Overview of observed precursor phases. Observed PP (a) and SS waveform stacks (b), respectively, for the caps shown in Fig. 1(c) and the numbering detailed in Table S1. Bootstrap uncertainties are shown as dashed lines. Blue/red colours represent positive/negative amplitudes, respectively. Systematic search for SdS (c) and PdP (d) reflections. Amplitudes (colour-coded) of local maxima in the observed waveform stacks are plotted as a function of picked traveltime (ordinate; relative to the free surface reflection) and number of traces per stack (abscissa). Larger/smaller dots represent smaller/larger bootstrap uncertainties associated with the detected reflection, respectively. The total number of detected reflections as a function of traveltime is displayed by the grey histogram. In all plots, theoretical arrival times of the 410-, 520- and 660-underside reflections (P410P/S410S, P520P/S520S, P660P/S660S) in the seismic reference model IASP91 (Kennett & Engdahl 1991) are shown in green.