Imaging laterally varying regional heterogeneities from seismic coda using a source-array analysis

Seismic coda is composed of scattered waves originated from various kinds of heterogeneities, whose locations are not confined in the great-circle ray-path direction between the source and receiver. We devise a technique to image laterally varying regional heterogeneities using single-station seismograms for clustered events. We analyze regional seismic records of the Borovoye seismic station for the Balapan nuclear explosions in Kazakhstan. Scattered waves that arrive coherently over the source arrays, are extracted by directional beamformings. The locations of scatterers responsible for the scattered waves are mapped from the beamforming direction, travel time and slowness (equivalently, phase velocity). The illuminated locations of heterogeneities are highly correlated with the structural variations in surface topography, crustal thickness and sedimentary thickness. The influence of the structural variations on seismic waves is quantified in terms of scattering intensity and quality factor. The scattering properties are observed to vary with phase due to the difference in frequency content and phase velocity. The proposed technique appears to be useful for a study of active tectonic regions with limited monitoring stations.