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Near-Field Spectra of Large Earthquakes

机译：大地震的近场光谱

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We have studied the spectra of large subduction earthquakes in Chile at short epicentral distances. In this work, we concentrate on the M-w 8.2, Iquique earthquake of 1 April 2014. For such a large event, in most stations we cannot separate P and S waves; which arrive very close together and interfere to produce the static displacement field. We find that the displacement spectra observed in all the accelerograms are significantly different from the usual far-field Brune spectrum. Displacement spectra have clear omega-1 decay at low frequencies that we prove to be associated with the dominant role of near and intermediate field waves in the ground motion. The origin of the omega-1 decay is that at short distances, displacement contains a finite static displacement. We confirm this spectral behavior by comparing the spectra computed from accelerograms integrated to velocity with co-located GNSS recordings. Both spectra are flat at low frequencies and the low-frequency asymptote of the velocity spectrum is proportional to the static displacement determined from GNSS. We explore the transition from these near-field records to far-field ones and find that the usual omega-2 spectrum appears only at large distances compared to the size of the dynamic rupture zone. For large earthquakes, the displacement spectra have a term proportional to the moment spectrum and another proportional to moment rate spectrum. The term proportional to moment dominates the low-frequency behavior of accelerograms in the near and intermediate field, so that they do not follow the omega-square decay of the moment rate spectrum.
机译：我们研究了智利的大型俯冲地震的光谱，在短暂的震中距离。在这项工作中，我们专注于2014年4月1日的M-W 8.2。对于如此大的事件，在大多数地区，我们不能分开P和S波浪;它非常靠近并干扰静态位移场。我们发现，在所有加速局中观察到的位移光谱与通常的远场近距离光谱有显着不同。位移光谱在低频下具有清晰的ω-1衰减，我们证明与地面运动中的近乎和中间场波的主导作用相关联。 Omega-1衰减的起源是，在短距离时，位移包含有限静态位移。通过将从集成的AcceleroGrams的频谱与共同位于GNSS录制的速度进行比较，我们通过比较来确认这种光谱行为。两个光谱在低频下均匀，并且速度谱的低频渐近速度与从GNSS确定的静态位移成比例。我们探讨了从这些近场记录到远场的过渡，并发现与动态破裂区域的大小相比，通常的ω-2光谱仅出现在大距离。对于大地震，位移光谱具有与矩比例成比例的术语和另一个与矩速率谱的比例。与时刻成比例的术语占据了近乎和中间场的加速局的低频行为，因此它们不遵循瞬间速率谱的欧米茄衰减。

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