model of volcanic topography including seafloor topography for Izu-Oshima Island, where many observation stations such as GNSS and tiltmeters have been installed by the JMA and the MRI. The volcanic deformation, such as displacements and tilt changes due to the expansion of a spherical pressure source located just below the summit of Miharayama, was calculated numerically for each observation station. The depth of the pressure source was set from 400 m above sea level to 1000 m below sea level.
The volcanic deformations obtained from the numerical calculations which were considered seafloor topography were compared with the results obtained when only the topography of the land area was taken into account. As a result, at the GNSS stations located near the coast, such as Okada and Senba installed by MRI, the amplitudes of the calculated displacements differed by about 10~20% between the cases where the seafloor topography was taken into account and the cases where the seafloor topography was not taken into account. The difference in displacement was larger when the pressure source was located near the sea level. On the other hand, the differences in displacement and tilt change obtained from the calculation results for the stations located around Miharayama, which were far from the coast, were less than 2% regardless of the depth of the pressure source. These results indicate that the estimated depth of the pressure source may be shallower if the pressure source is analyzed without considering the seafloor topography from the data of stations near the coast.
The mesh model of the volcanic topography and the numerical calculation of volcanic deformation with the topography were based on the volcanic deformation calculation program with the topography using the boundary element method. The mesh model was made from the 10-m mesh DEM of the Geospatial Information Authority of Japan (GSI) for the land area, and 500-m mesh bathymetry data of the Japan Oceanographic Data Center (JODC) for the sea area. We made a mesh model of volcanic topography including seafloor topography for Izu-Oshima Island, where many observation stations such as GNSS and tiltmeters have been installed by the JMA and the MRI. The volcanic deformation, such as displacements and tilt changes due to the expansion of a spherical pressure source located just below the summit of Miharayama, was calculated numerically for each observation station. The depth of the pressure source was set from 400 m above sea level to 1000 m below sea level.
The volcanic deformations obtained from the numerical calculations which were considered seafloor topography were compared with the results obtained when only the topography of the land area was taken into account. As a result, at the GNSS stations located near the coast, such as Okada and Senba installed by MRI, the amplitudes of the calculated displacements differed by about 10~20% between the cases where the seafloor topography was taken into account and the cases where the seafloor topography was not taken into account. The difference in displacement was larger when the pressure source was located near the sea level. On the other hand, the differences in displacement and tilt change obtained from the calculation results for the stations located around Miharayama, which were far from the coast, were less than 2% regardless of the depth of the pressure source. These results indicate that the estimated depth of the pressure source may be shallower if the pressure source is analyzed without considering the seafloor topography from the data of stations near the coast.