A Sharp Structural Boundary in Lowermost Mantle Beneath Alaska Detected by Core Phase Differential Travel Times for the Anomalous South Sandwich Islands to Alaska Path

Abstract

We report anomalous core phase PKPbc-PKPdf differential travel times relative to 1-D spherically symmetric model with a uniformly anisotropic inner core recorded by stations in Alaska for South Sandwich Islands (SSI) earthquakes. The data sample the inner core for the polar paths, as well as the lowermost mantle beneath Alaska. Our major observations are the following: (1) fractional travel time residuals of PKPbc-PKPdf increase rapidly within 2°, (2) a clear shift of the residual pattern could be seen for earthquakes with different locations, and (3) the residuals show systematic lateral variation: at the northern part, no rapid increase of residual can be seen. A structural boundary with a P wave velocity contrast of about 3% at the lowermost mantle beneath East Alaska is invoked to explain the observation, and the required strength of anisotropy in the quasi-western hemisphere of the inner core might be reduced if those anomalous data are excluded from analysis.