Non-steady Slow Rotation of the Inner Core from Repeating Earthquakes

The temporal changes of seismic waves sampling the inner core are well-established, particularly from high-quality waveform doublets (repeating earthquakes). The temporal changes have been interpreted as the rotation of the inner core shifting lateral velocity gradients in the uppermost inner core. However, some recent studies have argued against the rotation of the inner core from inconsistent rotation rates and favored a model of localized temporal variations at the inner core boundary (ICB). In this study, we carefully analyzed an updated database of highest-quality repeating earthquakes. We proposed an interpretation of non-steady and relatively slow rotation of the inner core, based on the following observations. First, the inconsistency of the rotation rates is not sufficient to rule out the inner core rotation, as the underlying short-wavelength structure is highly variable and difficult to resolve accurately. Second, the rotation is essential to account for the time-varying signals, as the source of the temporal variations is mostly from the interior rather than the ICB. The time-varying reflective PKiKP arrivals, the key evidence for localized temporal variations of the ICB, could be influenced by the coda of the precedent time-varying refractive PKIKP. Third, we found a new pathway of significant temporal changes with dense sampling from clusters of repeating earthquakes along the Chilean trench to a few stations in Kazakhstan. The rotation rate is only ~0.06 degree per year on average from 1995 to 2014, due to a very sharp and small-scale patch of lateral velocity gradient under North Atlantic. Last, the rotation of inner core seems to decelerate to nearly zero at some time between 2005 and 2009, as there are no obvious temporal variations from repeating earthquakes in the recent decade. The non-steady and relatively slow rotation of the inner core may reconcile the discrepancy of different studies and may explain the time-varying inner-core reflections even without considering temporal variations at the ICB.