Tomographic Inversion for Shear Velocity Beneath the North American Plate
Tomographic Inversion for Shear Velocity Beneath the North American Plate
复制标题
北美板块下方剪切速度的层析反演
DOI：
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发表时间：
1987
期刊：
影响因子：
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通讯作者：
S. Grand
中科院分区：
文献类型：
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作者：
S. Grand
层析反投影方案已被应用到S和SS旅行时间，以反演北美板块下方的剪切速度。数据范围从8°到80°，总共使用了3923个到达时间。第一波到达是直接从地震图上测得的，而后来到达的时间是用合成地震图通过波形相关技术测得的。初始模型在上部400公里处横向不均匀，以解释已知射线路径的一阶差异。该模型被分成若干块，横向尺寸为500公里乘500公里，纵向厚度各不相同。从地壳下到地幔深处约1700公里的结构获得了良好的分辨率。在上地幔中，在加拿大地盾正下方约400 km深度处发现了一个高速根，其中上级省的速度最高，根最深。据发现，美国东海岸在100至350公里深度的速度为中等，美国西部在这些深度的速度最慢。在400公里深度以下，发现的最重要的结构是从东加勒比海到美国北方的板状高速异常。在加勒比海下面，这种异常几乎是垂直的，从大约700公里到1700公里深。再往北，异常倾向东，在美国中部700公里深度处速度很高，在东海岸1100公里深度以下速度很高。异常的幅度约为1%。这种下地幔异常可能与北美之下的法拉隆板块过去的俯冲有关。
A tomographic back projection scheme has been applied to S and SS travel times to invert for shear velocity below the North American plate. The data range in distance from 8° to 80°, and a total of 3923 arrival times were used. First arrivals were measured directly off the seismograms, while the arrival times of later arrivals were found by a waveform correlation technique using synthetic seismograms. The starting model was laterally heterogeneous in the upper 400 km to account for the first-order differences in ray paths already known. The model was divided into blocks with horizontal dimensions of 500 km by 500 km and varying vertical thicknesses. Good resolution was obtained for structure from just below the crust to about 1700 km depth in the mantle. In the upper mantle a high-velocity root was found directly beneath the Canadian shield to about 400 km depth with the Superior province having the highest velocity and deepest root. The east coast of the United States was found to have intermediate velocities from 100 to 350 km depth and the western United States the slowest velocities at these depths. Below 400 km depth the most significant structure found is a slab-shaped high-velocity anomaly from the eastern Carribean to the northern United States. Beneath the Carribean this anomaly is almost vertical and extends from about 700 km to 1700 km depth. Further to the north, the anomaly dips to the east with high velocities at 700 km depth in the central United States and high velocities below 1100 km depth beneath the east coast. The anomaly is about 1% in magnitude. This lower-mantle anomaly may be associated with past subduction of the Farallon plate beneath North America.