Seismic near-surface investigations at the InSight landing site

Abstract

NASA’s InSight mission was the first mission to install a geophysical observatory on Mars, including a seismometer that allowed us to investigate the shallow subsurface with seismic techniques. To date, the seismic investigations of the subsurface at the InSight landing site included the analysis of the traveltimes of seismic waves emitted during the hammering of the heat probe, the compliance inversion of atmospheric pressure signals, and the inversion of frequency-dependent Rayleigh wave ellipticity from ambient vibrations. These results allowed us to establish a near-surface model consisting of a very low-velocity (vp » 120 m/s; vs » 60 m/s) layer at the top interpreted as low-density, high-porosity impact-fragmented regolith of sand-sized particles. Seismic velocities increase in the topmost metres with a boundary at a few meters depth, where the regolith transitions into fragmented basaltic lava. Seismic S-wave velocities >700 m/s below around 20 m depth are interpreted as shallow high-velocity Amazonian age basalt flows. A low-velocity (vs <500 m/s) zone at 30–75 m depth was associated with a sedimentary layer laying above older Amazonian or Hesperian age basalt flows with vs >1’500 m/s. A deep sedimentary layer has been proposed at around 175 m depth.