Bi-normalized response spectra and seismic intensity in Bucharest for 1986 and 1990 Vrancea seismic events

The Vrancea subcrustal earthquakes of August 30, 1986 and May 30, 1990 are the two most recent seismic events that have occurred in Romania with moment magnitudes M _W ≥ 6.9. The spectral analysis of the strong ground motions recorded in Bucharest reveals that despite small differences in magnitude between the 1986 and 1990 earthquakes, their frequency contents are very different, sometimes even opposing. The main focus of this study is to conduct a comparative analysis of the response spectra in terms of the bi-normalized response spectra (BNRS) proposed by Xu and Xie (2004 and 2007) for strong ground motions recorded in Bucharest during these two seismic events. The mean absolute acceleration and relative velocity response spectra for the two earthquakes are discussed and compared. Furthermore, the mean bi-normalized absolute acceleration and normalized relative velocity response spectra with respect to the control period T _C are computed for the ground motions recorded in Bucharest in 1986 and 1990. The predominant period T _P is also used in this study for the normalization of the spectral period axis. Subsequently, the methodology proposed by Martinez-Perreira and Bommer (1998) is applied in order to estimate the seismic intensity of the two events. The results are discussed and several conclusions regarding the possibility of using the bi-normalized response spectra (BNRS) are given.