Noise-Based Broadband Complex Site Response Functions for Virtual Seismogram Generation

Although accurate estimation of local site effects is essential for seismic risk assessment, most of empirical and numerical methodologies are hindered by significant limitations. In particular, they focus primarily on estimating the amplitude of site response, thereby failing to capture its full complex form. Moreover, most approaches neglect cross-coupling between ground motion components. This study proposes a new approach to overcome these limitations. It is based on seismic ambient noise recorded at three stations (Reference, Bridge, and Target) and on cross-coupling inversion. The method eliminates the need for active seismic sources and allows for the recovery of the full nine-component site transfer function, including both amplitude and phase information. Because noise sources differ at low and high frequencies, incident motions at the basin and reference stations also vary. Therefore, the Complex Site Response Function (CSRF) is calculated separately for low(<1 Hz) and high(>1 Hz) frequencies. The estimated CSRFs for low and high frequencies are rotated according to the earthquake back-azimuth at the reference station and the inter-station profile. By convolving the rotated CSRF with the reference site recordings, three-component virtual seismograms are synthesized for the target site. Finally, the low- and high-frequency virtual seismograms are merged to produce a single broadband seismogram. We applied the proposed method to seismic data from Kefalonia Island in the central Ionian Sea. The resulting CSRFs and synthesized virtual seismograms show good agreement with standard spectral ratios and observed ground motions, respectively.