Seismic hazard of Romania due to Vrancea earthquakes - How asymmetric is the strong ground motion distribution

Abstract

We present a short review of the most recent results coming from the numerical modeling of seismic hazard and interpretation of the new observations provided by the recently installed seismic stations in Romania. There are still controversial questions related to the asymmetric distribution of the ground motion radiated by the Vrancea intermediate-depth earthquakes and which is the main factor responsible for this particular distribution: seismic source, structural model, site effects or vulnerability. Our main goal is to provide a solution to this key problem, with direct implications upon the seismic hazard assessment. NE-SW elongation of the isoseismals and the maximum values in the Romanian Plain are well explained by the source radiation pattern and the average structural model. The attenuation toward NW is shown to be a frequency-dependent effect, much more important in the high frequency range (≯1 Hz). We conclude that the present seismic hazard of Romania, computed by the deterministic approach, fits well, as a first approximation, the ground motion distribution for the low-frequency band, and the apparent contradiction with the historically-based intensity maps arises mainly from a systematic difference in the vulnerability of the buildings in the intra- and extra-Carpathians regions.