5:15 PM - 7:15 PM
[SSS08-P01] Estimation of seismic velocity structure beneath Hokkaido
- High VP/VS region beneath Mt. Tokachi-dake due to dehydration reaction of the mantle wedge -
Keywords:Tomography, mantle wedge, dehydration
To accurately grasp the location of the magma chamber inside the Earth, detailed velocity model of the Earth's interior using seismic tomography are often used. In this study, we developed an analysis tool using SIRT (simultaneous iterative reconstruction technique), which minimize the variation in the slowness of each seismic wave propagation path. Although SIRT does not allow for the calculation of a resolution matrix, it makes the process easier and faster by eliminating the need to solve a huge sparse inverse matrix. Furthermore, the accuracy of the velocity structure estimation depends on the density of rays passing through the computational grid.
Tokachi-dake is in the center of Hokkaido, where the Pacific plate subducts from the southeast to the northwest of Hokkaido along the Hidaka Mountains, Kitami Mountains, and the Teshio Mountains. Active volcano Mt. Tokachi-dake began its activity 2 million years ago, and in the past 150 years alone it has erupted in the Ansei (1857), Meiji (1887-1888), Taisho/Showa (1926-1928), Showa (1962) and Showa/Heisei (1988-1989) eras. The eruption style is mainly Strombolian, the mountain body is composed of basalt and andesite, and it is thought that primary magma from the mantle is strongly involved.
The initial velocity structure model used in this analysis was iasp91 (Kennet et al., 1991). A section line was set along the subduction direction of the Pacific plate passing through Hokkaido, including Mt. Tokachi-dake. 14 seismic stations and epicenters within 15 km width from the section line that occurred in the 10-year period from 2013 to 2022 were extracted from the JMA catalog. The number of earthquakes used in the analysis was 10,562, and the total number of ray paths (P and S) was 33,077. The computational grid size was L:30km x D:30km. The analysis revealed a linear low VP/VS region south of Mt. Tokachi-dake, which corresponds to the landward plate. Furthermore, a conduit-like high VP/VS region was estimated from Mt. Tokachi-dake to a depth of more than 150 km, where seismic activity rapidly declines within the Pacific plate slab. This high VP/VS region is a structure indicating magma melting by dehydration in the mantle wedge beneath Hokkaido, and was not reveal in Matsubara et al. (2018), a representative velocity structure model for Japan.
Tokachi-dake is in the center of Hokkaido, where the Pacific plate subducts from the southeast to the northwest of Hokkaido along the Hidaka Mountains, Kitami Mountains, and the Teshio Mountains. Active volcano Mt. Tokachi-dake began its activity 2 million years ago, and in the past 150 years alone it has erupted in the Ansei (1857), Meiji (1887-1888), Taisho/Showa (1926-1928), Showa (1962) and Showa/Heisei (1988-1989) eras. The eruption style is mainly Strombolian, the mountain body is composed of basalt and andesite, and it is thought that primary magma from the mantle is strongly involved.
The initial velocity structure model used in this analysis was iasp91 (Kennet et al., 1991). A section line was set along the subduction direction of the Pacific plate passing through Hokkaido, including Mt. Tokachi-dake. 14 seismic stations and epicenters within 15 km width from the section line that occurred in the 10-year period from 2013 to 2022 were extracted from the JMA catalog. The number of earthquakes used in the analysis was 10,562, and the total number of ray paths (P and S) was 33,077. The computational grid size was L:30km x D:30km. The analysis revealed a linear low VP/VS region south of Mt. Tokachi-dake, which corresponds to the landward plate. Furthermore, a conduit-like high VP/VS region was estimated from Mt. Tokachi-dake to a depth of more than 150 km, where seismic activity rapidly declines within the Pacific plate slab. This high VP/VS region is a structure indicating magma melting by dehydration in the mantle wedge beneath Hokkaido, and was not reveal in Matsubara et al. (2018), a representative velocity structure model for Japan.