Seismic codas are usually characterized by the values of coda Q (Qc). However, interpretation of this quantity is often tricky because of its frequency dependence and acute sensitivity to subjective theoretical assumptions and processing parameters. Here, a simpler and physically more consistent parameterization of coda envelopes is proposed by noting that their temporal decay rates are often nearly frequency-independent. This weak frequency dependence shows that codas mostly consist of elastic reverberations and scattering on larger-scale structures, and the subwavelength-scale scattering and Q-type wave attenuation are weak. A recent study of the eastern Indian Shield by Singh et al. (in this journal) gives an illustration of such elastic coda. From that study, the inferred Qc steeply increases with frequency, lapse times, window lengths, and distances from the seismic station. However, we show that all of these dependencies of Qc represent a common artifact of the acquisition geometry and inversion procedure. In an alternate interpretation, we explain the same coda envelopes by two frequency-independent properties of the Earth’s subsurface: geometrical attenuation denoted γc,Earth and effective Q denoted Qc,Earth. Based on these parameters, the model becomes independent of theoretical assumptions and comparable to other areas, and the acquisition/inversion artifact is reduced. The estimated γc,Earth is above 0.01 s−1, which is also found in other areas of active tectonics. The effective attenuation is weak (Qc,Earth > 5700, likely below the measurable level), which is typical for stable tectonic areas. The data indicate near-surface resonances beneath the recording station. Effects of these resonances on coda envelopes also exceed those of Q-type attenuation. Thus, in the eastern Indian Shield and likely many other areas, coda envelopes are principally controlled by elastic structures such as crustal and near-surface layering, and not necessarily by the S-wave Q and uniformly-distributed random, small-scale scattering as commonly thought.

中文翻译：

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东印度地盾内地震尾波包络的弹性特征

地震尾波通常用尾波 Q (Qc) 的值来表征。然而，由于其频率依赖性和对主观理论假设和处理参数的高度敏感性，对该量的解释通常很棘手。在这里，通过注意到它们的时间衰减率通常几乎与频率无关，提出了一种更简单且物理上更一致的尾声包络参数化。这种弱频率依赖性表明尾声主要由较大尺度结构上的弹性混响和散射组成，亚波长尺度散射和 Q 型波衰减较弱。Singh 等人最近对东印度地盾的研究。（在本期刊中）给出了这种弹性尾声的说明。从该研究中，推断的 Qc 随频率、失效时间、窗口长度、以及距地震台的距离。然而，我们表明 Qc 的所有这些相关性代表了采集几何和反演过程的共同工件。在另一种解释中，我们通过地球地下的两个与频率无关的属性来解释相同的尾波包络：几何衰减表示为 γc,Earth，有效 Q 表示为 Qc,Earth。基于这些参数，该模型变得独立于理论假设并与其他领域具有可比性，并且减少了采集/反演伪影。估计的 γc,Earth 高于 0.01 s−1，这在其他活动构造区也有发现。有效衰减较弱（Qc,Earth > 5700，可能低于可测量水平），这是稳定构造区的典型特征。数据表明记录站下方的近地表共振。这些共振对尾声包络的影响也超过了 Q 型衰减的影响。因此，在东印度地盾和可能的许多其他地区，尾波包络主要受弹性结构（如地壳和近地表分层）控制，不一定受 S 波 Q 和均匀分布的随机小尺度散射控制，如一般认为。