The SEIS seismometer deployed at the surface of Mars in the framework of
the NASA-InSight mission has been continuously recording the ground
motion at Elysium Planitia for more than one martian year. In this work,
we investigate the seasonal variation of the near surface properties
using both background vibrations and a particular class of
high-frequency seismic events. We present measurements of relative
velocity changes over one martian year and show that they can be modeled
by a thermoelastic response of the Martian regolith. Several families of
high-frequency seismic multiplets have been observed at various periods
of the martian year. These events exhibit repeatable waveforms with an
emergent character and a coda that is likely composed of scattered
waves. Taking advantage of these properties, we use coda waves
interferometry to measure relative travel-time changes as a function of
the date of occurrence of the quakes. While in some families a
stretching of the coda waveform is clearly observed, in other families
we observe either no variation or a clear contraction of the waveform.
Measurements of velocity changes from the analysis of background
vibrations above 5Hz are consistent with the results from coda wave
interferometry. We identify a frequency band structure in the power
spectral density, that can be tracked over hundreds of days. This band
structure is the equivalent in the frequency domain of an
autocorrelogram and can be efficiently used to measure relative
travel-time changes as a function of frequency. The observed velocity
changes can be adequately modeled by the thermoelastic response of the
regolith to the time-dependent incident solar flux at the seasonal
scale. In particular, the model captures the time delay between the
surface temperature variations and the velocity changes in the
sub-surface. Our observations could serve as a basis for a joint
inversion of the seismic and thermal properties in the first meters
below InSIght.