Evidence for inner‐core rotation from possible changes with time in PKP coda

Abstract

Inner‐core‐grazing PKP waves from seven French nuclear tests on Mururoa Island recorded at NORSAR appear to change over a decade. We compared filtered and stacked signals from the explosions, which took place from 1977 to 1987 and were separated by less than 10 km. The PKP energy 3 to 10 s after the first arrivals differs greatly between the NORSAR subarrays, and evolves over time. The small time shifts of coda arrivals, a few tenths of a second, are qualitatively consistent with shifts predicted for point scatterers in an inner core that rotates in the range 0.05° to 0.10° per year. There are also visible changes in the envelope of the PKP energy. Although the alternative of systematic shifts in source location cannot be ruled out, the apparent time evolution of PKP supports claims of slow inner core rotation [Song and Richards, 1996; Creager, 1997; Laske and Masters, 1999; Vidale et al., 2000] and offers a powerful new tool for its assessment.