Seismic velocity and attenuation structures in the top of the Earth's inner core

Abstract

We collect a global data set of PKIKP and PKiKP phases recorded by the Global Seismic Network and many regional seismic arrays to study seismic structure in the top of the Earth's inner core. The PKIKP and PKiKP observations show different characteristics between those sampling the “eastern” hemisphere (40°E–180°E) of the inner core and those sampling the “western” hemisphere (180°W–40°E). PKIKP phases (1) arrive about 0.4 s earlier than the theoretical arrivals based on Preliminary Reference Earth Model (PREM) for those sampling the eastern hemisphere of the inner core and about 0.3 s later for those sampling the western hemisphere (131°–141°); (2) bifurcate at smaller epicentral distances for those sampling the eastern hemisphere, compared to those sampling the western hemisphere; and (3) have smaller amplitudes for those sampling the eastern hemisphere. Waveform modeling of these observations suggests two different types of models for the two “hemispheres” of the top of the inner core, with a model in the eastern hemisphere having a P velocity increase of 0.765 km/s across the inner core boundary, a small radial velocity gradient of 0.000055 (km/s)/km, and an average Q value of 250, and a model in the western hemisphere with a P velocity increase of 0.633 km/s across the inner core boundary, a radial velocity gradient of 0.000533 (km/s)/km and an average Q value of 600. The hemispherical difference of seismic structures may be explained by different geometric inclusions of melt and/or different alignments of iron crystals with anisotropic properties in both velocity and attenuation. We speculate that this large‐scale pattern of seismic heterogeneities may be caused by a large‐scale heat flow anomaly at the bottom of the outer core and/or different vigorousness of convection in the top of the inner core between the two hemispheres.

A method of spherical harmonic analysis in the geosciences via hierarchical Bayesian inference

“…5, left column). All this is different from the observed east-west dichotomy from the PKPbc-df data set, according to which the quasi-eastern hemisphere is faster than the quasiwestern hemisphere (Tanaka & Hamaguchi 1997;Wen & Niu 2002). This likely confirms that the effects from the lowermost mantle are too strong for our assumption to be valid, i.e.…”

Short wavelength topography on the inner-core boundary

“…6C) and the 10-km wavelength of the topography, our observations are in agreement with an eastward rotation of the inner core with respect to the mantle at a rate of Ϸ0.12-0.15°per year (see Materials and Methods), compatible with scattered-wave-and normal-mode-based estimates (11,21). The viscosity at the top of the inner core would have to be large enough to sustain such topography over a time scale of decades (4), or it could be sustained by small-scale convection in the top layers of the inner core (22). (Fig.…”

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

“…The relative amplitude differences between PKP precursors and PKIKP are also affected by inner core attenuation (Wen and Niu, 2002;Waszek et al, 2015) or topography at the CMB (Doornbos, 1978;Bataille and Flatté, 1988). To test the inner core effect, we checked PKIKP turning points of the earthquakes in the three sub-regions, and found that they were highly random and intermixed with each other.…”