The Effect of Velocity Inversions on H/V

We analyzed the phenomenology of microtremor H/V curves under inversions in the shear-wave velocity (Vs) profile in the subsoil. Under no Vs inversion the spectral signature of the H/V peaks is found to be ‘eye-shaped’ with the horizontal components higher than the vertical. Conversely, under negative velocity gradients, numerous of differences emerge. I) A H/V ratio below 1 is observed for a wide range of frequencies, due to the decrease of the horizontal components below the vertical one. II) In the presence of persistent H/V < 1, small bumps in the H/V ratio given by local minima in the vertical spectral component may represent the relics of the peaks indicating resonances and stratigraphic discontinuities. As a consequence, in the presence of velocity inversions the H/V > 2 SESAME (2004) criterion fails but a stratigraphic interpretation may still be possible. III) The H/V curves should always be interpreted together with the single component spectra. IV) Microtremor H/V measurements for stratigraphic/microzonation purposes on stiff artificial soils, (asphalt, concrete, cement, pavements) should always be avoided since the latter often produce velocity inversions. This may have consequences in the intermediate to high frequency domain (> 1 Hz) also in the application of reference site methods, like Hsite/Hbedrock, to microtremor. Theoretical modeling confirms these experimental findings.