Deterministic Approach for the Seismic Microzonation of Bucharest

Abstract

The mapping of the seismic ground motion in Bucharest, due to the strong Vrancea earthquakes is carried out using a complex hybrid waveform modeling method which combines the modal summation technique, valid for laterally homogeneous anelastic media, with finite-differences technique, and optimizes the advantages of both methods. For recent earthquakes, it is possible to validate the modeling by comparing the synthetic seismograms with the records. We consider for our computations the frequency range from 0.05 to 1.0 Hz and control the synthetic signals against the accelerograms of the Magurele station, low-pass filtered with a cut-off frequency of 1.0 Hz of the 3 last major strong (Mw > 6) Vrancea earthquakes. Using the hybrid method with a double-couple seismic source approximation, scaled for the source dimensions and relatively simple regional (bedrock) and local structure models, we succeeded in reproducing the recorded ground motion in Bucharest at a satisfactory level for seismic engineering. Extending the modeling to the entire territory of the Bucharest area, we construct a new seismic microzonation map, where five different zones are identified by their characteristic response spectra

Broadband frequency-dependent amplification of seismic waves across Bucharest, Romania

“…7). A similar frequency-dependent behaviour for PGA is indicated in Cioflan et al (2004). They model wave propagation of Vrancea events across Bucharest, and their model S3 (2D profile) includes the dipping Quaternary and Mesozoic sedimentary layers.…”

“…Their results reveal maximum amplitudes in NE Bucharest at 0.1-0.3 Hz, in the city centre at 0.5-0.7 Hz and in SW Bucharest at 0.7-0.9 Hz for P-waves. Interestingly, the spatial frequency-dependent effect is less pronounced for radial S-waves and negligible for transverse Swaves (Cioflan et al 2004). …”

“…As Bucharest is built on the deep foreland basin of the Southern Carpathian arc (Fig. 1), the seismic wave amplitudes become amplified in the sedimentary layers (Cioflan et al 2004;Mandrescu et al 2007). The whole sedimentary rock sequence reaches a depth of about 8-10 km with up to several tens of meters of weakly consolidated fluvial sediments at the surface.…”

“…Recent strong motion studies, conducted to determine the peak ground acceleration (PGA) for risk assessment, revealed a strong spatial and frequency-dependent variability of PGA across the city (e.g. Wirth et al 2003;Aldea et al 2004;Cioflan et al 2004;Bartlakowski et al 2006) as well as a dependence on the absolute seismic amplitude (Mândrescu et al 2004).…”

Multidisciplinary co-operation in building design according to urbanistic zoning and seismic microzonation

“…The 1977 pattern was infirmed by the PGA values and the spectral content of the instrumental records for the 1986 (M w =7.2) and 1990 (M w =6.9) earthquakes (Mândrescu and Radulian, 1999). Recently, Cioflan et al (2004) performed a study based on instrumental observation, interpretation of the site effects, and numerical modelling, calibrated with recorded seismograms. The results were extended to city regions with similar geological structure, dividing the city area into nonconcentrical zones, to which representative response spectra were assigned.…”