11:15 AM - 11:30 AM
[SSS08-03] Magma feeding system beneath the Kirishima volcano based on 3-D velocity structure
Keywords:Magma feeding system, Seismic tomography, Kirishima volcano
Kirishima volcano, located in the southern Kyushu, is a Quaternary volcanic complex consisting of more than 20 volcanoes. Shinmoe-dake in the Kirishima volcano experienced a sub-plinian eruption in January 2011, followed by other magmatic eruptions from 2017 to 2018, and a phreatic eruption was also observed at Iwo-yama in April 2018. In order to understand the magma-feeding process at the Kirishima volcano, it is important to estimate the subsurface structure beneath the volcano. In this study, we conducted 3-D velocity tomography using natural earthquakes to estimate the seismic wave velocity structure at depths shallower than 20 km beneath the volcano in order to advance our understanding of magma pathways. Yukutake et al. (2024, SSJ) reported preliminary analysis results using the earthquakes that occurred in and around the Kirishima volcano from the end of January 2011 to the end of 2017. In this presentation, we also added new earthquake data after 2021 to further improve the resolution in the target area.
In addition to data from 124 permanent seismic stations, we used seismic waveform data from the temporal seismic stations established by Kyoto University and Kyushu University in the northern and western regions of the Kirishima volcano, respectively. Waveform records were retrieved based on the JMA unified earthquake catalog. The arrival times of the P and S waves were picked by the PhaseNet code (Zhu and Beroza 2019). The manual picking data by the JMA was also used. The Double-difference Tomography method (Zhang and Thurber, 2003) was used to estimate the velocity structure. The 1D velocity structure of the Kirishima volcano by Mikada (1996) was used as the initial structure. The grids for the tomography analysis were set up with 4 km intervals horizontally and 3 km intervals vertically to a depth of 20 km beneath the Kirishima volcano. Through the checkerboard resolution test, we confirmed that the additional data improved the resolution of velocity structure, especially from 10 to 20 km depth under the Kirishima volcano, compared to that in our previous study.
In the estimated velocity structure, Low Vp, Low Vs, and High Vp/Vs regions are identified at a depth of 6 km beneath the region between Karakuni-dake and Shinmoe-dake, suggesting the presence of a magma reservoir. This result is consistent with a model of siliciclastic magma reservoir beneath Shinmoe-dake, based on petrological analysis (Suzuki et al., 2013). Low Vs and High Vp/Vs regions are inferred in the source region of the deep low-frequency earthquakes at a depth around 20 km below Ohachi, suggesting the presence of a magma reservoir at the root of the Kirishima volcano. A Low Vp and Low Vp/Vs region was estimated at depths of 10 km to 15 km. This velocity structure can be interpreted as an area existing of gas-filled cracks. On the other hand, surface wave tomography analysis has estimated a strong radial anisotropy (VSV < VSH) in this region, suggesting the presence of a sill-like magma (Nagaoka et al., 2020). Vp/Vs may be estimated to be apparently low due to the seismic waves with the oblique incident angle into such an anisotropic medium (Wang et al., 2012). We will evaluate the resolution of the inferred structure and further develop the interpretation of the velocity structure.
In addition to data from 124 permanent seismic stations, we used seismic waveform data from the temporal seismic stations established by Kyoto University and Kyushu University in the northern and western regions of the Kirishima volcano, respectively. Waveform records were retrieved based on the JMA unified earthquake catalog. The arrival times of the P and S waves were picked by the PhaseNet code (Zhu and Beroza 2019). The manual picking data by the JMA was also used. The Double-difference Tomography method (Zhang and Thurber, 2003) was used to estimate the velocity structure. The 1D velocity structure of the Kirishima volcano by Mikada (1996) was used as the initial structure. The grids for the tomography analysis were set up with 4 km intervals horizontally and 3 km intervals vertically to a depth of 20 km beneath the Kirishima volcano. Through the checkerboard resolution test, we confirmed that the additional data improved the resolution of velocity structure, especially from 10 to 20 km depth under the Kirishima volcano, compared to that in our previous study.
In the estimated velocity structure, Low Vp, Low Vs, and High Vp/Vs regions are identified at a depth of 6 km beneath the region between Karakuni-dake and Shinmoe-dake, suggesting the presence of a magma reservoir. This result is consistent with a model of siliciclastic magma reservoir beneath Shinmoe-dake, based on petrological analysis (Suzuki et al., 2013). Low Vs and High Vp/Vs regions are inferred in the source region of the deep low-frequency earthquakes at a depth around 20 km below Ohachi, suggesting the presence of a magma reservoir at the root of the Kirishima volcano. A Low Vp and Low Vp/Vs region was estimated at depths of 10 km to 15 km. This velocity structure can be interpreted as an area existing of gas-filled cracks. On the other hand, surface wave tomography analysis has estimated a strong radial anisotropy (VSV < VSH) in this region, suggesting the presence of a sill-like magma (Nagaoka et al., 2020). Vp/Vs may be estimated to be apparently low due to the seismic waves with the oblique incident angle into such an anisotropic medium (Wang et al., 2012). We will evaluate the resolution of the inferred structure and further develop the interpretation of the velocity structure.