10:45 AM - 12:15 PM
[SVC31-P11] Estimation of three-dimensional S-wave velocity structure of Hachijojima Island using ambient noise by dense seismic observations
Keywords:Hachijojima Island, Dense seismic observation, Ambient noise, S-wave velocity structure
In order to prevent and mitigate volcanic disasters, it is important to detect signs of an eruption from observation data and to provide accurate warnings. To correctly interpret observation data, it is essential to know the subsurface conditions in advance. Although volcanic activity at Hachijojima Island is continuously monitored 24 hours a day, few studies have been conducted on the subsurface structure, and the subsurface structure has not been clarified with sufficient resolution. In this study, we estimated the 3D S-wave velocity structure of Hachijojima Island using ambient noise from dense seismic observations conducted at Hachijojima Island.
Ambient noise by a temporary dense seismic observation conducted at Hachijojima Island and Hachijo-Kojima Island from September 2019 to March 2020 was used for the analysis. First, for 946 pairs of 44 observation points, the wave fields propagating between the observation points were obtained by calculating the cross-correlation function from ambient noise of each observation point. The positive and negative symmetry of the wavefield can confirm that noise sources are evenly distributed in space. Next, the dispersion curves of Rayleigh waves were obtained by the extended SPAC method using the observation points centered at each observation point within a radius of 1.5 km for Nishiyama and a radius of 3.5 km for Higashiyama. Using the obtained dispersion curves, the 1D S-wave velocity structure was estimated by Simulated Annealing inversion. Finally, the 1D S-wave velocity structure was interpolated to estimate the 3D velocity structure of Hachijojima Island.
As a result, we obtained a 3-D S-wave velocity structure to a depth of 4 km, which has a higher spatial resolution than previous studies. The S-wave velocity structure at a depth of 0.5 km is consistent with the geological structure, with lower velocities in Nishiyama compared to Higashiyama. In the plain between Nishiyama and Higashiyama, a constant velocity surface with Vs=1.5 km/sec was observed to a depth of 1.5-2 km, indicating that the low-velocity region was continuous to a greater depth than the surrounding area. From the southeast of Nishiyama to Higashiyama, the iso-velocity surface of Vs=1.5km/sec corresponds to the upper surface of the Neogene stratum. A high-velocity zone was observed from just below Nishiyama to the northwestern side of Nishiyama. The high-velocity region at a depth of 2-5 km beneath Nishiyama is consistent with the P-wave high-velocity region beneath Nishiyama shown in previous studies.
Ambient noise by a temporary dense seismic observation conducted at Hachijojima Island and Hachijo-Kojima Island from September 2019 to March 2020 was used for the analysis. First, for 946 pairs of 44 observation points, the wave fields propagating between the observation points were obtained by calculating the cross-correlation function from ambient noise of each observation point. The positive and negative symmetry of the wavefield can confirm that noise sources are evenly distributed in space. Next, the dispersion curves of Rayleigh waves were obtained by the extended SPAC method using the observation points centered at each observation point within a radius of 1.5 km for Nishiyama and a radius of 3.5 km for Higashiyama. Using the obtained dispersion curves, the 1D S-wave velocity structure was estimated by Simulated Annealing inversion. Finally, the 1D S-wave velocity structure was interpolated to estimate the 3D velocity structure of Hachijojima Island.
As a result, we obtained a 3-D S-wave velocity structure to a depth of 4 km, which has a higher spatial resolution than previous studies. The S-wave velocity structure at a depth of 0.5 km is consistent with the geological structure, with lower velocities in Nishiyama compared to Higashiyama. In the plain between Nishiyama and Higashiyama, a constant velocity surface with Vs=1.5 km/sec was observed to a depth of 1.5-2 km, indicating that the low-velocity region was continuous to a greater depth than the surrounding area. From the southeast of Nishiyama to Higashiyama, the iso-velocity surface of Vs=1.5km/sec corresponds to the upper surface of the Neogene stratum. A high-velocity zone was observed from just below Nishiyama to the northwestern side of Nishiyama. The high-velocity region at a depth of 2-5 km beneath Nishiyama is consistent with the P-wave high-velocity region beneath Nishiyama shown in previous studies.