Abstract The earthquake coda is the residual vibration energy long after the main arrivals of an earthquake. Previous studies of coda waves have focused on high frequencies and small earthquakes, which are generally attributed to the scattering from the Earth's internal heterogeneities. Here we examined the early coda of great earthquakes at long periods (50 to 300 s) at time window of 10,000 to 50,000 s after the origin time. We used 10 major earthquakes (magnitude > 8.0) in recent decades that were recorded at global seismic networks. We found that the coda energy can be well represented by a simple exponential decay model for the early coda. The variation of the initial coda energy with distance is dominated by the effect of geometric spreading, suggesting that the scattering of the Earth's internal heterogeneities contributes little to the long-period early coda. The energy decay rate is largely insensitive to earthquake source (location, magnitude, focal mechanism, or rupture process), station location, or 3D variation of the earth's internal structure, making it ideal to constrain the average global attenuation structure. A small modification of the quality factor based on a global reference model can fit the coda energy decay well. The long-period coda energy may potentially provide a new way to determine the seismic moment of a major earthquake.

中文翻译：

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大地震长周期尾波的振幅和衰减

摘要 地震尾波是地震主要到达后很久的残余振动能量。之前对尾波的研究集中在高频和小地震上，这通常归因于地球内部异质性的散射。在这里，我们在起源时间后 10,000 到 50,000 s 的时间窗口检查了长时间（50 到 300 s）大地震的早期尾声。我们使用了近几十年来在全球地震网络中记录的 10 次大地震（震级 > 8.0）。我们发现尾声能量可以用早期尾声的简单指数衰减模型很好地表示。初始尾能量随距离的变化主要受几何扩散效应的影响，表明地球的散射是 s 内部异质性对长期早期尾声贡献很小。能量衰减率在很大程度上不受地震源（位置、震级、震源机制或破裂过程）、台站位置或地球内部结构的 3D 变化的影响，使其成为约束平均全球衰减结构的理想选择。基于全局参考模型的品质因子的小修改可以很好地拟合尾波能量衰减。长周期尾波能量可能为确定大地震的地震矩提供一种新的方法。使其成为约束平均全局衰减结构的理想选择。基于全局参考模型的品质因子的小修改可以很好地拟合尾波能量衰减。长周期尾波能量可能为确定大地震的地震矩提供一种新的方法。使其成为约束平均全局衰减结构的理想选择。基于全局参考模型的品质因子的小修改可以很好地拟合尾波能量衰减。长周期尾波能量可能为确定大地震的地震矩提供一种新的方法。