The Effect of Velocity Inversions on H/V

Abstract

We analyzed the phenomenology of microtremor H/V curves under inversions in the shear-wave velocity (V s ) profile in the subsoil. Under no V s 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 H site /H bedrock , to microtremor. Theoretical modeling confirms these experimental findings.

Seismic site response of unstable steep slope using noise measurements: the case study of Xemxija Bay area, Malta

“…This causes the HVSR values to drop below 1 over a wide frequency range (e.g. Castellaro and Mulargia 2009;Panzera et al, 2011b). The origin of the resonance peak was confirmed by carrying out 1-D modelling, computing synthetic HVSR curves (Fig.…”

Dynamic characteristics of an active coastal spreading area using ambient noise measurements—Anchor Bay, Malta

“…4(a) shows H/V curves from similar geology (UCL overlying BC) at various sites over the Maltese islands, all showing a clear peak between 1.0 and 2.0 Hz, and a dip in the spectral ratio below 1.0 over a wide frequency range. Di Giacomo et al (2005) and Castellaro & Mulargia (2009) interpret this dip in the H/V ratio in terms of a shallow shear-wave velocity inversion, which in this case corresponds to the interface between the UCL and BC. The interpretation of other features of this peak in terms of Rayleigh wave ellipticity and/or trapping of SH waves in the lowvelocity layer is the subject of an ongoing study using numerical modelling.…”

“…It should be noted that a mechanical vibration mode, although clearly identified in terms of polarization strike and linearity, need not necessarily exhibit an H/V resonance peak, the reason being that such modes may show individual maxima of all spectral components (vertical and two horizontals) at these frequencies, as opposed to the stratigraphy-related peaks (in this case the 1.0-2.0 Hz peak) for which the individual spectra form an 'eye-shape' due to the lowering of the vertical spectra with respect to the horizontal ones (Castellaro & Mulargia 2009). Fig.…”

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities

“…Results obtained by the HVNR confirm the findings from earthquake data analysis and set into evidence that we are dealing with a geological setting more complex than a simple 1-D layered structure, for which the noise spectral ratio method was originally proposed. The presence of lava flows at the surface imply the existence of possible velocity inversions that give origin to H/V spectral amplitude lower than one unit (Castellaro and Mulargia, 2009;Di Giacomo et al, 2005) and the existence of amplification in the vertical component of the ground motion.…”