Evaluating Contributions to SK(K)S Splitting from Lower Mantle Anisotropy: A Case Study from Station DBIC, Cote D'Ivoire

Abstract

Measurements of seismic anisotropy constitute a very important tool for examining patterns of flow and mineral properties in the Earth's mantle. A popular strategy for gaining insight into upper mantle processes is to examine the splitting of SK(K)S phases and interpret them in terms of upper mantle anisotropy and deforma- tion; in such studies, any contribution to splitting from anisotropy in the lower mantle is usually ignored. Here we present measurements of SKS and SKKS splitting at Global Seismograph Network station DBIC in the Cote D'Ivoire, which exhibits a very unusual pattern of shear-wave splitting. The splitting pattern is dominated by null measurements over a wide range of back azimuths, with non-null measurements found over a very limited back-azimuthal range; we also identified examples of dis- crepant SKS-SKKS splitting for the same event-station pair. Splitting at DBIC has previously been interpreted in terms of upper mantle anisotropy, but we argue that this splitting pattern can best be explained by an apparently isotropic upper mantle with a contribution from anisotropy in the lower mantle, likely in the D layer. Using station DBIC as a case study, we discuss the potential pitfalls in interpreting SKS-splitting measurements and suggest a set of best practices to decrease the like- lihood of misinterpreting shear-wave-splitting results at a seismic station. Online Material: Summary of events used in the study, with their associated splitting parameters.