Characterizing ground motion of historical earthquakes. Study of Sant'Agata del Mugello combining building archaeology, earthquake engineering and seismology

A fault is charged during (hundred-) thousand years, then the accumulated elastic energy is released in few seconds when an earthquake occurs. To correctly assess seismic hazard it is of capital importance to study the seismic history. Over the last decades approaches such as historical seismology, archaeoseismology and paleoseismology have been developed chasing alternative sources of information. Among them, historical buildings witnessed ancient earthquakes as 'stone seismometers' recorded in their walls as structural disorders and repairs. I develop an innovative methodology connecting building archaeology, seismology and earthquake engineering. I aim to show that archaeological characterization of post-seismic repairs on historical buildings can successfully infer key ground motion characteristics of historical earthquakes. The test case is the medieval church of Sant'Agata del Mugello, an exceptional site with many historical sources describing the damages induced by past earthquakes, and their renovation. The site is located in the Mugello basin (Central Apennines, Italy, Tuscany), characterized by a moderate seismicity. The largest known events occurred in 1542 (Mw 6) and 1919 (Mw 6.3). I first trace the seismic history of the church by combing a stratigraphic analysis of the building with an in-depth study of historical texts. A CAD-based model of the current church is designed from a laser scanner survey. A CAD-model of the church before and after each historical earthquake is then extrapolated from the current church and its deduced constructive history. I have developed an ad hoc meshing code to generate a finite element mesh from the CAD-based model. We perform two ambient vibration testing survey in the church. 8 modes of vibration (natural frequency, modal shapes and damping ratio) are estimated. A Vibration-Based model updating based on the identified experimental parameters and the constructive history of the church allows to calibrate the numerical model of the church in its linear part. A continuum damage model is used to identify the limit of the linear model of the church. I then focus on the study of the 1919 non damaging earthquake. A collection of waveforms compatible with the seismotectonic context is selected, corrected, and used to solicit the updated linear digital model of the church. I show preliminary results to discuss the ground motion characteristics of the 1919 earthquake. (author)