No abstract

Seismic velocity is the geophysical property that has a key role in characterizing dynamic processes and the state of the stress around the faults, providing valuable information regarding the change in the tectonic regime. The stress in the crust is an important indicator of the possible occurrence of a major earthquake, and the variation of seismic velocities, in time, can provide a clearer picture on the tectonic processes taking place in the region. In the crust, velocities change before, during, and after earthquakes through several mechanisms related to fault deformations, pore pressure, stress changes, and recovery processes. In this study, we investigate the possible correlation between the changes of seismic velocities (Vp/Vs) in time and the occurrence of moderate size crustal and intermediate depth earthquakes from the Vrancea region. Our findings show that there are no significant variations in Vp/Vs for the intermediate depth earthquakes, while crustal events have decreased seismic activity prior to the main earthquake and no high Vp/Vs anomalies. Our results indicate key aspects, and such analyses should be carried out in real-time to continuously explore any unusual pattern pointed out by the seismic velocity changes. Vp/Vs and their standard errors can also be used to describe seismic activity patterns that shape the tectonic evolution of the area.

<p>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.</p>