Wide-scale detection of earthquake waveform doublets and further evidence for inner core super-rotation

We report on more than 100 earthquake waveform doublets in five subduction zones. These doublets are important for studying earthquake physics and the precision of location estimates contained in standard earthquake catalogs, as well as the study of temporal changes of the inner core. We find six different paths through the inner core for which is it possible measure accurately any changes in the travel time of PKIKP waves. We find three paths for which such change is no more than 0.005 s/year; and three paths with significant change, on the order of 0.1 s/year or more. Such a pattern of observations showing both presence and near absence of inner-core traveltime change can be explained by the geometry and relative direction of ray path, lateral velocity gradient, and inner-core particle motion, due to an eastward super-rotation of a few tenths of a degree per year.