3-D Seismic velocity structure and stress regime under Japan Islands

我们利用地震层析成像技术研究了日本列岛的三维壳幔结构和地震构造。揭示了地壳大地震与构造不均匀性的密切关系。大多数大的地壳地震发生在低速区或其周围，这些低速区代表了孕震地壳的薄弱部分。在火山区，地壳弱化是由活火山和弧岩浆引起的。在弧前地区，大地震震源区存在流体，对破裂成核起了作用。流体来源于俯冲菲律宾海板块的脱水作用。这些结果表明，大地震并不是发生在任何地方，而是发生在地球物理方法能够探测到的异常区域。在俯冲带下方的过渡带深处可见强而宽的快速异常，这表明大部分板片物质在最终坍塌到下地幔之前停滞在过渡带中。在地幔的大部分地区的主要热点区，特别是在夏威夷、冰岛、南太平洋和非洲，地幔柱状缓慢异常清晰可见。热点下的慢异常通常不呈直柱状，而是呈弯曲状，表明地幔柱并非固定在地幔中，而是可以被地幔流偏转。因此，热点并不是真正固定的，而是可以在表面上徘徊。

We have used seismic tomography to study the 3-D crust and mantle structure and seismotectonics of the Japan Islands. A close relationship between large crustal earthquakes and structural heterogeneities is revealed. Most of the large crustal earthquakes occurred in or around the areas of low velocity that represent weak sections of the seismogenic crust. In volcanic areas the crustal weakening is caused by active volcanoes and arc magma. In forearc areas fluids are suggested in the large earthquake source zones, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods.A new approach was used to conduct global tomographic inversions to study the deep structure of subducting slabs and mantle plumes. Strong and wide fast anomalies are visible in the transition zone depths under subduction regions, which suggests that most of the slab materials are stagnant in the transition zone before finally collapsing down to the lower mantle. Plume- like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, South Pacific and Africa. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the surface.