17:15 〜 18:30
[SCG55-P02] Characteristics of the oceanic plate near the Nosappu fracture zone revealed by the extensive seismic structure studies
キーワード：海洋プレート、断裂帯、地震探査、マントル異方性
The global compilation studies on the seismic structure of the oceanic crust, such as Christeson et al. (2019) and Grevemeyer et al. (2018), indicate that the fast spreading oceanic crust, in comparison with the slow spreading oceanic crust, is laterally uniform in thickness and seismic velocities, and has relatively higher seismic velocities within both the oceanic layer 2 and layer 3. The northwestern part of the oceanic Pacific plate off northeast Japan arc was formed at a fast spreading ridge more than 100 million years ago. We JAMSTEC have conducted extensive controlled-source seismic surveys in this region and confirmed that the oceanic crust is quite simple and laterally uniform in most areas. The typical thickness of the oceanic crust is approximately 7 km and the seismic velocities at the top of the oceanic layer 3 is 6.5 km/sec in most areas. These features are well consistent with the global compilation studies.
However, our data show that the oceanic crust become thinner when it approaches to the Nosappu fracture zone. As the crust become thinner, seismic velocities within the oceanic crust become lower and the boundary between the layer 2 and layer 3, which is estimated by the changes in the vertical velocity gradient, becomes unclear. These characteristics are not consistent with those of the fast spreading oceanic crust but somewhat similar to those of the slow spreading oceanic crust. In addition to the oceanic crust, we found that the oceanic mantle shows strange characteristics toward the Nosappu fracture zone. Generally, the topmost oceanic mantle formed at the fast spreading center shows strong seismic anisotropy, which can be explain by the alignments of the olivin crystal due to the fast mantle flow at the spreading center. Since the mantle flow is expected to be perpendicular to the ridge axis, the fast axis of the seismic anisotropy is perpendicular to the ancient ridge axis which is represented by the magnetic anomaly lineation. In the northwestern part of the oceanic Pacific plate, the topmost oceanic mantle shows a strong anisotropy, more than 8 percent, and the fast axis is perpendicular to the magnetic anomaly lineation in most areas (Kodaira et al., 2014). However, when it approaches to the Nosappu fracture zone, the direction of the fast axis seems to rotate by more than 45 degrees although the strength of the seismic anisotropy suggested by the shear wave splitting is roughly the same (about 8 percent). If the strong anisotropy is caused by the mantle flow at the spreading center, our observation implies that the mantle flow near the Nosappu fracture zone was oblique to the ridge axis when the oceanic plate was formed. The oblique mantle flow might be explained by the existence of the transform fault (the current Nosappu fracture zone) near the spreading center and might be responsible for the slow-spreading-like oceanic crust there.
In this presentation, we will show the existing seismic data and seismic structures that support our hypothesis. In addition, we would like to propose an integrated marine geophysical observation, including seismic, electromagnetic, and gravity, to test our hypothesis.
However, our data show that the oceanic crust become thinner when it approaches to the Nosappu fracture zone. As the crust become thinner, seismic velocities within the oceanic crust become lower and the boundary between the layer 2 and layer 3, which is estimated by the changes in the vertical velocity gradient, becomes unclear. These characteristics are not consistent with those of the fast spreading oceanic crust but somewhat similar to those of the slow spreading oceanic crust. In addition to the oceanic crust, we found that the oceanic mantle shows strange characteristics toward the Nosappu fracture zone. Generally, the topmost oceanic mantle formed at the fast spreading center shows strong seismic anisotropy, which can be explain by the alignments of the olivin crystal due to the fast mantle flow at the spreading center. Since the mantle flow is expected to be perpendicular to the ridge axis, the fast axis of the seismic anisotropy is perpendicular to the ancient ridge axis which is represented by the magnetic anomaly lineation. In the northwestern part of the oceanic Pacific plate, the topmost oceanic mantle shows a strong anisotropy, more than 8 percent, and the fast axis is perpendicular to the magnetic anomaly lineation in most areas (Kodaira et al., 2014). However, when it approaches to the Nosappu fracture zone, the direction of the fast axis seems to rotate by more than 45 degrees although the strength of the seismic anisotropy suggested by the shear wave splitting is roughly the same (about 8 percent). If the strong anisotropy is caused by the mantle flow at the spreading center, our observation implies that the mantle flow near the Nosappu fracture zone was oblique to the ridge axis when the oceanic plate was formed. The oblique mantle flow might be explained by the existence of the transform fault (the current Nosappu fracture zone) near the spreading center and might be responsible for the slow-spreading-like oceanic crust there.
In this presentation, we will show the existing seismic data and seismic structures that support our hypothesis. In addition, we would like to propose an integrated marine geophysical observation, including seismic, electromagnetic, and gravity, to test our hypothesis.