A Study on P Wave Heterogeneity in the Outermost Core from Long Period Body Waves

The core-mantle boundary (CMB) is the largest seismic discontinuity in the interior of the Earth, separating dynamic systems with remarkable contrasts in composition and material properties. We examined seismic structure of the core-side of the CMB to obtain evidence for stable staratification and lateral heterogeneity in the outermost core. Travel times of long period SKS, S2KS, S3KS phases recorded at the GDSN and WWSSN stations were analyzed for 71 deep earthquakes (depth > 200 km).
We constructed a velocity model of the outer core applying the Herglotz-Wiechert method to travel times of SKS and S2KS phases. Inhomogeneity index h estimated from this model inicated that outermost core in depth ranges of about 100 km from the CMB was characterized by h being greater than unity and these from 100 thorough 400 km was by h being smaller than unity, suggesting mild chemical inhomogeneity persisted in the outermost region in the core.
In order to reveal lateral heterogeneity in the outermost core, we analyzed the residuals of the differential travel times of S2KS-SKS and S3KS-S2KS. We applied spherical hamonics analysis to the geographical distribution of the residuals corrected for the lower mantle heterogeneity and found that pattern of degree 1 was a predominant feature. The average residuals were +0.6 seconds in one hemisphere including the Pacific Ocean (the P-hemisphere) and -0.6 seconds in the other hemisphere incluing the Atlantic Ocean and Africa (the A-hemispehre). We estimated that average perturbations of velocities in the outermost 200 km in the core were -0.3 and +0.3% in the P- and A- hemispheres, respectively.
We discussed geophysical implications of radial and lateral heterogeneities in the outermost core with regard to the characteristics such as stable staratication, temperature distribution and symmetrical axis. If the outermost core was chemically inhomogeneous and adiabatic, then temperatures immediately benath the CMB were relatively low in the P-hemisphere and high in the A-hemisphere. This pattern of temperature distribution was consistent well with that derived from geomagnetic study. Two poles of the symmetrical axis for the heterogeneity in the outermost core were located at the cental Pacific (5¡N, 160¡W) and centeal Africa (5¡S, 20¡E), which were close to the poles found in the lower mantle and crust. This suggested a hypothesis that outermost core should be regarded as some origins of activity at the Earth's surface through the mantle.
Seismological evidences obtained in the present study strongly suggested the existence of the stably stratified layer and lateral heterogeneity in the outermost core, which could provide the first clue as what is whole global tectonics including the core, mantle and crust.