Seismic detection of oceanic crust in Earth’s lower mantle and its relation to large-scale mantle structure

Abstract:

Seismology permits direct observation of structure within the Earth. Different seismological techniques allow features of a range of sizes to be resolved. Observations of unpredicted high frequency energy (e.g. > 1 Hz) provide evidence for the presence of small-scale heterogeneities (e.g. 1-13 km) within the Earth. Small-scale heterogeneities with anomalous elastic properties compared to the surrounding medium can scatter seismic waves thus generating unpredicted seismic arrivals that carry information about the scattering heterogeneities. Using small aperture seismic arrays and high-resolution processing methods we observe scattered P-waves that travel into the core (along a PKP path), but upon exiting the core are scattered back into the core and to the surface along a second path. The paths from the earthquake-to-scatterer and scatterer-to-array are both out of the great-circle plane. We call this PK*KP, and use it to deterministically locate heterogeneities within the lowermost 300 km of the mantle across nearly the whole globe. We find scattering heterogeneities to be distributed unevenly across the CMB both laterally and vertically with the majority of heterogeneities located close to the CMB and heterogeneities higher up into the mantle limited to a few regions. Heterogeneities concentrate within regions associated with the edges of the Large Low Shear Velocity Provinces in the lower mantle (LLSVPs). Using broadband records and analysis in different frequency bands we find scattered energy to be strongest in the 2-4 Hz band indicative of strong heterogeneities of ~4-7 km in size, although the heterogeneities show a wide spectrum of scales. The size and distribution of heterogeneities agrees with predictions of the scale and dynamic behaviour of viscously entrained subducted basaltic oceanic crust in the lowermost mantle. Thus, the observed scattering heterogeneities likely reveal the influence of convective processes on crust subducted into the lowermost mantle.