Linear and Nonlinear Site Response from S-wave HVSR: Observations in the New Madrid Seismic Zone and from the Ridgecrest Earthquake Sequence

Because of the reduction of shear moduli under large strain, i.e. nonlinearity, the frequencies of peak shaking can decrease during strong ground motions, particularly at sites underlain by soft sediments. Also, nonlinearity increases damping, thus causing high-frequency ground motions to be attenuated more than those at lower frequencies. Borehole site transfer functions and soil-to-reference-site spectral ratios can be used to measure this effect when pairs of ground-motion spectra have similar source and path terms. In recent studies, single-station S-wave horizontal-to-vertical spectral ratios (HVSR) have been used to both estimate site response and to identify nonlinear response characteristics, which is particularly useful when a suitable reference site is unavailable. In this study, we calculated mean S-wave HVSRs and moving-window full-waveform HVSRs at 10 broadband stations, two of which are bedrock-penetrating boreholes, in the New Madrid Seismic Zone (NMSZ). Due to the lack of large ground motions in NMSZ, we also performed the same analyses on ground-motions recorded at seven stations in the Indian Wells Valley, Calif. area from the 2019 Ridgecrest earthquake sequence. Peak-ground accelerations in the Ridgecrest sequence dataset range from weak (< 0.05 g) to strong motions (up to 0.58 g). The results show that, as expected, no nonlinearity can be seen from the HVSRs of weak S-wave records in NMSZ and near Ridgecrest, Calif. However, the HVSRs of strong S-wave records from the Ridgecrest sequence at soil sites reveal nonlinear response characteristics: the peak ratios are reduced at higher-frequencies—f > 2-4 Hz—relative to those from weak-motions. These observations suggest that nonlinearity occurred at soil sites during strong shaking from the Ridgecrest sequence.