10:15 AM - 10:30 AM
[SSS09-21] Ground structure identification by using earthquake ground motion records at S-net sites based on Diffuse Field Concept for earthquake
Keywords:S-net, Horizontal-to-Vertical spectral Ratio of earthquake , ground structure
In the last decade, several ground motion observation networks were constructed in the ocean area around Japan. DONET was installed near Kumano-nada sea and Kii-suido channel, and is recording ground motions and some pressures from 2011. S-net was installed off the pacific coast of Kanto, Tohoku and Hokkaido, and started recording at 2016. The underground structure of the ocean area has been investigated, but the detailed velocity structure in the ocean area is not well constructed yet compared to terrestrial area because of the difficulty of measurement. The ground motion record observed at the seafloor station such as DONET and S-net is effective to reflect the actual ground motion characteristics to estimate the velocity structure.
In this study we applied Diffuse Field Concept for earthquake (DFCe, Kawase et al., 2011) to the observed ground motions at S-net sites and identified the velocity structures. We first corrected the misalignment of axes of records according to Takagi et al. (2019), and obtained the ground motion records with North-South, East-West and vertical components. Then we selected earthquake motions whose epicentral distance is shorter than 200 km, whose MJMA is larger than 5.5, and whose PGA is smaller than 50 Gal.
Previous studies proposed the influence of the reflected wave from the sea surface on the seafloor ground motion record. To confirm the influence, we calculated the running spectrum and Stockwell Transform of the vertical component and investigated the time dependence of the spectral amplitude. If the effect of the reflected wave is obvious, it is expected that the trough appears at a certain frequency following the quarter wave length law. However, no remarkable trough was found on the running spectrum and Stockwell transform, therefore the reflected wave was not strong in our dataset.
Horizontal-to-Vertical spectral Ratio of earthquake (EHVR) was calculated by using the selected ground motion records. The obtained EHVR at S2N01, for example, has a gentle peak at 0.3 Hz, a trough at 0.6 Hz, and a sharp peak at 10 Hz. We inverted the observed EHVR to estimate one-dimensional velocity structure based on DFCe. As mentioned above, we didn’t consider the reflected wave from sea surface in the inversion and applied the inversion method which was developed for the terrestrial sites. We don’t have a priori information of velocity structure in the sea area, then we assumed an arbitrary ten layer model. We identified S-wave velocities, P-wave velocities and thicknesses of the layers without setting searching ranges. The inverted velocity structures represented the peak at 0.3 Hz by using the layers of 700 to 2000 m depth, and the peak at 10 Hz by the shallowest layer.
We will investigate the seafloor ground motion records in more detail, estimate velocity structures at all the S-net sites, and the overall structures of off pacific coast from Kanto to Hokkaido.
In this study we applied Diffuse Field Concept for earthquake (DFCe, Kawase et al., 2011) to the observed ground motions at S-net sites and identified the velocity structures. We first corrected the misalignment of axes of records according to Takagi et al. (2019), and obtained the ground motion records with North-South, East-West and vertical components. Then we selected earthquake motions whose epicentral distance is shorter than 200 km, whose MJMA is larger than 5.5, and whose PGA is smaller than 50 Gal.
Previous studies proposed the influence of the reflected wave from the sea surface on the seafloor ground motion record. To confirm the influence, we calculated the running spectrum and Stockwell Transform of the vertical component and investigated the time dependence of the spectral amplitude. If the effect of the reflected wave is obvious, it is expected that the trough appears at a certain frequency following the quarter wave length law. However, no remarkable trough was found on the running spectrum and Stockwell transform, therefore the reflected wave was not strong in our dataset.
Horizontal-to-Vertical spectral Ratio of earthquake (EHVR) was calculated by using the selected ground motion records. The obtained EHVR at S2N01, for example, has a gentle peak at 0.3 Hz, a trough at 0.6 Hz, and a sharp peak at 10 Hz. We inverted the observed EHVR to estimate one-dimensional velocity structure based on DFCe. As mentioned above, we didn’t consider the reflected wave from sea surface in the inversion and applied the inversion method which was developed for the terrestrial sites. We don’t have a priori information of velocity structure in the sea area, then we assumed an arbitrary ten layer model. We identified S-wave velocities, P-wave velocities and thicknesses of the layers without setting searching ranges. The inverted velocity structures represented the peak at 0.3 Hz by using the layers of 700 to 2000 m depth, and the peak at 10 Hz by the shallowest layer.
We will investigate the seafloor ground motion records in more detail, estimate velocity structures at all the S-net sites, and the overall structures of off pacific coast from Kanto to Hokkaido.