The frequency-dependent amplification for rock sites, as well as apparent source spectra, were studied using an earthquake ground-motion database collected in Romania. Almost all events occurred in the Vrancea focal zone (Southeast Carpathians), which is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume. The seismic activity ranges within an almost vertical stripe in depths between 60 and 170 km. The database used includes several hundred records from more than 100 small magnitude (MW 3.5–5.3) earthquakes that occurred in 1996–2004 and six acceleration records obtained during four large (MW 7.2, 6.9, 6.3, and 6.0) earthquakes. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical (H/V) Fourier spectral ratio of the S-wave phase (Lermo and Chavez-Garcia, 1993). The apparent source spectra were obtained from horizontal components using site (H/V), propagation path (geometric spreading and anelastic attenuation), and near-surface attenuation (kappa-factor) corrections.
Seismic radiation (within the frequency range from 0.2–0.3 to 15–20 Hz) during the earthquakes of various magnitudes in the Vrancea zone may be described by an omega-square single-corner-frequency spectral model (ω2 Brune spectrum). The stress parameter Δσ, which controls high-frequency spectral amplitudes, increases with magnitude from 20–30 bars for MW ≤3.5 up to 200–250 bars for MW 4.8–5.3, and up to 1000 bars for the case of large (MW >6.0–6.5) events. Empirical amplification (H/V) for rock sites, showing stability for particular locations, reveal high variability from station to station, which together with variations of values of kappa, reflects influence of local geologic and geomorphologic factors.