Temporal changes of S-wave velocities at shallow depth on Mars are derived using seismic data from the InSight mission. Autocorrelation functions are computed for three-component seismic recordings to retrieve zero-offset reflection seismograms. Observed S-wave reflection phase with two-way travel time of ~1.2 s and its multiples indicate an interface at ~200 m depth. Daily relative travel time changes (dt/t) with ~5% variations are correlated well with the surface temperature. A top ~1m-thick regolith layer produces a delay of about one Martian day between the dt/t and surface temperature. Assuming the travel time changes are produced primarily in the top ~18 m sand layer, the daily velocity variations in that layer are ~40%. The dominant mechanisms driving the changes are thermoelastic strain in the shallow structure generating the time delays and possible material failures in the regolith layer.