Thick layers of unconsolidated sediments are widespread in the Japan island’s surface, and their frictional properties significantly affect the shallow slip behavior during earthquakes. However, laboratory data on the properties of the shallow zone remain limited. We collected tuff breccia samples from deep borehole cores of the Miocene “Green Tuff” formation, a major surface cover, and performed velocity-stepping friction tests on them under in situ stresses of 2 to 20 MPa to assess the velocity-dependence of frictional strength. The samples exhibit predominantly frictionally stable, velocity-strengthening behavior over the range of normal stress tested, which supports the hypothesis that shallow sediment layers are seismically quiescent. The result is consistent with the low seismicity and attenuation of coseismic slip in the shallow zone observed for the regional earthquakes. Microstructural observations of the postmortem samples through optical and scanning electron microscopies indicate a fabric transition from boundary shear localization to distributed cataclastic flow with increasing normal stress. Our laboratory investigation on depth-variable distribution of friction velocity dependence of a shallow sediment layer would provide further insight into the mechanical role for earthquake rupture dynamics and shallow seismicity.