环渤海地区的地震层析成像与地壳上地幔结构

Seismic tomography and velocity structure in the crust and upper mantle around Bohai Sea area

利用环渤海地区的天然地震P波到时资料,采用纬度和经度方向分别为0.5°×0.6°的网格划分,反演了该地区地壳上地幔的三维P波速度结构.初步结果表明,环渤海地区地壳上地幔的速度结构具有明显的横向不均匀性:京津唐地区地壳中上部的速度异常反映了浅表层的地质构造特征,造山带和隆起区对应于高速异常,坳陷区和沉积盆地对应于低速异常;地壳下部出现大规模的低速异常与华北地区广泛存在的高导层相对应,估计与壳内的滑脱层和局部熔融、岩浆活动有关;莫霍面附近的速度异常反映了地壳厚度的变化及壳幔边界附近热状态的差异;上地幔顶部大范围的低速异常可能是上地幔软流层热物质大规模上涌所致.

Three-dimensional P wave velocity images of the crust and upper mantle beneath Bohai Sea and its adjacent regions are reconstructed through a tomographic inversion by using P wave arrival times from local and remote earthquakes, with a grid spacing of 0.5°×0.6° in latitude and longitude directions. The velocity images reveal an evident lateral heterogeneity of the curst and upper mantle of the Bohai Sea regions. Velocity anomalies in upper-mid crust reflect the character of surface geology： orogenic belt and uplift areas have high velocities, while depressions and sedimentary basins have low velocities; Widely-distributed low velocities in the mid-lower crust beneath North China correspond to high conductive layers in this region, which may be caused by partial-melting and magma activity. Velocity anomalies near Moho reflect the variation of crust thickness and the thermal state in crust-mantle boundary, while low velocity anomalies in uppermost mantle may be related with the upweUing of hot materials from asthenosphere.