Stress and strain fields in the Iberian Peninsula and adjacent Mountain Ranges

Iberia represents the westernmost tectonic region involved in the Eurasia-Nubia convergence, playing a key role in shaping the plate tectonics of the westernmost Mediterranean. It is affected by alpine deformation in the Pyrenees in the north and the Gibraltar Arc in the south, alongside other internal mountain ranges. Toward the south, the region is further deformed in the Atlas and Tell Cordilleras.

This study aims to analyze and compare the active stress and strain fields with seismicity and active faults to discuss the geodynamic processes, determine the main active structures, and assess how stresses are accommodated, whether seismically or aseismically.

The stress field is derived from an extensive compilation of available crustal earthquake focal mechanism solutions across the region. The data are inverted using the STRESSINVERSE software (Vavryčuk, 2014) on a 0.5° spaced grid, requiring a minimum of eight focal mechanism for cell. The geodetic dataset includes nearly 500 continuous GNSS stations, with time series spanning up to 25 years, along with 25 episodic GNSS stations. Data processing is performed using GAMIT/GLOBK (Herring et al., 2010), following the methodology outlined by Palano et al. (2020). The resulting velocity field is enhanced with other available velocity fields to increase station density. The strain field is estimated on a 0.5° grid according with the methodology illustrated by Shen et al. (2015). Finally, to compare the stress and strain fields, s_Hmax and e_Hmin are estimated.

The results show that the region is affected by NW-SE compression, causing irregular deformation. Shortening of up to 4–5 mm/yr, parallel to the compression, is mainly concentrated in southern Iberia, along the Eurasia-Nubia plate boundary, accompanied by frequent low-to-moderate seismicity. In southwestern Iberia and in the Tell Cordillera, the NW-SE compression can result in moderate-to-high seismicity. Meanwhile, both central-northern Iberia and the Atlas Cordillera undergo limited deformation under the general NW-SE compression. The first is characterized by zones of low seismicity linked to normal and strike-slip faults. The Atlas Cordillera, in contrast, exhibits sporadic but moderate-to-high magnitude seismicity related to thrusting. The stress pattern significantly changes in the westernmost Gibraltar Arc and in the Pyrenees. The Gibraltar Arc characterises by a slightly rotated NNE-SSW striking compressional axis, while the shortening aligns E-W. The former observation and the absence of seismicity in the front of the arc suggest aseismic displacement to the west of the Gibraltar Arc, perpendicular to the Eurasia-Nubia convergence. Finally, the data show that the shortening in the Pyrenees has ceased, and stress suggest a N-S extension, likely related to isostatic readjustment of the mountain range.

• Herring, T. A., et al. (2010). GAMIT Reference Manual, GPS analysis at MIT, Release 10.4, Dept. of Earth Atmos. and Planet. , Mass. Inst. of Technol., Cambridge, MA, 171pp.
• Palano, M., et al. (2020). Geopositioning time series from offshore platforms in the Adriatic Sea. Scientific Data, 7(1), 373.
• Vavryčuk, V. (2014). Iterative joint inversion for stress and fault orientations from focal mechanisms.Geophysical Journal International, 199(1), 69-77.
• Shen, Z. K., et al. (2015). Optimal interpolation of spatially discretized geodetic data. Bulletin of the Seismological Society of America, 105(4), 2117-2127.