Evidence of nonlinear site response in HVSR from SMART1 (Taiwan) data

Deamplification of strong motion and the increase of the effective period of soil deposits are typical nonlinear effects; this paper seeks them in SMART-1 array data by applying the horizontal-to-vertical spectral ratio (HVSR) technique. The recordings, from four soil and one rock stations, represent 23 earthquakes (M subscript L 4.9-7.0); peak ground acceleration (PGA) varies between 20-260 cm/s superscript 2. For each station, mean HVSR curves are calculated for two PGA ranges: < 75 cm/s superscript 2 and > 100 cm/s superscript 2 (weak and strong motion). At the soil stations, the "weak" (linear) and "strong" (nonlinear) responses are significantly different. Below 1-1.8 Hz, the nonlinear response exceeds the linear one. Above 2 Hz, the nonlinear response drops below the linear one and above 4-6 Hz, below unity (deamplification). From 10 to 16 Hz, the two responses converge. One soil site shows significant negative correlation between resonance frequency and ground acceleration. Such behaviour agrees with other empirical studies and theoretical predictions. The results imply that the HVSR technique is sensitive to ground-motion intensity and can be used to detect and study nonlinear site response.