Abstract Earthquakes in intraplate regions are often scattered and difficult to associate with specific geological structures, requiring detailed surveys to identify active faults. Improving knowledge of local seismicity in these areas is essential for accurate seismic hazard assessment and for safe planning of economic activities that could affect subsurface conditions. Here, we present a detailed analysis of seismicity in the Basque‐Cantabrian Zone (BCZ) and adjacent areas of the Pyrenean‐Cantabrian mountain chain in northern Iberia, where prior knowledge was limited and historical earthquakes with estimated magnitudes exceeding five have occurred. Our survey used two dense networks of broadband seismometers operating between 2014 and 2020, enabling us to locate more than 1,200 local earthquakes and compute over 40 new focal mechanisms. We observed generally dispersed seismicity, concentrated primarily in the transition between the BCZ and the Western Pyrenees. The Ebro and Duero basins were practically aseismic. In the BCZ, notable seismicity occurs along a major south‐dipping Mesozoic normal fault, as well as in association with salt diapirs and blind faults likely affecting the Paleozoic basement. In the Western Pyrenees, seismicity is concentrated along a steeply dipping fault we interpret as the Ollín fault, while two seismic crises in the South Pyrenean Zone appear related to blind faults. In the northern Iberian Range, seismicity occurs both above and below the Cameros thrust. Analysis of the new focal mechanisms indicates a predominantly extensional stress regime in the BCZ, with localized strike‐slip components in several areas, including the South Pyrenean Zone.
Olivar‐Castaño et al. (2026) studied this question.