14:15 〜 14:30
[SCG45-34] Correlation between plate boundary structure and distribution of shallow slow earthquakes in the western Nankai Trough
キーワード：南海トラフ、プレート境界浅部、P波速度構造、波形インバージョン、スロー地震、海底地震計
We present high-resolution P-wave velocity models of the forearc and the plate boundary beneath the westernmost Nankai Trough in southwest Japan. The study area covers regions where a variety of slow earthquakes, such as low-frequency tremors (Yamashita et al., 2015; 2021) and very low-frequency earthquakes (Tonegawa et al., 2020), are confirmed to have occurred on the updip side of the large interpolate earthquakes (Yagi et al., 1998; 2001) and in the flank of subducted seamounts associated with Kyushu-Palau Ridge. This tectonic setting offers a superb location to link our understanding of structural heterogeneities and hydrological properties at and around the plate boundary to slip behaviors at subduction faults.
In 2021, we carried out seismic refraction surveys along two lines: The HYU03 profile is the eastern extension of another along-trough seismic line of our previous studies (HYU01) that crosses the source regions of low-frequency tremors distributed between the deep and shallow subducting seamounts (Arai et al., under review). The HYU04 profile extends from the seaward side of the Nankai Trough just east of the Kyushu-Palau Ridge to the forearc basin east off Miyazaki prefecture and also transects the shallow tremor region in the across-trough direction. For the data acquisition, 50 ocean bottom seismographs (OBS) were deployed at 1.7-2 km intervals on each profile and recorded acoustic signals generated from the 10,600 cu.in. airgun arrays onboard the R/V Kaimei of JAMSTEC.
We constructed P-wave velocity models by applying the advanced full waveform inversion analyses to the OBS refraction data. We then compared the P-wave velocity model with the seismic reflection image on the identical line for structural interpretations. The velocity models reveal significant lateral variations in upper-plate seismic velocity and demonstrate that tremors and very low-frequency earthquakes are distributed in the regions where the plate boundary and the overriding plate have P-wave velocities lower than 4.0 km/s while slow earthquakes seems inactive where the upper-plate velocities become higher than the threshold value. The plate boundary exhibits some locations with velocity reversals in the depth direction, which indicate that thick low-velocity layers may be formed. These results support that shallow slow earthquakes occur in regions that sustain a substantial amount of fluid. We also discovered numerous low-velocity columns that vertically elongate inside the overriding plate. These low-velocity columns extend from near seafloor and are linked to the plate boundary that hosts the slow earthquakes. Based on the observations, we suggest that these low-velocity columns may play a role of conduits that facilitate upward migration of fluids at depths to the surface and thus modulate the fluid pressure and slip behaviors at the shallow parts of the subduction faults.
In 2021, we carried out seismic refraction surveys along two lines: The HYU03 profile is the eastern extension of another along-trough seismic line of our previous studies (HYU01) that crosses the source regions of low-frequency tremors distributed between the deep and shallow subducting seamounts (Arai et al., under review). The HYU04 profile extends from the seaward side of the Nankai Trough just east of the Kyushu-Palau Ridge to the forearc basin east off Miyazaki prefecture and also transects the shallow tremor region in the across-trough direction. For the data acquisition, 50 ocean bottom seismographs (OBS) were deployed at 1.7-2 km intervals on each profile and recorded acoustic signals generated from the 10,600 cu.in. airgun arrays onboard the R/V Kaimei of JAMSTEC.
We constructed P-wave velocity models by applying the advanced full waveform inversion analyses to the OBS refraction data. We then compared the P-wave velocity model with the seismic reflection image on the identical line for structural interpretations. The velocity models reveal significant lateral variations in upper-plate seismic velocity and demonstrate that tremors and very low-frequency earthquakes are distributed in the regions where the plate boundary and the overriding plate have P-wave velocities lower than 4.0 km/s while slow earthquakes seems inactive where the upper-plate velocities become higher than the threshold value. The plate boundary exhibits some locations with velocity reversals in the depth direction, which indicate that thick low-velocity layers may be formed. These results support that shallow slow earthquakes occur in regions that sustain a substantial amount of fluid. We also discovered numerous low-velocity columns that vertically elongate inside the overriding plate. These low-velocity columns extend from near seafloor and are linked to the plate boundary that hosts the slow earthquakes. Based on the observations, we suggest that these low-velocity columns may play a role of conduits that facilitate upward migration of fluids at depths to the surface and thus modulate the fluid pressure and slip behaviors at the shallow parts of the subduction faults.