We investigate the elastic and anelastic response of the crust and upper mantle across Alaska to mass loading by ocean tides. GPS-inferred surface displacements recorded by the Plate Boundary Observatory network are compared with predictions of deformation associated with the redistribution of ocean water due to the tides. We process more than 5 yr of GPS data from 131 stations using a kinematic precise point positioning algorithm and estimate tidal contributions using harmonic analysis. We also forward calculate load-induced surface displacements by convolving ocean-tide models with load Green’s functions derived from spherically symmetric Earth models. We make the comparisons for dominant tidal harmonics in three frequency bands: semidiurnal (M₂), diurnal (O₁) and fortnightly (M_f). Vector differences between predicted and observed ocean tidal loading (OTL) displacements are predominantly sub-mm in magnitude in all three frequency bands and spatial components across the network, with larger residuals of up to several mm in some coastal areas. Accounting for the effects of anelastic dispersion in the upper mantle using estimates of Q from standard Earth models reduces the residuals for the M₂ harmonic by an average of 0.1–0.2 mm across the network and by more than 1 mm at some individual stations. For the relatively small M_f tide, the effects of anelastic dispersion (<0.03 mm) are undetectable within current measurement error. Incorporating a local ocean-tide model for the northeastern Pacific Ocean reduces the M₂ vertical residuals by an average of 0.2 mm, with improvements of up to 5 mm at some coastal stations. Estimated RMS observational uncertainties in the vertical component for the M₂ and O₁ tides are approximately ±0.08 mm at the two-sigma level (±0.03 mm in the horizontal components), and ±0.21 mm for the M_f harmonic (±0.07 mm in the horizontal components). For the M₂ harmonic, discrepancies between predicted and observed OTL displacements exceed observational uncertainties by about one order of magnitude. None of the ocean tide and Earth model combinations is found to reduce the M₂ residuals below the observational uncertainty, and no single forward model provides a best fit to the observed displacements across all tidal harmonics and spatial components. For the O₁ harmonic, discrepancies between predicted and observed displacements are generally several-fold larger than the observational uncertainties. For the M_f harmonic, the discrepancies are roughly within a factor of two of the observational uncertainties. We find that discrepancies between predicted and observed OTL displacements can be significantly reduced by removing a network-uniform tidal-harmonic displacement, and that the remaining discrepancies exhibit some regional-scale spatial coherency, particularly for the M₂ harmonic. We suggest that the remaining discrepancies for the M₂, O₁ and M_f tides cannot be fully explained by measurement error and instead convey information about deficiencies in ocean-tide models and deviations from spherically symmetric Earth structure.

中文翻译：

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阿拉斯加预测和观测到的海洋潮汐载荷的比较

我们研究了阿拉斯加的地壳和上地幔对海洋潮汐的质量载荷的弹性和非弹性响应。将板块边界天文台网络记录的 GPS 推断表面位移与与潮汐引起的海水重新分布相关的变形预测进行比较。我们使用运动学精确点定位算法处理来自 131 个站点的超过 5 年的 GPS 数据，并使用谐波分析估计潮汐贡献。我们还通过将海洋潮汐模型与从球对称地球模型导出的载荷格林函数进行卷积来正向计算载荷引起的表面位移。我们对三个频段的主要潮汐谐波进行了比较：半日 (M2)、日 (O₁) 和每两周 (M_f)。预测和观测到的海洋潮汐载荷 (OTL) 位移之间的矢量差异在所有三个频段和整个网络的空间分量中的幅度主要为亚毫米，在一些沿海地区具有高达几毫米的较大残差。使用来自标准地球模型的 Q 估计值来考虑上地幔中非弹性色散的影响，会使​​整个网络中 M2 谐波的残差平均减少 0.1-0.2 毫米，在某些单独的台站减少超过 1 毫米。对于相对较小的 M_f 潮汐，在电流测量误差内无法检测到非弹性色散 (<0.03 mm) 的影响。结合东北太平洋的局部海洋潮汐模型将 M2 垂直残差平均减少 0.2 毫米，在某些沿海站可改善高达 5 毫米。估计 M2 和 O₁ 潮汐垂直分量的 RMS 观测不确定性在二西格玛水平上约为 ±0.08 毫米（水平分量为 ±0.03 毫米），而 M_f 谐波为 ±0.21 毫米（水平为 ±0.07 毫米）组件）。对于 M2 谐波，预测和观测到的 OTL 位移之间的差异超过观测不确定性大约一个数量级。没有发现海洋潮汐和地球模型组合可以将 M₂ 残差降低到观测不确定性以下，并且没有一个单一的前向模型提供对所有潮汐谐波和空间分量的观测位移的最佳拟合。对于 O₁ 谐波，预测位移和观测位移之间的差异通常比观测不确定性大几倍。对于 M_f 谐波，这些差异大致在两个观测不确定性因素的范围内。我们发现通过去除网络均匀潮汐谐波位移可以显着减少预测和观察到的 OTL 位移之间的差异，并且剩余的差异表现出一些区域尺度的空间一致性，特别是对于 M2 谐波。我们认为 M₂、O₁ 和 M_f 潮汐的剩余差异不能完全用测量误差来解释，而是传达有关海洋潮汐模型缺陷和与球对称地球结构偏差的信息。其余的差异表现出一些区域尺度的空间连贯性，特别是对于 M 2 谐波。我们认为 M₂、O₁ 和 M_f 潮汐的剩余差异不能完全用测量误差来解释，而是传达有关海洋潮汐模型缺陷和与球对称地球结构偏差的信息。其余的差异表现出一些区域尺度的空间连贯性，特别是对于 M 2 谐波。我们认为 M₂、O₁ 和 M_f 潮汐的剩余差异不能完全用测量误差来解释，而是传达有关海洋潮汐模型缺陷和与球对称地球结构偏差的信息。