Recently, Zhou (2022) reported temporal change of seismic velocity in the Earth’s outer core based on relative travel time differences of SKS phase between some “doublets”. The study further suggested existence of a possible 2-3% density deficit in the outer core and a localized transient flow with a speed of ~40 km/year. We examine the seismic data of the best-quality “doublet” (event pair 19970503-20180910) reported in the study. We relocate the “doublet” based on a master-event relocation method (Wen , 2006) using the seismic data of the compressional waves that travel outside the outer core, including P or Pdiff , pP or pPdiff , pPn, PP or Pdiff Pdiff , and PcP waves recorded at the global seismographic network. The later event (20180910) is found to be located 14.20 km away, 204.33°NW, of the earlier event (19970503) with a source depth 1.45 km deeper. After correction for the effects of relative source location and origin time, SKS signals exhibit no discernable relative travel time differences between the two events at the frequency band ≥0.2 Hz at all the four most anomalous stations (COLA, INK, ULN, YAK) reported in Zhou (2022). However, SPdKS-SKPdS phases, which start bifurcating from the SKS phases at the distance range of those four reported anomalous stations, exhibit evident changes of waveform and travel time between the events. The “SKS signals” used inZhou (2022), which had a 50-s time window and were filtered from 0.01-0.05 Hz, contain signals of SKS and SPdKS-SKPdS phases. It is the changes of SPdKS-SKPdS phases, not that of SKS phases, that generate the apparent time shift in the low-frequency filtered “SKS signals” reported in Zhou (2022). The SPdKS-SKPdS phases of those reported anomalous stations sample a lowermost mantle region populated with ultra-low velocity zones (ULVZs). The separation of the two events is large and the SPdKS-SKPdS phases would sample ULVZs with slightly different paths between the two events, yielding different waveform and travel time (Wen & Helmberger , 1998). We conclude that there is no observable temporal change of seismic properties in the Earth’s outer core in the seismic data used in Zhou (2022) and the reported relative travel time difference in the “SKS signals” in Zhou(2022) is caused by waveform and relative travel time changes in SPdKS-SKPdS phases due to slightly different sampling paths to the ULVZs at the bottom of the mantle between the events.

Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquakes during 2010‒2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40‒100 days with amplitudes up to 1e−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4‒10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake-induced pore pressure change and elastic deformation due to an earthquake-triggered aseismic slip on a nearby fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation.

The properties of the inner core boundary (ICB) carry important information about the growth of the solid inner core and solidification of the liquid outer core. In this presentation, we study the ICB by analyzing the spectrum difference of PKiKP (a compressional wave reflected from the ICB) and PcP (a compressional wave reflected from the core-mantle boundary) phase pair. Our dataset includes the seismic data recorded by the Hi-net in Japan for two earthquakes occurring in Myanmar on 4th September 2009 and 4th February 2011, with their PKiKP waves sampling the ICB beneath mid-eastern China. Little waveform and spectrum differences are observed between PKiKP and PcP waves in the seismic data. By comparing spectrum shapes of the seismic data and the synthetics of models with various topographic fluctuations and transitional thicknesses of ICB, we conclude that the studied ICB region is flat and sharp. Within the resolvability of the seismic data, the ICB topographic fluctuation of the region is less than 200 m in height and the transition of the boundary is thinner than 100 m in thickness.