11:00 AM - 1:00 PM
[SVC31-P09] Three-dimensional velocity structure beneath the Hakusan volcano estimated by Double-Difference tomography
Keywords:Hakusan, magma reservoir
The Hakusan Volcano is an active volcano that has historically erupted many times, and there are swarm earthquakes beneath it in recent years. Takahashi et al. (2004) conducted seismic tomography focusing on the Hakusan volcano and found a low-velocity and high-Vp/Vs region at a depth of 10-14 km, which is regarded as a magma reservoir. In this study, we apply the Double-Difference (DD) tomography method (Zhang and Thurber, 2003) to the recent travel time data to re-estimate the 3D seismic velocity structure beneath the Hakusan volcano and clarify the characteristics of the magma chamber.
We divided the area of 34.15°N-38.15°N, 134.75°E-138.7°°E, and 0-450 km depth into a 0.05°×0.05°×3 km grid and selected the earthquakes with the largest magnitude in each grid. A total of 3975 regular earthquakes and 139 low-frequency earthquakes were used for the analysis. The number of P-wave and S-wave travel time data is 46602 and 44084, respectively, and the number of differential travel time data of P-wave and S-wave is 138455 and 114741, respectively. The one-dimensional velocity structure was determined by the JHD method (Kissling et al., 1994) and was used as the initial model. The DD tomography was performed on a grid of 10 km in the horizontal direction and 4 km in the depth direction around the Hakusan volcano.
The result of DD tomography shows a low-Vp, low-Vs, and high-Vp/Vs region at a depth of 11 km beneath the Hakusan volcano, which is consistent with the result of Takahashi et al. (2004). We confirmed that this low-Vp, low-Vs, and high-Vp/Vs region is reliable from the checkerboard and spike tests. Assuming equilibrium melting of rock in the low Vp, low Vs, and high Vp/Vs region at 11 km depth, the fluid volume fraction is estimated to be about 4% from the model of Takei (2002). Umino et al. (2018) estimated three stages of magma at different depths for the eruptive process of the latest eruption of the Hakusan volcano. We suggest that the low-Vp, low-Vs, and high-Vp/Vs region obtained in this study at a depth of 11 km is considered to correspond to the magma in the middle of the three stages.
Acknowledgments: We used data provided by JMA.
We divided the area of 34.15°N-38.15°N, 134.75°E-138.7°°E, and 0-450 km depth into a 0.05°×0.05°×3 km grid and selected the earthquakes with the largest magnitude in each grid. A total of 3975 regular earthquakes and 139 low-frequency earthquakes were used for the analysis. The number of P-wave and S-wave travel time data is 46602 and 44084, respectively, and the number of differential travel time data of P-wave and S-wave is 138455 and 114741, respectively. The one-dimensional velocity structure was determined by the JHD method (Kissling et al., 1994) and was used as the initial model. The DD tomography was performed on a grid of 10 km in the horizontal direction and 4 km in the depth direction around the Hakusan volcano.
The result of DD tomography shows a low-Vp, low-Vs, and high-Vp/Vs region at a depth of 11 km beneath the Hakusan volcano, which is consistent with the result of Takahashi et al. (2004). We confirmed that this low-Vp, low-Vs, and high-Vp/Vs region is reliable from the checkerboard and spike tests. Assuming equilibrium melting of rock in the low Vp, low Vs, and high Vp/Vs region at 11 km depth, the fluid volume fraction is estimated to be about 4% from the model of Takei (2002). Umino et al. (2018) estimated three stages of magma at different depths for the eruptive process of the latest eruption of the Hakusan volcano. We suggest that the low-Vp, low-Vs, and high-Vp/Vs region obtained in this study at a depth of 11 km is considered to correspond to the magma in the middle of the three stages.
Acknowledgments: We used data provided by JMA.