南阿拉斯加地壳及上地幔结构成像研究

通过反演562891个纵波和156321个横波走时数据，第一次同时获得了阿拉斯加地区地壳及上地幔的纵波与横波速度以及泊松比图像，为更好地认识阿拉斯加地区的深部地震结构、太平洋板块与亚库塔特板块的俯冲几何形态提供了科学依据.成像结果表明P波和S波速度图像与泊松比结构具有很好的一致性，强的高速度和低泊松比异常沿着阿拉斯加俯冲带延伸至200 km深度，该高速度和低泊松比异常体与俯冲带的地震空间分布吻合，因此，我们认为该高速体为俯冲的太平洋板块和亚库塔特板块.从地震空间分布发现，大部分大地震（M>6.5）发生在高速度与低速度异常交界处，可能反映了俯冲板块之间强耦合作用.在俯冲带的地幔楔显示出广泛的低速度和高泊松比异常，并且这些异常与岛弧火山的位置相对应，这与大洋板块俯冲所形成的岩浆入侵作用有关.研究结果表明在南阿拉斯加俯冲带，俯冲板块的俯冲角度从兰格尔块体下方的平坦变成在布里斯托尔湾下方的陡峭，这与亚库塔特板块俯冲在兰格尔块体下方和太平洋板块俯冲在布里斯托尔湾下方有关.在基奈半岛和科迪亚克岛连接处的上地幔位置存在强烈的低速与高泊松比异常体，使该处的大洋俯冲板块变薄.这一现象可能与亚库塔特板块和太平洋板块相互碰撞作用以及软流圈强烈的上升流入侵有关.

Structure imaging of the crust and upper mantle in south of Alaska

For the first time we used 562891 P- and 156321 S-wave travel-time data to invert for the V_p and V_s structures simultaneously in the crust and upper mantle in south of Alaska. At the same time, the Poisson's ratio images were calculated from the inverted velocity models. Our tomographic results provide crucial information for a better understanding of seismic structures and the geometry of the subducting slabs under south of Alaska. The P-wave velocity is similar to that of the S-wave velocity and coincides well with the Poisson's ratio image. Strong high velocity (V_p, V_s) and low Poisson's ratio (σ) anomalies down to 200 km depth are clearly imaged along the Alaska subduction zone, indicating the subducting Pacific and Yakutat slabs. The high-velocity and low-σ zone is well consistent with the spatial distribution of the earthquakes along the subducting slab. Most of large earthquakes (M>6.5) are located in the boundaries between the high- and low-velocity zones, suggesting strong interplate coupling of the subducting slabs. Wide low-velocity anomalies with high Poisson's ratio are revealed in the mantle wedge of the subduction zone, which might reflect the magmatic flow associated with the slab subduction. Our study indicates the dipping angle of the slab changes from steep in the Wrangell Block to flat under the Bristol Bay in south of Alaska, which is associated with the Yakutat plate under the Wrangell Block and the Pacific plate subducting under the Bristol Bay. A strong low-velocity body in the upper mantle is observed under the conjunction of the Kenai Peninsula and the Kodiak Island, suggesting the collision of the Yakutat and Pacific plates as well as the strong upwelling of the mantle asthenosphere that might have thinned the subducting plate there.