Locating scatterers in the mantle using array analysis of PKP precursors from an earthquake doublet

Abstract

Well separated individual PKP precursors observed at the Yellowknife seismic array (YK) for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation. Based on the comparison of the waveforms of stacked individual precursors and those of PKIKP phases, we are able to determine that most of these precursors originate from scattering of the PKPbc (rather than the PKPab) branch above the B caustic on the receiver side. This allows a reliable location of the scatterers in the lower mantle. Their depths range from 2890 km (the CMB) to 2270 km, scattering angles range from 45.8° to 16.0°, and surface projections range from southern Ontario to northern Saskatchewan in Canada. These locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north America. This suggests that the subducted slab remnants under north America have retained their compositional signature. The fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended, which should be confirmed using broadband data. Average differences in measured slowness and back-azimuth of the doublet precursors are as small as 0.08 s/deg and 1.4 deg, respectively. Our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays.