Was the Pays de Bray fault an active structure during the Upper Pleistocene? The contribution of the “Beauvais La Justice” palaeolithic site (France)

Abstract

The Paris Basin, long regarded as a region of very low seismicity, shows new evidence of recent tectonic activity. Re-examination of the pedosedimentary sequence uncovered during the 1993 excavation of the Paleolithic site of Beauvais “La Justice” reveals a fault system cutting through deposits dating from the Upper Pleistocene ( ∼ 60 to 45 ka), as well as their underlying strata (Paleocene and Upper Cretaceous). These faults, with a total vertical offset up to 25 cm, were mapped across the excavation and interpreted as tectonic in origin, excluding periglacial, karstic, or anthropogenic causes. Their location and geometry suggest a link to deeper crustal structures, potentially associated with the Pays de Bray anticline and related fault. This discovery aligns with broader reassessments of intraplate seismic hazard in France, prompted by the 2019 M ∼ 5 Le Teil earthquake which demonstrated that moderate events could produce surface ruptures. The BLJ reinterpretation challenges the level of our knowledge of the fault activity and of their contribution to seismic hazard in the Paris Basin, mirroring how Le Teil reshaped perception of seismic hazard in the Rhône Valley. The Pays de Bray fold and fault—or a secondary structure—may have generated the observed deformations, raising questions about its potential for larger earthquakes. The study emphasizes the need for further investigations to refine hazard assessments. It also highlights the value of interdisciplinary collaboration, to uncover hidden tectonic activity and improve seismic hazard models in low-seismicity regions.