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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 (σ_ic) of travel time measurements from all possible sources except the inner core temporal changes. We set 2σ_icas 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 3σ_ic) 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.

 

TheH‐κmethod (Zhu & Kanamori, 2000,https://doi.org/10.1029/1999JB900322) has been widely used to estimate the crustal thickness (H) and the ratio ofPtoSvelocities (V_P/V_Sratio,κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy.H‐κstacking in case of azimuthal or radial anisotropy with flat Moho has been proposed but not for cases with plunging anisotropy and dipping Moho. Here we propose a generalizedH‐κmethod calledH‐κ‐c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations ofPsand its crustal multiples with back azimuth (θ). Systematic synthetic tests show that the arrival time variations can be well fitted by cosθand cos2θfunctions even for very complex crustal structures. Correcting for the back azimuthal variations significantly enhancesH‐κstacking. We verify the feasibility of theH‐κ‐c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliableH‐κvalues. Large differences inH(up to 5.0 km) andκ(up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world.

 

SUMMARY Due to the partly diffuse character of ambient noise, the retrieval of amplitude information and attenuation from noise cross-correlations has been difficult. Here, we apply the temporal reweighting method proposed by Weaver & Yoritomo to seismic data from the USArray in the central-midwest US. The results show considerable improvements in retrieved Green's functions in both symmetry and causality. The reweighting is able to make the effective incident noise field more isotropic (though not yet truly isotropic). It produces more robust amplitude measurements and also makes both the causal and anticausal parts usable. This suggests that it could be widely applicable for retrieval of Green's functions from ambient noise for attenuation study. The results also suggest an alternative measure of signal-to-noise ratio that complements the conventional one.