The existence of an inner core super-rotation questioned by teleseismic doublets

The first evidence for a differential rotation of the Earth's inner core with respect to the mantle comes from the detection of a variation, over 30 years, of the differential travel time anomalies of seismic core phases [Song, X., Richards, P.G., 1996. Seismological evidence for differential rotation of the Earth's inner core. Nature, 382, 221-224]. The comparison of teleseismic doublets provides a powerful means to measure such differential travel times and to test their temporal variation. Contrary to direct measurements of differential travel times, this technique does not require to have an accurate location of the seismic events. This method is first checked on examples of doublets for which no differential travel time is expected. These tests allow us to point out the difficulties, such as polarity reversals, which may be encountered in the use of the doublet method. It is then applied to deep Tonga events recorded in France and to the South Sandwich Islands events recorded at station College (COL), Alaska, which led Song and Richards to propose an inner core rotation. For the first path, we find no time variation, in agreement with previous studies. For the path from South Sandwich Island to COL, the doublet analysis shows that no significant temporal change in PKP travel time can be detected within the resolution of the method, which is about 0.05 s. The variation of the residuals observed by Song and Richards, about 0.3 s over 30 years, is in a large extent ascribable to hypocenter mislocations. Consequently, present seismological observations do not detect the inner core differential rotation because, if it exists, it is smaller than about 0.2°/year, the present detection capacity of seismology.