The Deep Bedrock in Rome, Italy: A New Constraint Based on Passive Seismic Data Analysis

• Published in: Pure and applied geophysics 2019-06, Vol.176 (6), p.2395-2410
• Main Authors: Marcucci, Sandro, Milana, Giuliano, Hailemikael, Salomon, Carlucci, Giorgia, Cara, Fabrizio, Di Giulio, Giuseppe, Vassallo, Maurizio
• Format: Article
• Language: English
• Subjects: Bedrock; Data analysis; Data processing; Earth and Environmental Science; Earth Sciences; Earthquakes; Ellipticity; Free surfaces; Geophysics/Geodesy; Ground motion; Impedance; Limestone; Pleistocene; Resonance; Sedimentary basins; Seismic activity; Seismic analysis; Seismic arrays; Seismic data; Seismic surveys; Seismic velocities; Seismological data; Shaking; Shear; Subsoils; Urban planning; Velocity; Velocity distribution; Vibration; Vibration analysis; Wave velocity
• ISSN: 0033-4553; 1420-9136
• DOI: 10.1007/s00024-019-02130-6

Seismic resonance inside sedimentary basins severely influences ground shaking at the free surface in case of earthquakes. Starting from few observations of a low-frequency resonance in the historical center of Rome, Italy, we performed several single-station ambient vibration measures to verify and estimate the resonance frequency in a wide area of the city by Horizontal-to-Vertical spectral ratio method. We verified a stable low-frequency peak in the range 0.3–0.4 Hz. Recordings of August 24th 2016, Mw 6.0 Amatrice earthquake, available both inside and outside the basin of Rome, confirm the presence of high-energy components at frequencies of 0.2–0.4 Hz within the basin. These observations support the hypothesis of a deep seismic impedance contrast responsible for the low frequency resonance. To infer the depth range of subsoil deposits related to this impedance contrast, we analyzed ambient vibration data recorded by 2-D seismic arrays aiming at retrieving the shear-wave velocity structure up to relevant depths. To increase the investigation depth (up to 2000 m), we jointly inverted for Rayleigh-waves dispersion and ellipticity curves and resonance frequency. The shear-wave velocity profile shows two main discontinuities at depths of about 500 m and 1800 m that can be related to the bottom of the Plio-Pleistocene filling of the Rome basin and to the top of the basal limestone formation, respectively. These results fill a gap of knowledge about the deep velocity structure in the city that may be helpful for ground-motion scenario studies.