Fig. 2: Magnitude–distance distribution and FMD of recorded VF and HF events.

a, VF events with chirps (w chirps; yellow triangles) and without chirps (w/o chirps; blue triangles), as well as HF events (purple dots) and selected BB events (red X’s). Confirmed impact events¹⁵ are marked with red circles. A separation between chirp and non-chirp VF events is shown with a yellow dashed line. See the discussion in ‘Acoustic signal in VF coda’. Noise percentile levels (Extended Data Fig. 3 and ‘Background seismic noise’) are given by the solid (33rd percentile), dotted (67th percentile) and dashed (99th percentile) lines. As we assume that VF-like wave propagation trapped in the crust is possible only to 45° distance, the noise level at longer periods, relevant for the BB events, is used for larger distances. An imaged crater not detected by SEIS (grey diamond, 35 m crater constrained to 2 d; https://www.uahirise.org/ESP_076590_2180) was used to assess the noise levels. b, Cumulative number of events over all of Mars. The Gutenberg–Richter fits for ≥M_W were estimated as 2.0 (HF, purple dots and dashed red line), and 2.2 and 2.5 (VF and BB, red X’s and dashed and dotted lines, faded lines for M_W < magnitude of completeness). The blue-shaded area highlights the uncertainty between the different fits. VF and BB events are scaled to a global number from different cutoff distances, using a magnitude-dependent area correction, as explained in ‘Seismology event normalization’.