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 wi th respect to the mantle comes from the detection of a variation, over 30 y ears, of the differential travel time anomalies of seismic core phases [Son g, X., Richards, P.G., 1996. Seismological evidence for differential rotati on of the Earth's inner core. Nature, 382, 221-224]. The comparison of tele seismic doublets provides a powerful means to measure such differential tra vel times and to test their temporal variation. Contrary to direct measurem ents 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 doublers for which no differential travel time is expected. Th ese tests allow us to point out the difficulties, such as polarity reversal s, which may be encountered in the use of the doubler method. It is then ap plied to deep Tonga events recorded in France and to the South Sandwich Isl ands events recorded at station College (COL), Alaska, which led Song and R ichards to propose an inner core rotation. For the first path, we find no t ime variation, in agreement with previous studies. For the path from South Sandwich Island to COL, the doubler analysis shows that no significant temp oral 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 ascribab le to hypocenter mislocations. Consequently, present seismological observat ions do not detect the inner core differential rotation because, if it exis ts, it is smaller than about 0.2 degrees/year, the present detection capaci ty of seismology. (C) 2000 Elsevier Science B.V. All rights reserved.