Temporal Changes of the Inner Core From Globally Distributed Repeating Earthquakes

Abstract

Time‐dependent travel times of seismic waves traversing the inner core from repeating earthquakes provided compelling evidence for an inner core differential motion. Here we conducted a systematic search for strong repeating earthquakes in the last three decades to examine the global pattern of temporal changes of the inner core. We performed extensive analyses on the quality of the repeating earthquakes and quantified the error (σic) of travel time measurements from all possible sources except the inner core temporal changes. We set 2σic as a threshold for judging whether an inner core temporal change is significant. No significant temporal changes were found in most parts of the inner core, but large temporal changes (over 3σic) were observed beneath four regions in Northern Hemisphere (North Atlantic, Northeast Pacific, Russian Far East/Sea of Okhotsk, and Europe/North Africa), besides the well‐known Central America anomaly in previous studies. Most large temporal changes were associated with time lapses of over 6 years and smaller distances, possibly resulting from the rotation shifting the laterally varying top 300 km of the inner core. A new path sampling North Atlantic suggested a small‐scale and steep lateral velocity gradient of the inner core and a slow eastward inner core rotation of 0.051°/year. Small‐scale lateral variations may reconcile large difference in the estimates of the inner core rotation rate. We also observed enigmatic very large abrupt temporal changes (as short as 44 days), which may be related to disturbances caused by the great Sumatra earthquakes.

Inner Core Rotation Captured by Earthquake Doublets and Twin Stations

“…We did make a few assumptions that the mantle influence in the ddt is small, that each local gradient is linear, and that the IC lateral shifts are from the rigid body rotation of the IC around the spin axis of the Earth, all of which seem reasonable. The new rate is also consistent with relatively low rates (0.05–0.10°/yr) in recent studies (Vidale, 2019; Yang & Song, 2020b) and remarkably close to the most recent estimate of 0.125°/yr from IC scattering waves by Wang and Vidale (2022). Such a slow rotation can also reconcile with a marginal rate of 0.13 ± 0.11°/yr from the normal‐mode data (Laske & Masters, 2003).…”

“…The IC temporal changes have been explained by an IC differential rotation that shifts laterally varying IC structures (Creager, 1997; Vidale, 2019; Vidale et al., 2000; Yang & Song, 2020b; Zhang et al., 2005). However, besides the driving force from the electromagnetic torque, the core‐mantle gravitational coupling tends to lock the IC at the position of gravitational equilibrium (Buffett, 1997).…”

Misinterpreted Seismic Evidence for Localized Rapid Changes of the Inner Core Boundary Surface

“…We need to clarify that the IC lateral gradient is not our "postulation," but a well-established observation reported by many previous studies (e.g., Creager, 1997;Song, 2000;Yang & Song, 2020b). The lateral velocity gradient is the dependence of travel time anomaly on the lateral sampling location.…”

“…While citing only the papers from their own group (Wen, 2006;Yao et al, 2015Yao et al, , 2019Yao et al, , 2021, TW23 ignored the disadvantageous evidence reported in many other studies, for example, to name a few: temporal changes of PKP DF (e.g, Song & Poupinet, 2007;Song & Richards, 1996;Sun et al, 2006;Yang & Song, 2020bZhang et al, 2005), inner-core scattering waves (e.g., Pang & Koper, 2022;Wang & Vidale, 2022), and antipodal PKIIKP arrivals (Tsuboi & Butler, 2020). Yang and Song (2020a) have systematically evaluated the temporal change of DF and CD with global doublets.…”

“…Wen (2006) found the temporal changes of the CD arrival times based on a relative relocation, which searches out the relative location and origin time of the two repeaters by minimizing the relative time shifts between the non-IC waveforms at the same stations. The total error of time shift from measurement precision with cross-correlation and tiny mantle heterogeneity is generally about 0.01 s (Yang & Song, 2020b;Yao et al, 2015). However, waveform misalignments, after removing the influences of origin time errors and event separations based on relative relocations, are also commonly observed in non-IC mantle arrivals in AAK and many other global stations from several hundred doublets, and are used to quantify the clock errors .…”