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In this study, information carried by the ambient seismic field is exploited to extract impulse response functions between two seismic stations using one as a ‘virtual’ source. Interferometry by deconvolution method is used and validated by comparing the extracted ambient noise impulse response waveforms with records of moderate magnitude earthquakes (from Mw 4 to 5.8) that occurred close to the virtual source station in Japan. As the information is only available at low frequencies (less than 0.25 Hz), the ambient seismic field approach is coupled to a non-stationary stochastic model to simulate time domain accelerograms up to 50 Hz. This coupling allows the predicted ground motion to have both the deterministic part at low frequencies coming from the source and the crust structure and the high-frequency random contribution from the seismic waves scattering. The resulting combined accelerograms for an Mw 5.8 event show a good agreement with observed ground motions from a real earthquake. [ABSTRACT FROM PUBLISHER]

Ground motions of the 2011 Tohoku earthquake recorded at Onahama port (Iwaki, Fukushima prefecture) rank among the highest accelerations ever observed, with the peak amplitude of the 3-D acceleration vector approaching 2g. The response of the site was distinctively non-linear, as indicated by the presence of horizontal acceleration spikes which have been linked to cyclic mobility during similar observations. Compared to records of weak ground motions, the response of the site during the Mw 9.1 earthquake was characterized by increased amplification at frequencies above 10 Hz and in peak ground acceleration. This behaviour contrasts with the more common non-linear response encountered at non-liquefiable sites, which results in deamplification at higher frequencies. We simulate propagation of SH waves through the dense sand deposit using a non-linear finite difference code that is capable of modelling the development of excess pore water pressure. Dynamic soil parameters are calibrated using a direct search method that minimizes the difference between observed and simulated acceleration envelopes and response spectra. The finite difference simulations yield surface acceleration time-series that are consistent with the observations in shape and amplitude, pointing towards soil dilatancy as a likely explanation for the high-frequency pulses recorded at Onahama port. The simulations also suggest that the occurrence of high-frequency spikes coincided with a rapid increase in pore water pressure in the upper part of the sand deposit between 145 and 170 s. This sudden increase is possibly linked to a burst of high-frequency energy from a large slip patch below the Iwaki region. [ABSTRACT FROM AUTHOR]