Probing the Inner Core with P′P′

Abstract:

Geophysical observations of the inner core today improve our understanding not just of the processes occurring in the core at the present, but also those that have occurred in the past. As the inner core freezes it may record clues as to the state of the Earth at the time of growth, although the texture of the inner core may also be modified through post-solidification mechanisms. The seismic structure of the inner core is not simple; the dominant pattern is one of anisotropic and isotropic differences between the Eastern and Western 'hemispheres' of the inner core. Additionally, there is evidence for an innermost inner core, layering of the uppermost inner core, and possibly super-rotation of the inner core relative to the mantle.

Most body wave studies of inner core structure use PKP-PKIKP differential travel times to constrain velocity variations within the inner core. However, body wave studies are inherently limited by the geometry of fixed sources and stations, and thus there are some areas of the inner core that are relatively under-sampled, even in today’s data-rich world. Here, we examine the differential travel times of the different branches of P′P′ (PKIKPPKIKP and PKPPKP), comparing the arrival time of inner core sensitive branch, P′P′df, with the arrival times of branches that only reach the outer core. By using P′P′ we are able to exploit alternative ray geometries and sample different regions of the inner core to those areas accessible to studies which utilize PKIKP.

We use both linear and non-linear stacking methods to make observations of small amplitude P’P’ phases. These measurements match the broad scale hemispherical pattern of anisotropy in the inner core.