Seismic coda is composed of scattered wavelets originated from various heterogeneities. The phase composition of regional seismic coda still remains unknown, despite its use for several decades. This is caused partly because the ray paths of scattered wavelets in coda are not on the great-circle path between a source and receiver. We examine the constituent original phases of regional coda with the help of a source-array analysis. A set of uniform sources that are essential for a source-array analysis is organized with underground nuclear explosions. Strong Rg-origin energy is observed in the coda at frequencies of 0.2-0.8 Hz, and it lasts more than 700 sec until the end of records. The coherent energy in the coda reduces with frequency. It constitutes about 20% of the total coda energy at frequencies of 0.2-0.4 Hz, and 12% at frequencies of 0.4-0.8 Hz. The other 80% of coda energy in 0.2-0.8 Hz is mixed with complex phases from various untraceable origins. The Rg energy is the most influential component in the construction of low-frequency regional coda. On the other hand, the coda at higher frequencies, 0.8-3.2 Hz, is observed to be mixed with complex phases that cause the wave field to be diffused. The observation of Rg-origin energy at the regional coda suggests that scattered energy from phase coupling of Rg is not significant compared to Rg-to-Rg scattered energy.
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