TEXTURE OF THE UPPERMOST INNER CORE FROM FORWARD- AND BACK-SCATTERED SEISMIC WAVES

Abstract

Body waves interacting with the boundary of the solid inner core at narrow and wide angles of incidence provide independent constraints on a heterogeneous texture that may originate from the process of solidification. The equatorial, quasi-eastern hemisphere, of the uppermost 50-100 km of the inner core is characterized by a higher isotropic P wave velocity, lower Qα's inferred from PKIKP, and simpler PKiKP pulses compared to adjacent regions in the western hemisphere and polar latitudes. Compared to this region, the adjacent western (primarily Pacific) equatorial region is characterized by higher Qα's and a higher level of coda excitation following PKiKP. Lateral variations in both inner core Qα inferred from transmitted PKIKP and inner core heterogeneity inferred from the coda of reflected PKiKP can be modeled by lateral variations in a solidification fabric. In an actively crystallizing eastern equatorial region, characterized by upwelling flow in the outer core, fabrics that explain strong attenuation and the absence of attenuation and velocity anisotropy in short range (120°-140°) PKIKP and weak PKiKP codas have an anisotropy of scale lengths with longer scale lengths in the vertical direction, perpendicular to the inner core boundary. In less actively solidifying regions in the equatorial western hemisphere, longer scale lengths tend to be more parallel to the inner core boundary, consistent with outer core flow tangent to the inner core boundary or viscous shearing and recrystallization in the horizontal direction away from more actively crystallizing regions in the eastern hemisphere. This texture is less effective in attenuating PKIKP by forward scattering. Lateral variation in the equatorial western hemisphere between vertical versus horizontal oriented plate-like textures may explain lateral variations from weak to strongly back-scattered PKiKP coda and from strong to weak velocity and attenuation anisotropy in short range PKIKP. © 2007 Elsevier B.V. All rights reserved.