Crustal structure of Mars from the first observation of surface waves

Kurzfassung

Surface waves had not been observed on any marsquake records during the first 3 years of seismic monitoring on Mars. We report the first detection of surface waves on Mars from two meteoroid impacts. The observed surface waves allow us to expand the current understanding of crustal structure on Mars beyond the crustal layering inferred beneath the InSight landing site. The ground-truth identification of the two events as impacts removes all uncertainties related to hypocentral depth and location. We measure the group velocities in the 8 – 15 s period range which is primarily sensitive to crustal structure between ~5 – 30 km depth. At greater depths, we lose sensitivity due to lack of dispersion measurements at long periods highly contaminated by wind noise. We find that the low VS layer extending down to 10 km depth in the shallow crust of Elysium Planitia may not exist globally on Mars. Instead, the average crustal velocities along the R1 paths are considerably faster and are likely to be more representative of average crustal structure. This implies either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves, suggesting a higher crustal density than inferred beneath the lander. Our findings are further supported by more prominent surface wave energy observed in the recent marsquake record, Mars’ largest quake ever recorded during the InSight mission.