A map of inner core scatterers from beamforming the LASA array

Abstract:

Earth’s inner core (IC) is a central but poorly understood frontier. Understanding its structure, motion, and history all pose formidable challenges. Temporal changes and roughly hemispherical structure of seismic velocity variation, anisotropy, thickness of the upper isotropic layer, and attenuation have been estimated. Interpretation of these heterogeneous IC features has so far been incomplete and often contradictory. The features have been variously attributed to layering during formation, IC boundary deposition and melting, internal convection, anisotropic texturing, partial melt near the boundary, and perhaps modulated by interactions with the thermal boundary conditions of the overlying mantle.

We will report on refined beam-forming techniques, applied to 1970s data from the dense Large Aperture Seismic Array (LASA) to explore the distribution of fine-scale IC heterogeneity. Back-scattering of high-frequency (>1Hz) waves from fine-scale IC heterogeneity has been recognized for some time [Vidale and Earle, 2000] but has many possible explanations. We have recently found that large arrays have good resolution to map the heterogeneity causing the backscattering [Vidale, 2019].

We show below a preliminary model of the scatterers in the northwest direction from LASA. It agrees with existing models with greater scattering in the eastern hemisphere, and suggests that the scatterers are strongest in the shallow IC, and more concentrated locally in some areas than others. We are currently adding data from additional networks. Our goal is to address the geological history and geophysical processes near the center of the Earth.

Figure - Preliminary IC scattering strength distribution at two depths. The black lines mark a boundary between the eastern and western hemispheres [Yu et al., PEPI, 272, 2017].