Amplitude and decay of long-period coda of great earthquakes

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.