抄録
The heterogeneity and anisotropy in the lower mantle such as seismic P wave velocities and geoid anomalies point to a fundamental Pacific/African bipolarity. The ridge/subduction zone systems have some relationship with the hemispheric hotspots grouped through the mantle convection and so may have nearly antipodal axes under the influence of convection flow. A global seismicity map suggests that most of earthquake epicenters are distributed on a part of great circles over the Earth's sphere. We propose a method for detecting great circles from the global epicenter distribution by using the Hough transformation. As a result, we extract two dominant great circles which intersect perpendiculaly each other at the equator as an axis of symmetry. One of these two great circles is passing through the northern part of the circum-Pacific seismic zone and the Africa-Antarctic plate boundary, while the other is passing through the Eurasian seismic belt. The reason of the orthogonality and the symmetry of these two great circles to the equator is discussed qualitatively in comparison with the long-wavelength geoidal anomalies and the Earth's rotation.
