New Insights into the Lateral‐Strength Variations and Depth to the Brittle‐Ductile Transition Zone in NW Iberia

Abstract The northwest of Iberia experiences important tectonic activity that has led to several episodes of seismic swarms. Despite substantial knowledge of the surface structures that could be responsible for this activity, their distribution at depth remains unclear. Geophysical research and rheological data display important discrepancies concerning the deep structure of the area. In order to shed light on the processes that control deep seismicity and their relationship with surface structures, we combine the analysis of seismic activity in northwest Iberia with the interpretation of sandbox experiments simulating tectonic structures similar to those observed. The structural interpretation of the modeling results was conducted by analyzing the surface particle image velocimetry field. Additionally, yield strength and brittle‐ductile transition zone (BDTZ) maps were constructed from model sections. The modeling results show a direct relationship between the main active structures developed along pre‐existing weak zones and the observed variations in crustal thickness. Moreover, these thickened zones correspond to the sectors where the BDTZ in the crust deepens. Therefore, the anomalous thickness and lateral variations of the seismogenic zone cannot be solely explained by rheological or structural changes but result from a combination of factors such as fluid pressure and the location of fractures at depth, which controls the fluid migration responsible for the failure of crustal faults. These factors can trigger seismic activity at depth and may control the observed thickness variations in the BDTZ.