Journal of Geophysical Research: Solid Earth Uppermost mantle seismic velocity structure beneath USArray
Journal of Geophysical Research: Solid Earth Uppermost mantle seismic velocity structure beneath USArray
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我们在整个USAray足迹下应用Pn层析成像，以成像美国下方的最上层地幔速度结构和各向异性以及地壳厚度限制。在美国东部的稀疏源分布和由此产生的较长的射线路径提供了新的挑战，并证明包括额外的参数，占地幔盖的速度梯度。在大尺度上，Pn速度在美国东部比西部高，但我们观察到新马德里地震带周围和阿巴拉契亚山脉东部以下的速度较低的补丁。对于美国中部和东部以下的大部分地幔盖，我们发现一个中等的正速度梯度。在美国西部，我们观察到胡安·德富卡俯冲带区域有一个中等梯度，但该区域的南部和东部没有明显的梯度。在各向异性方面，我们发现Pn快轴通常与SKS分裂方向不一致，这表明上地幔各向异性在垂直方向上发生了显著变化。特别是在美国西部的SKS的快速偏振方向的圆形图案是不太明显的Pn的结果，并在美国东部的主要Pn快速方向是大约南北，而SKS快速偏振取向大致平行于绝对板块运动方向。
We apply Pn tomography beneath the entire USArray footprint to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the United States. The sparse source distribution in the eastern United States and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid. At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe patches of lower velocities around the New Madrid seismic zone and below the eastern Appalachians. For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient. In the western United States, we observe a moderate gradient in the region of the Juan de Fuca subduction zone, but no significant gradient to the south and east of this region. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting significant vertical changes in anisotropy in the upper mantle. In particular the circular pattern of the fast polarization direction of SKS in the western United States is much less pronounced in the Pn results, and in the eastern US the dominant Pn fast direction is approximately north-south, whereas the SKS fast polarizations are oriented roughly parallel to the absolute plate motion direction.