Andy COMBEY thesis defense - Archaeoseismological approach in the Heartland of the Incas (Cusco, Peru). Potentialities and limitations for the current seismic hazard assessment and the past earthquake risk perception.

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We are proud to anounce the thesis defense of our CDP Risk PhD Students. Andy will defend his thesis entitled Archaeoseismological approach in the Heartland of the Incas (Cusco, Peru). Potentialities and limitations for the current seismic hazard assessment and the past earthquake risk perception, on Tuesday, March 15th, 2022 at 2.00pm, INRAE Amphitheatre. Like the 11 thesis of the Cross Disciplinary Programme Risk, his work has been co-directed : ISTerre laboratory and AE&CC Labex.

ABSTRACT

Considering the increasing exposure of modern societies to earthquakes, an accurate assessment of the seismic hazard is of major significance. Identifying the earthquake evidence in the archaeological record is now considered a relevant approach for bridging the gap between palaeoseismology and historical/instrumental seismicity. However, this field of research remains confined to the Mediterranean area.
Unlike the Pacific margin, subject to frequent and well-defined megathrust earthquakes, the Cusco region in Peru is affected mainly by crustal seismicity, moderate but diffuse. The latter represents a latent danger and a significantly underestimated hazard. The region is also the heartland of the Incas and the focal point of their monumental stone architecture, allegedly earthquake-resistant. This heritage thus offers a unique opportunity to expand the seismic catalogue, better understand the dynamics of regional faults and improve our understanding of the perception/management of earthquakes in Inca times. In this pioneering PhD work, we present the results of the first large-scale archaeoseismological study in South America. The cross-disciplinary approach we conducted includes an extensive survey of 17 archaeological sites, the design/development of a field data collection-management system and the reinterpretation of a legendary story coming from the pre-Columbian oral tradition. In addition, we developed a methodology allowing the analysis of the dynamic response of massive stone structures, through a case study during the Le Teil seismic crisis (2019) in France, and prior to its application in Cusco.
First, with more than 3,000 earthquake features registered, the results show the potential of the Inca buildings to record earthquakes. Regarding the sensitivity threshold of this type of construction, the historical data suggest a seismic intensity close to VIII. The great majority of the damage recorded in the archaeological remains demonstrates thus the occurrence of one, or more, ground motions, stronger than those reported since 1650. Through the combined interpretation of field and ethnohistorical data, we support the existence of, at least, one major earthquake (Mw6.5) during the Inca imperial phase (1400-1533 CE), associated with the surface rupture of the Tambomachay-Pachatusan fault segment bordering the northern part of the Cusco Basin. Finally, our work emphasizes the interest of ambient vibration based techniques to characterize the dynamic behaviour of archaeological structures and question the earthquake-proof design of Inca architecture.
More broadly, this manuscript lays the foundation for a renewed analysis of the interactions
between pre-Columbian populations and the seismicity, and argues for greater development of the palaeo- and archaeoseismology in the Andean Highlands, and on a larger scale in South America where this approach has not yet been implemented.

Keywords
Inca - Archaeoseismology - Peru - Cusco - Seismic hazard - Palaeoearthquakes