Fig. 1: Overview of VF events.

a, Envelopes of recorded VF quality B events sorted by distance, plotted from 120 s before to 1,100 s after the event. The envelopes are for the vertical component between 2.1 and 2.7 Hz, where, due to a local subsurface resonance at 2.4 Hz, the signal-to-noise ratio is typically highest^47,48. They are normalized by their maximum amplitudes in that frequency band, aligned on the first (Pg) arrival. Marked are second (Sg) arrivals (blue line), chirp arrivals (where present; ‘Acoustic signal in VF coda’, black line) and the moveout of the speed of sound on Mars (240 m s⁻¹, ref. ⁴¹, grey line). Confirmed impact events (red) are placed at the distance where the craters were found¹⁵. b, Velocity spectrograms for events S0334a (top) and S0793a (bottom) between 0.5 and 10 Hz. Phase arrivals are marked with vertical dashed lines. c, Displacement power spectral density (PSD) for a VF event (top, S0334a) and an HF event (bottom, S0933a). VF events have excess energy in the horizontal components (north (middle) and east (right)) at high frequencies compared with the vertical component. All three components of HF events have a similar spectral shape. The resonance peak at 2.4 Hz is visible most strongly on the vertical. All phase picks, plus event start and end, are taken from the MQS catalogue²¹.