The shear wave velocity underneath Bucharest city, Romania, from the analysis of Love waves

Abstract

S U M M A R YFrom the dispersion of Love waves, we infer models of shear wave velocity structure underneath Bucharest (Romania) at depths down to 2 km that can contribute to seismic hazard estimation. Waves from eight regional events recorded during 10 months with a network of 34 seismic broad-band stations of the URban Seismology (URS) experiment are used. Although these events provide poor azimuthal coverage the data reliably constrain a shear wave velocity model with an interface between the Neogene and the Cretaceous sediments that is dipping northwards towards the Carpathian mountains. Array processing techniques that account for non-uniform wave propagation are used to estimate the dispersion of structural phase velocity. From this, we infer subsurface structure at three different latitudes. The Neogene sediments are represented by a gradient layer with no significant lateral variation. Shear wave velocity increases from approximately 400 m s −1 near the surface to 1 km s −1 at 1 km depth and 5 km in the south, and to 1.35 km s −1 at 1.5 km depth and 5 km in the north from the centre of Bucharest, respectively. For the half-space representing the Cretaceous sediments, we obtain shear wave velocities of 2.7-2.9 km s −1 . The results are consistent with results from boreholes and shallow seismics for the near-surface structure and results from receiver function studies and crustal refraction seismic studies for the deeper structure. The details of the Neogene layer comprising a vertical gradient fill a gap in existing models of the subsurface structure of Bucharest and can contribute to modelling of seismic hazard for the city. Since the signalto-noise ratio restricted useful data to the frequency range from 90 to 290 mHz, the inversion could not constrain the near-surface velocity independently. Due to strong trade-off between near-surface velocity and depth of half-space, the latter had to be introduced as a priori data from previous studies.

Analysis of the seismic wavefield in the Moesian Platform (Bucharest area) for hazard assessment purposes

“…For this stacking, the recordings of the respective events were normalized by their total recorded energy. The comparison with the Love wave dispersion curves obtained from ambient vibrations (Manea et al 2016) and with the ones proposed by Sèbe et al (2009) shows a good agreement (Fig. 5 left).…”

“…For the first event (Fig. 3, left), an intermediatedepth event that occurred in the Vrancea area (021), the energy is distributed between 0.5 and 6 Hz and is not generating surface waves due to the large focal depth (Sèbe et al 2009). Conversely, for the regional shallow event located on Bulgarian territory (351), surface waves at low frequencies strongly dominate the signal (Fig.…”

Time domain classification and quantification of seismic noise in an urban environment

“…An amplitude and site effect study by Sudhaus & Ritter (2009), using waveforms of the URS data set, found the same effect of latitude‐dependent ground motion amplification across the Bucharest area. Sudhaus & Ritter (2009) relate this effect to resonance effects in the unconsolidated sediments above the distinct dipping Neogene–Cretaceous boundary from 800 m depth in the south to 2000 m depth in the north (Mandrescu et al 2004; Sèbe et al 2009). Also for the URS data set Ziehm (2006) applied a comprehensive H/V analysis for the Bucharest area.…”

A single Rayleigh mode may exist with multiple values of phase-velocity at one frequency

“…There is only one value of Fourier phase at each frequency. In cases where multiple overtones interfere in the seismogram, phase-matched filter techniques can be used to isolate a single mode in the waveform signal (Herrin & Goforth 1977;Russell et al 1988;Sèbe et al 2009). These techniques essentially apply a deconvolution with a reference dispersion pref(f ) in order to concentrate the seismic energy of the respective mode in a small time window.…”