Near-surface ambient-noise seismic tomography of Bucharest, Romania

Bucharest is a densely populated urban region affected by strong earthquakes generated in the Vrancea seismogenic area. Studying and understanding its underground structure can help constrain seismic risk and improve estimates of seismic hazard and resilience. To obtain a 3D image of the near - surface structure beneath Bucharest, we analyse ambient noise records from 34 broadband seismic stations that operated throughout the city in 2004 ( the URS – URban Seismology network). We cross - correlate daily vertical component seismograms to obtain virtual Rayleigh waveforms and extract the phase velocity dispersion curves between pairs of stations using an automated Bessel - function analogue algorithm for periods between 2s and 10s. Dispersion curves are then combined in a fast marching seismic tomography (FMST) to estimate the lateral distribution of phase velocities at discrete periods. These are then jointly inverted with horizontal-to-vertical spectral ratios using Simulated Annealing methods under the assumption of a diffuse field to obtain shear wave velocity profiles with depth beneath each station. Preliminary results reveal seismic heterogeneities beneath Bucharest and offer fundamental constraints on the anomalous ground motion amplification and its relationship with complex geological structures from the uppermost crust.