Historical earthquakes in the Holy Cross Mountains, Poland, true or false? Unveiling insights through archaeoseismology

The Holy Cross Mountains are an intraplate range with a limited historical seismicity record. The only documented earthquakes include the February 6, 1837 M 4.3 event, which caused ground cracks, and swarm events from February 1932 (M ∼ 3.5), likely triggered by the Holy Cross Fault (HCF) or sub-perpendicular faults. The apparent lack of older destructive earthquakes in historical catalogs motivated us to conduct archaeoseismological research to improve seismic hazard assessment, risk mitigation, and urban planning strategies, ultimately benefiting local communities. We focused on the 12th-century Collegiate church of Saint Martin in Opatów, located near the Holly Cross Fault (HCF). We report numerous damage features, such as leaning, bulging, and twisted walls, dropped keystones in Romanesque and Gothic portals, strike-slip displacements of these portals, surplus, oversized buttresses, and walled-up portals. While some deformations may result from humid loess instability and war destructions, our data, combined with historical records, suggest two to three seismic events in the past 800 years as a cause. We argue these deformations were co-seismically triggered by either large far-field events, like the 1259 AD earthquake, or local, shallow small-magnitude events significantly amplified by site effects. This indicates potential seismic activity in the Holy Cross Mountains during Medieval times. The absence of historical records does not imply the absence of earthquakes.