Mapping current Iberian tectonic stress regimes using Gutenberg–Richter b-values

Introduction: The primary objective of this paper is to demonstrate how the spatial mapping of the b-value (the slope of the Gutenberg–Richter magnitude–frequency relation), integrated with the established framework relating b-values to tectonic regimes—specifically, b < 1 for compressional regimes and b > 1 for extensional regimes—provides key insights into the present-day regional stress field across the Iberian Peninsula. Materials and methods: A comprehensive, publicly available seismic catalogue comprising 114,485 earthquakes recorded between 2000 and 2024 was used in this study. The methodology involves calculating b-values on a high-resolution grid. For each grid cell, the seismic data within a defined radius centred on the node were used to compute the initial b-value. These preliminary estimates were subsequently smoothed using a moving-window mean filter. Finally, the classification of b-values as either greater than or less than unity (b > 1 or b < 1) was applied to the resulting robust estimates. Results: The findings indicate that, despite a regional tectonic setting dominated by plate-scale compression, a substantial portion of Iberia exhibits an extensional regime that warrants a clear geodynamic explanation. Extensional regimes are predominantly observed in the high mountain belts of the Pyrenees and the Betic Cordillera, as well as across several headwater catchments of the major Iberian river basins (the Duero, Tajo, Guadiana, and Guadalquivir). Conclusions: While the compressional stress regime is governed by the convergence between the African and Eurasian plates transmitted across Iberia, two hypotheses are proposed to explain the localized extensional regimes within the peninsula. First, we hypothesize that extension in the high mountain ranges of the Pyrenees and the Betic Cordillera is driven by gravitational collapse. Second, extension across the headwater catchments of the major Iberian rivers is attributed to denudational isostatic rebound.