Evidence of nonlinear site response in horizontal-to-vertical spectral ratio from near-field earthquakes

The issue is addressed as to whether the horizontal-to-vertical spectral ratio (HVSR) method is sensitive to the amplitude of ground motion from near-field earthquakes. Twenty-one three-component accelerograms from two closely located similar soil sites in the town of Lefkas are used. The recordings represent 17 earthquakes covering a wide range of magnitudes, epicentral distances and azimuths. Peak horizontal accelerations (PGA) and velocities (PGV) lie in the ranges 20-540 cm/s superscript 2 and 1.4-55.2 cm/s. For each HVS ratio, the site's fundamental-resonance frequency, f subscript res, is determined visually. Linear correlation analysis shows that f subscript res is strongly (negatively) correlated to PGA and PGV (r between -0.7 and -0.8); no correlation is found with resonance amplitude or epicentral distance. It is shown that the observed correlation is attributable to soil nonlinearity and indicate how weak-motion estimates of f subscript res can be corrected for use in assessing site response during strong shaking.