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Constituents of Vertical-component Coda Waves at Long Periods

机译：垂直分量尾波在长时间内的成分

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Aki (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude 7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than 35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f-k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh waves for synthesizing the envelope. At lapse times later than 20,000 s–35,000 s, higher modes with phase velocities around 20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus earthquake. The small coda attenuation factor from (1.90±0.23) × 10−3 to (2.38±0.32) × 10−3 estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very large lapse times.
机译：Aki（1969）首先将局部地震的尾波建模为散射面波的叠加。本文试图在很长的时间间隔内长时间澄清表面波尾声的成分。对于7级或7级以上的大地震，从地震发生开始，垂直分量振荡在90到180 s的时间内持续存在超过20个小时。尽管可以通过假设分布在整个地球上的异质性来假设不相干的散射瑞利波来成功建模尾声包络线的早期部分，但是观测到的尾声包络线的后期部分（大约晚于35,000 s）在系统上比理论预测的振幅更大。为了弄清这种差异的原因，我们使用f-k功率谱分析研究了日本F-net记录的三场大地震的垂直分量地震图的组成。我们发现，在每个包络的较早部分，速度约为3.7 km / s的直接和散射基模瑞利波占主导地位。它证明了使用基本瑞利波的散射模型来合成包络是合理的。在低于20,000 s–35,000 s的间隔时间中，具有约20 km / s的相速度的较高模式将成为主导。对于焦点较深的地震，发现过渡到较高模式的主导地位的时间变得更早。根据在全球分布的IRIS台站记录的后期尾随包络估算出的小尾声衰减因子从（1.90±0.23）×10−3 到（2.38±0.32）×10−3 也与衰减因子一致根据PREM的球体模式我们可以解释为，由于较大的速度色散和/或散射，高模波在较大的经过时间处均匀分布，并且由于下地幔的衰减较小，它们在基本模波中占主导地位。发现Aki提出的尾声衰减测量即使在很长的时间间隔内也非常有用。

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