The lack of equipartitioning in global body wave coda

Correlation of the coda of large earthquakes can be used to retrieve body waves at teleseismic distances. This retrieval depends on waves propagating with equal energy in all directions (equipartitioning). We carry out a beamforming analysis of body waves at periods of 10 and 40 s recorded at USArray and show that the late coda, up to 10 h after the Okhotsk earthquake, is dominated by waves propagating in the great circle direction; late coda waves are not equipartitioned. This implies for seismic interferometry that teleseismic body waves can only be extracted from the coda when the earthquake and the receivers are located on a great circle. This happens automatically when the employed stations are antipodal or when the autocorrelation of waves is used. The use of long-period ambient seismic noise relies on the distribution of sources because scattering only marginally randomizes directions. This provides insight into recent observations of core phases extracted from seismic interferometry.