Characterization of a sedimentary basin using geophysical measurements: example of the Mugello basin, Italy

The ambition of the French ACROSS (ArChaeology, inventory of RecOnstruction, Seismology and Structural engineering, across-project.github.io) project is to study past earthquakes through their impact on historical buildings. Based on an interdisciplinary study, we aim at developing an approach to assess the seismic ground motions required to explain building repairs and disorders, or their absence. The Mugello area is characterized by a 500 m deep, 10 km large and 20 km long sedimentary basin bordered by two fault systems. This region has witnessed significant seismic events in the past, notably in 1542 (M 5.7) and 1919 (M 6.2), leading to extensive damage throughout the area. These events resulted in disorders, repairs and restorations to historical buildings situated in the area; among them, five bell towers are studied in the project. Different geophysical and geotechnical measurements were conducted in the past to characterize the basin properties. We reprocessed some of these data and conducted new geophysical campaigns to complement them. Seismic noise measurements were processed using the HVSR method to get the fundamental resonance frequency of the basin. Some of these measurements were acquired near 3 bell towers studied in the project and processed using Ambient Vibration Array (AVA) f-k beamforming techniques to characterize local 1D shear wave velocity profiles. Active MASW (Mutichannel Analysis of Surface Waves) measurements were also carried out at these locations to better characterize profiles in the subsurface. These data were processed using the Geopsy software (Wathelet et al., 2020), interpreted based on geological knowledge and on already existing deep boreholes and used to build a 3D model of the basin depth. To complement these measurements, we installed broadband stations nearby the 5 bell towers in 2021 and 2022. We computed amplification using regional earthquake recordings and a reference station close to a bell tower located outside the basin. This empirical amplification will be compared with results obtained from numerical simulations. This will help to validate the 3D basin model considered to compute surface ground motion due the rupture of the Sieve or the Ronta fault system that may have caused past earthquakes (see abstract of C. El Khoury et al., session GRM-4).