Surrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ∼1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region.

中文翻译：

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哥斯达黎加南部的地震各向异性证实了上地幔从太平洋流向加勒比

被俯冲的板块和大陆龙骨包围，在很大程度上防止了太平洋上地幔与周围地区的混合。一个可能的出口在中美洲地峡的南部以下，那里是地震各向异性，火山喷发同位素组成随时间变化的区域性观测以及对动态地形的考虑，都表明上地幔从太平洋流向加勒比。我们从远震剪切波的双折射观察中得出了哥斯达黎加南部上地幔地震各向异性性质的新约束。快和慢分量相差约1 s，快波沿40°–50°（东北）方向极化，近似正交于中美洲收敛边缘。