Different branches of seismic waves generated by an earthquake pass through different parts of the deep Earth (A). Seismograms from two earthquakes 10 years apart appear similar (B) indicating that the earthquakes must be nearly identical. But a detailed examination shows that the part of the seismic wave that passed through the inner core (PKP(DF)) made the trip faster in 2003 than in 1993 (C). Image Credit: Zhang, Song, et al
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