Regional differences in upper mantle heterogeneity fron coherency measurements on array data

Abstract

When a ray-path passes through an homogeneous earth, the energy arriving at a seismic array should be relatively coherent with the signal complexity being dependent mainly on the source function. When the signal, however, encounters both lateral and vertical inhomogeneities along its path, much of its energy will be scattered such that the energy arriving at the array will be relatively incoherent. The signal complexity should therefore depend not only on the source function but also on the scattering characteristics of the ray-path. Measurements of the relative amounts of coherent and incoherent energy arriving at an array for different travel paths should therefore reveal regional differences in upper mantle heterogeneity. Approximately 300 earthquake recordings at the Gauribidanur medium aperture seismic array (GBA) in southern India (distance 15°–36°, azimuth 0°–360°): and over 65 earthquake recordings at the Yellowknife seismic array (YKA) in western Canada (distance 12°–30°, azimuth 180°–285°) were selected for this study. The analyses showed that there is relatively more heterogeneity for the continental upper mantle region to the north of GBA than for the corresponding oceanic structure to the south-west and southeast, with the structure above 400 km tending to be more complex than the structure below that depth. The YKA analyses showed that, in contrast to the GBA results, the upper mantle structure under the north-western part of the North American continent is progressively more heterogeneous with increasing depth between 400 and 750 km.