Illustration of the solid medium deformation due to a fluid pressure perturbation with four horizontal apparent velocities (1500, 660, 340 and 20 m s⁻¹), corresponding to the four scenarios in Section 3. For each horizontal apparent velocity, we compute the frequency-domain vertical compliance at the surface (central column). The real and imaginary parts of the compliance are plotted in dashed and solid curves, respectively. We then convert the compliance to the time domain to represent the ground-motion waveform (right-hand column). The elastic parameters of this solid medium are in Table 1. From these parameters, we compute the phase velocities of the Rayleigh wave and the leaky-mode surface wave (a). The same figure for the horizontal compliance is Fig. S1.