Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

Fig. 4 : Area of archaeoseismological investigations. (A) Zenithal view of deformed walls, and (B) structural diagram of the faulted zone, and impacted infrastructures. (C) Focus (red box on (B)) on Wall 2 displaying two phases of damage: a sinistral offset at the base (phase 1), and a tilting of the structure in the upper part (phase 2). In 2021, while conducting excavations at the lower site, unexpected signs of damage and disorders were identified on the archaeological remains, specifically on walls (Wall 1 and Wall 2, Fig. 4). Further exploratory field surveys in 2023 revealed that the archaeological deformation featureswere collocatedwith left-lateral kinematics in the bedrock. The facing of the two walls displays obvious misalignments, visible in both plan and zenithal views (Fig. 4b). At this point, Wall 1 exhibits a minimum left-lateral offset of 10 cm, taking the wall orientation without deformation as reference. Deformation features on Wall 2 potentially imply two phases of deformation. The base of the wall is left- laterally shifted with an offset of 25 cm (phase 1), assuming an initial straight shape when it was built (indicated by the green line, Fig. 4c). Some of the blocks are broken, and a mortar was used to reinforce this part of the wall, which could have been used then as a foundation of the upper wall. Later, this latter wall was tilted (phase 2), dipping 70° to the East (Fig. 4c). All the Roman remains are sealed by a backfill layer, from the early third century. The disorders observed on the walls, and the geological deformation features observed in the bedrock, are consistent with each other and with regional observations. The sinistral displacements observed on the walls could be the result of one or more ruptures during earthquake(s) on a segment of MVF faults that needs to be further investigated. C O N C L U S I O N S The preliminary results reported here suggest the occurrence of seismotectonic events during Antiquity along the northern segments of the MVF. Observations of recent markers found in the paleoseismological trench and in the Lac d'Antre sediment cores also indicate recent seismic activity (Antic period). Surface-rupture faulting could have damaged and offset Gallo-roman remains, and strong local shaking during those events could have coevally damaged the constructions. If confirmed, the 20–30 cm wall offset could fit with the deformation characteristic of a shallow to very shallow moderate- magnitude earthquake, capable of causing strong shaking in the vicinity of the fault.