Evolution of the correlation wavefield extracted from seismic event coda

The seismic correlation wavefield constructed from stacked cross-correlograms of event signals displays a wide range of features as a function of inter-station distance. The character of such correlation arrivals changes markedly with the segment of the wavefield employed. All such correlation arrivals arise from the interaction of seismic phases that have a common slowness at the pair of stations being correlated. It takes some time before a clear correlation field is established, but after 1 h from event initiation a weak version of the regular wavefield emerges accompanied by many phases that have no counterpart in the direct source excitation. Such arrivals are produced by the interaction of seismic phases with common propagation legs, and have time-distance behaviour controlled by the differences in accumulated phase. The regular phases fade with time and then distinct arrivals in the correlation field arise when there are many ways in which combinations of seismic phases have the same difference in propagation legs. There are many more such possibilities for steeply travelling waves in the late coda, so that a relatively stable correlation field develops. The properties of the correlation field as a function of time can be well described by using a representation in terms of generalized rays supplemented by the contribution from the fundamental mode Rayleigh wave.