Characterizing ground motion of historical earthquakes - Study of Sant’Agata del Mugello combining building archaeology, earthquake engineering and seismology
The defense tooke place on Tuesday, February 9, 2021 at 2:00 p.m. by videoconference from the Salle Froidevaux (E314) of the Ecole Normale Supérieure (24 rue Lhomond, 75005 Paris).
This interdisciplinary thesis was carried out in collaboration between the “laboratoire de géologie” and the laboratory “Archéologie et philologie d’Orient et d’Occident” of the Ecole Normale Supérieure (ENS, PSL University), the laboratory “Bureau d’Evaluation des Risques Sismiques pour la Sûreté des Installations” (BERSSIN) and the “Laboratoire de la Modélisation des Performances Sismiques” (LMAPS) of the “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN). This thesis is defended within the Doctoral School 560 STEP’UP (Sciences de la Terre et de l’Environnement et Physique de l’Univers de Paris).
The jury was composed of 8 members:
- Jean-François SEMBLAT (Ecole Nationale Supérieure de Techniques Avancées, Institut Polytechnique de Paris):
- Gianmarco DE FELICE(Department of Engineering, Roma Tre University)
- Cédric GIRY (Laboratoire de Mécanique et Technologie, Ecole Normale Supérieure Paris-Saclay)
- Pascal BERNARD (Institut de Physique du Globe de Paris)
- Laura PECCHIOLI (Humboldt Universität)
- Hélène LYON-CAEN (Laboratoire de Géologie, Ecole Normale Supérieure)
- Maria LANCIERI (Bureau d’évaluation des risques sismiques pour la sûreté des installations, Institut de Radioprotection et de Sûreté Nucléaire)
- Hélène DESSALES (Archéologie et philologie d’Orient et d’Occident, Ecole Normale Supérieure): Invited member
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.