Observed precursor traveltimes and amplitudes. (a) Map of interpolated differential S410S–S660S traveltimes relative to the global mean. Each cap is represented by a dot at the corresponding mean bounce point location. (b) Comparison of P410P and S410S traveltimes (blue) and amplitudes (orange). A 1:1-relation is depicted by the solid line. The dashed line represents the approximate P- over S-wave velocity ratio of the upper mantle (slope of 1.82). Pearson’s correlation coefficient is denoted by χ.