Inner Core Rotation Captured by Earthquake Doublets and Twin Stations

Abstract

The Earth's solid inner core (IC) and mantle are separated by the fluid outer core. Differential rotation of the IC relative to the mantle was expected from the electromagnetic torque between the magnetic field generated in the fluid outer core and the conducting solid IC (Glatzmaier & Roberts, 1995;Gubbins, 1981). The prediction was first observed by Song and Richards (1996) from temporal changes of seismic waves through the IC, which generated broad interests as well as rigorous debates over the years. The existence of IC temporal changes was subsequently supported (

Comment on “Inner Core Rotation Captured by Earthquake Doublets and Twin Stations” by Yang and Song

“…In conclusion, Yang and Song (2022) presented no seismic evidence that would require an inner core differential rotation with respect to the mantle. On the contrary, it adds another example of invalid logic of claiming existence of inner core differential rotation and faulty postulations of inner core differential rotation and “a local velocity gradient” at the top of the inner core to a pile of seismic evidence in the literature that are contradictory to the hypothesis of inner core differential rotation (Yao et al., 2015, 2019, 2021).…”

“…Since two neighboring stations would exhibit similar patterns of temporal change of inner core phases, but possibly in different magnitudes, it is not surprising that the temporal change rate (i.e., annual temporal change) observed at one station (AAK) correlates with the difference of temporal changes observed at the two neighboring stations. Judged from that the differential PKiKP‐PKIKP times are always shorter in AAK observations than KZA observations (Figure 3, Yang and Song (2022)) and the temporal change is in the trend of shorting the differential travel time (Figure 3a, Yang and Song (2022)) or PKiKP arrives progressively earlier over time, the largest temporal change is observed at AAK and the inner core surface of AAK data sampling is closer to the center of the temporal change. As the sampling regions of the inner core surface for HORS and KZA data have a similar distance away from the major region of temporal change of the inner core surface, these two stations experience similar waveform changes from the temporal change of the inner core surface.…”

“…The close proximity of the AAK and KZA data sampling paths in the inner core surface and the Fresnel zone (bottom‐right inset, Figure 1a) effects would suggest that the inner core phases observed at these two stations will exhibit similar manners of temporal change, albeit in different magnitudes depending on the details of the inner core surface change. The “spatial ddt” in Yang and Song (2022), or the difference of the differential PKiKP‐PKIKP (CD‐DF) times measured in the neighboring stations, is simply the difference of the individual temporal PKiKP changes observed in the two stations. Since two neighboring stations would exhibit similar patterns of temporal change of inner core phases, but possibly in different magnitudes, it is not surprising that the temporal change rate (i.e., annual temporal change) observed at one station (AAK) correlates with the difference of temporal changes observed at the two neighboring stations.…”

“…Thus, 07 HORS waveform can be used to replace 07 KZA waveform in the core evidence of Yang and Song (2022). If we follow the logic of Yang and Song (2022) that waveform similarity between the neighboring stations across the doublet indicates an inner core shift, the inner core patch sampled by 99 AAK data would have simultaneously shifted eastward (in more exact, SE45°) to the position sampled by 07 KZA data (as Yang and Song (2022) claimed) and westward (in more exact, NW48°) to the position sampled by 07 HORS data! This contradictory conclusion indicates that there is no validity of the logic presented in Yang and Song (2022) in concluding the existence of inner core differential rotation based on waveform similarity between neighboring stations across a doublet.…”

“…Instead, a localized and episodic inner core surface change provides a physically consistent explanation to the seismic data. The core evidence of their claiming inner core differential rotation relied on the waveform similarity between the recording of AAK in the earlier event of the doublet (99 AAK) and the recording of KZA in the later event of the doublet (07 KZA) (Figure 2c in Yang and Song (2022)). Because of the similarity, they concluded "This is clear evidence that the KZA path samples the vicinity of the patch of the IC that was sampled by the AAK path earlier for the corresponding pair."…”