16:00 〜 16:15
[SCG44-32] Interpretations of shallow structures in Hyuga-Nada, Southwest Japan: Effect of Kyushu-Palau Ridge subduction
キーワード：沈み込み帯、海山沈み込み、スロー地震、日向灘
Subduction of seamount modulates the upper plate controlling margin structure, faulting patterns and fluid mass transport. Kyushu-Palau Ridge (KPR), a chain of seamounts, subducts in Hyuga-Nada, southwest Japan. In this area, shallow tremor and VLFE characteristically distributed around subducting seamount, appearing to act as a barrier for the slow earthquakes.
In this study, we examined distributions of shallow deformation structures and their spatial relationships to the subducting seamount in Hyuga-Nada, to understand the modulation of the upper plate due to KPR subduction. Addition to that, we also examined the abnormal fluid pressure ratio (lambda star) converted from velocity in the seismic profiles using physical properties of sediments obtained from laboratory experiments. The detail method will be provided in the other presentation in this JpGU.
Philippine sea plate subducts at the Hyuga-Nada in a NW direction and KPR directs to NWN. The locations of the seismic lines are in the West (HYU-02) parallel to subduction direction, the top (HYU-22) and the East (HYU-18) of the inferred subducting seamount parallel to KPR.
The basic architecture of the three forearc regions constitutes 1) 10s km scale wavelength of folding within the main geological body of wedge, 2) slope basin in the syn-cline of the folds, 3) fractures at the anti-cline of the folds.
Unconformity is abundant in the landward slope basins but no unconformity in the seaward slope basins. At the top of the seamount, HYU-22 has many fractures both in the basement and slope basins. The complex structures including Mass Transfer Deposit (MTD) are observed in the landward side of slope basin. In the west of the KPR, HYU-02 shows relatively tiny fractures abundant both in the basement and slope basin. Complex structures of slope basin are observed in the landward side. While fracturing is most intense in the top of the KPR especially in the landward side, the weakest deformation is identified in the seaward and the eastern side of HYU-18. Summarizing these features from the three profiles, we may be able to define “simple basin” in the seaward side and “complex basin” in the landward side. The boundary between the simple and complex basins seems to be located at the leading side of the subducting seamount, suggesting that the shallow structures are modulated by the subducting seamount.
Abnormal fluid pressure ratio (lambda star) is defined as the difference between pore fluid pressure and hydrostatic pressure normalized by the difference between lithostatic and hydrostatic pressures. Lambda star indicates consolidation state of sediments, how much pore fluid pressure close to lithostatic pressure or hydrostatic pressure at a depth below seafloor.
Small value of lambda star is observed at the surface of seafloor throughout the profiles, but the thickness is thinner in the complex basin even though the basin sediment is very thick. Small basins in the simple basin have relatively thick layers with small lambda star, which is consistent with the simple basin structure especially in HYU-18. These features suggest that the relatively new sediments have pore fluid pressure close to the hydrostatic pressure. Larger value of lambda star is characteristically observed in the shallower portion of the simple basin, which may indicate that the high pore fluid pressure is expected in the seaward side. Although the resolution of velocity from the seismic profiles is needed to be carefully examined in the near future, there are some consistencies between shallow structures and abnormal fluid pressure ratio, which may indicate the relationship shallow structures and consolidation state of the sediments modulated by the subducting seamount.
In this study, we examined distributions of shallow deformation structures and their spatial relationships to the subducting seamount in Hyuga-Nada, to understand the modulation of the upper plate due to KPR subduction. Addition to that, we also examined the abnormal fluid pressure ratio (lambda star) converted from velocity in the seismic profiles using physical properties of sediments obtained from laboratory experiments. The detail method will be provided in the other presentation in this JpGU.
Philippine sea plate subducts at the Hyuga-Nada in a NW direction and KPR directs to NWN. The locations of the seismic lines are in the West (HYU-02) parallel to subduction direction, the top (HYU-22) and the East (HYU-18) of the inferred subducting seamount parallel to KPR.
The basic architecture of the three forearc regions constitutes 1) 10s km scale wavelength of folding within the main geological body of wedge, 2) slope basin in the syn-cline of the folds, 3) fractures at the anti-cline of the folds.
Unconformity is abundant in the landward slope basins but no unconformity in the seaward slope basins. At the top of the seamount, HYU-22 has many fractures both in the basement and slope basins. The complex structures including Mass Transfer Deposit (MTD) are observed in the landward side of slope basin. In the west of the KPR, HYU-02 shows relatively tiny fractures abundant both in the basement and slope basin. Complex structures of slope basin are observed in the landward side. While fracturing is most intense in the top of the KPR especially in the landward side, the weakest deformation is identified in the seaward and the eastern side of HYU-18. Summarizing these features from the three profiles, we may be able to define “simple basin” in the seaward side and “complex basin” in the landward side. The boundary between the simple and complex basins seems to be located at the leading side of the subducting seamount, suggesting that the shallow structures are modulated by the subducting seamount.
Abnormal fluid pressure ratio (lambda star) is defined as the difference between pore fluid pressure and hydrostatic pressure normalized by the difference between lithostatic and hydrostatic pressures. Lambda star indicates consolidation state of sediments, how much pore fluid pressure close to lithostatic pressure or hydrostatic pressure at a depth below seafloor.
Small value of lambda star is observed at the surface of seafloor throughout the profiles, but the thickness is thinner in the complex basin even though the basin sediment is very thick. Small basins in the simple basin have relatively thick layers with small lambda star, which is consistent with the simple basin structure especially in HYU-18. These features suggest that the relatively new sediments have pore fluid pressure close to the hydrostatic pressure. Larger value of lambda star is characteristically observed in the shallower portion of the simple basin, which may indicate that the high pore fluid pressure is expected in the seaward side. Although the resolution of velocity from the seismic profiles is needed to be carefully examined in the near future, there are some consistencies between shallow structures and abnormal fluid pressure ratio, which may indicate the relationship shallow structures and consolidation state of the sediments modulated by the subducting seamount.