Origin of Temporal Changes of Inner-core Seismic Waves

Abstract:

Time-varying PKIKP waves (traversing the inner core) from repeating earthquakes (doublets) provide compelling evidence for the temporal changes of the inner core and have mostly been interpreted with the model of inner core rotation shifting its heterogeneous structures. On the other hand, at the distance range between 128° and 142°, PKiKP waves (reflecting from the inner core boundary, ICB) from some repeating earthquakes occasionally exhibit temporal changes as well, which has been interpreted as evidence for the changes of the ICB over several years. The model of temporal changes at the ICB without any rotation is also favored by some seismic studies. The two models provide different constraints to the internal dynamics of the earth, so that it is important to distinguish the origin of the time-varying inner-core waves. We used 15 pairs of repeating earthquakes with clear PKP arrivals and later SKP (or PP) arrivals to investigate the temporal changes of PKIKP and PKiKP. Using SKP/PP phase as reference avoids possible clock errors of seismic stations and helps distinguish the source of the anomalies. Our results show that the changes are mostly from the PKIKP phase instead of the PKiKP phase. The later-arriving PKiKP phase may also be affected by the temporal changes of the PKIKP phase and its coda. A complication of using SKP as a reference is the sensitivity of the differential time between SKP and PKIKP (or PKiKP) to the relative depth of the doublet events. But because of the small depth difference (within 100-150 m), the influence is not significant. The results suggest that the temporal changes come from the body of the inner core and rule out the possibility of ICB as the sole origin. The inner core rotation provides the simplest and most reasonable explanation to the observations.