In Southeast Alaska, extreme uplift rates are primarily caused by glacial isostatic adjustment (GIA), as a result of ice thickness changes from the Little Ice Age to the present combined with a low-viscosity asthenosphere. Previous GIA models adopted a 1-D Earth structure. However, the actual Earth structure is likely more complex due to the long history of subduction and tectonism and the transition from a continental to an oceanic plate. Seismic evidence indeed shows a laterally heterogenous Earth structure. In this study, a numeral model is constructed for Southeast Alaska, which allows for the inclusion of lateral viscosity variations. The viscosity follows from scaling relationships between seismic velocity anomalies and viscosity variations. We use this scaling relationship to constrain the thermal effect on seismic variations and investigate the importance of lateral viscosity variations. We find that a thermal contribution to seismic anomalies of 10% is required to explain the GIA observations. This implies that non-thermal effects control seismic anomaly variations in the shallow upper mantle. Due to the regional geologic history, it is likely that hydration of the mantle impact both viscosity and seismic velocity. The best-fit model has a background viscosity of 5.0 × 10¹⁹ Pa-s, and viscosities at ∼80 km depth range from 1.8 × 10¹⁹ to 4.5 × 10¹⁹ Pa-s. A 1-D averaged version of the 3-D model performed slightly better, however, the two models were statistically equivalent within a 2σ measurement uncertainty. Thus, lateral viscosity variations do not contribute significantly to the uplift rates measured with the current accuracy and distribution of sites.

中文翻译：

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3-D 地球结构对小冰河时代后阿拉斯加东南部冰川均衡调整的影响

在阿拉斯加东南部，极端抬升率主要是由冰川均衡调整 (GIA) 引起的，这是从小冰河时代到现在的冰层厚度变化以及低粘度软流圈的结果。以前的 GIA 模型采用一维地球结构。然而，由于俯冲和构造作用的悠久历史以及从大陆板块到大洋板块的过渡，实际的地球结构可能更加复杂。地震证据确实显示了横向异质的地球结构。在这项研究中，为阿拉斯加东南部构建了一个数字模型，该模型允许包含横向粘度变化。粘度遵循地震速度异常和粘度变化之间的比例关系。我们使用这种比例关系来限制热效应对地震变化的影响，并研究横向粘度变化的重要性。我们发现需要 10% 的对地震异常的热贡献来解释 GIA 观测。这意味着非热效应控制了浅层上地幔的地震异常变化。由于区域地质历史，地幔的水合作用很可能会影响粘度和地震速度。最佳拟合模型的背景粘度为 5.0 × 10 地幔的水合作用很可能会影响粘度和地震速度。最佳拟合模型的背景粘度为 5.0 × 10 地幔的水合作用很可能会影响粘度和地震速度。最佳拟合模型的背景粘度为 5.0 × 10¹⁹ Pa-s，约 80 km 深度的粘度范围从 1.8 × 10 ¹⁹到 4.5 × 10 ¹⁹ Pa-s。3-D 模型的 1-D 平均版本表现稍好，但是，在 2 σ测量不确定度内，这两个模型在统计上是等效的。因此，横向粘度变化对以当前精度和站点分布测量的抬升率没有显着影响。