Heterogeneity of the Earth's inner core boundary from the characteristics of the reflected waves: numerical models and observations

Olga Usoltseva1, Vladimir Ovtchinnikov1

1Sadovsky Institute of Geosphere Dynamics, Russian Academy of Sciences


Detailed estimates of the structure of the Earth's inner-outer core transition are necessary for a deeper understanding of the mechanisms that generate and sustain the Earth's magnetic field. Occurring at the inner core boundary (ICB) the dynamic processes (growth of the inner core (IC), release of the light elements) cause the presence of irregularities at the IC surface. The existing concepts (hemispherical asymmetry: crystallization in the West and melting in the East, isotropic and anisotropic mosaic patches) imply different scale and strength of these features at the ICB. We have analyzed the influence of the different ICB topography classes on the seismic wave propagation around PKiKP arrival time (PKiKP is the wave reflected from the IC surface) at epicentral distances up to 50°. Modeling was performed for a 2Hz source using the spectral-element method Specfem2D. The height of the mountains and troughs varied from 1 to 3 km, the recurrence period from 14 to 30 km, and the relief was specified as sinusoidal, stepped, sine modulus, and alternating sine modulus and smooth boundary. Numerical calculations were performed for the modified model ak135. Since topography decreases PKiKP amplitude of ~1.5 times at maximum elevation of 1 km, ~2.5 at 2 km, ~4.5 at 3 km. Step topography decreases PKiKP amplitude of ~1.5 times. If there is sine modulus topography with 1 km height then smaller amplitude changes are observed, and if the alternation of sine modulus and smooth boundary have place then even smaller changes are. Model amplitude dependencies were compared with more than 100 measured PKiKP amplitudes from the 24.05.2013 deep earthquake in the Sea of Okhotsk at 14:56 UTC, recorded by Geophysical Survey of RAS stations, the XL temporary network in Mongolia and the permanent seismic network in Alaska at 30-40°. Comparison shows that a gradient zone 5-10 km thick at the top of the inner core or sinusoidal terrain roughness of 2-3 km high is possible in the latitudinal belt 50-60°N beneath the western tip of the Pacific Ocean. Beneath East Asia the presence of roughness 1-2 km high is likely. Comparison of real, model and noise spectrograms indicates that the area under East Asia is characterized by greater ruggedness of the terrain. The new data extends the region probed by PKiKP waves from other work and completes the initial map of heterogeneities [Wang, Vidale, 2022].