Spatial model of slip partitioning and Coulomb stress in the Central Alborz using radar interferometry

Adnani,, S. A.; Arian, M.; Solgi, A.; Shirazian, M.; Heidari, R.

doi:10.22061/jrsgr.2026.113440.1137

Document Type : Original Research Paper

Authors

¹ Department of Earth Sciences, SR. C., Islamic Azad University, Tehran, Iran

² Department of Geomatics Engineering, Civil Engineering Faculty, Shahid Rajaee Teacher Training University

https://doi.org/10.22061/jrsgr.2026.113440.1137

Abstract

Background and Objectives: Accurate monitoring of crustal deformation in transpressional tectonic systems is a fundamental challenge in geodynamics and seismotectonic assessment because strike-slip faulting and thrusting occur simultaneously and mechanically interact across adjacent fault segments. In the Central Alborz, oblique convergence between the Arabian and Eurasian plates has resulted in the partitioning of deformation among the Mosha–Fasham, North Tehran Thrust, Purkan–Vardij, Emamzadeh Davud, and Taleghan fault systems. Consequently, the assumption of uniform fault slip cannot adequately represent the spatial and depth-dependent heterogeneity of the deformation and stress fields. This study aims to develop an integrated framework based on remote sensing, geodesy, and fault mechanics to reconstruct the interseismic deformation field, partition the slip rate into stri ke-slip and dip-slip components, and calculate Coulomb failure stress changes throughout the fault network of the Central Alborz.
Methods: : C-band synthetic aperture radar data acquired by Envisat from 2003 to 2012 and Sentinel-1A and Sentinel-1B from 2014 to 2022 were used to generate differential interferograms and retrieve surface displacements along the satellite line of sight. Interferometric synthetic aperture radar processing was performed in SARscape and included selecting image pairs with suitable spatial baselines, removing the topographic phase using the Shuttle Radar Topography Mission digital elevation model, applying Goldstein and adaptive filters, unwrapping the interferometric phase, and correcting orbital errors using ground control points. To assess geodetic consistency and impose boundary conditions, the eastward and northward horizontal velocity components, together with their one-sigma uncertainties, were obtained from 45 Global Positioning System and Global Navigation Satellite System stations in a Eurasia-fixed reference frame. The radar, geodetic, seismic, and fault-geometry datasets were spatially co-registered in the International Terrestrial Reference Frame 2000 using a seven-parameter Bursa–Wolf transformation. The velocity field was subsequently transformed between reference frames using the Euler pole of the Central Alborz. Twenty-two fault segments were represented in Universal Transverse Mercator Zone 39 North and discretized into regular (1 × 1)-km elements. The final model comprised 8,248 elements and 16,496 unknown parameters corresponding to the strike-slip and dip-slip components. The model was solved using the boundary element method and dislocation Green’s functions for a homogeneous, isotropic elastic half-space. A crustal shear modulus of 30 gigapascals and an effective friction coefficient of 0.4 were adopted, and Coulomb failure stress changes were calculated at depths of 5, 10, and 15 km. To evaluate the consistency of the model outputs, the radar-interferometry-derived deformation field was compared with ground-based observations in areas of overlapping data coverage, while the modeled stress pattern was compared with the spatial distribution of filtered earthquake events extracted from homogenized seismic catalogs.
Findings: The slip distribution on the modeled fault surfaces does not follow a symmetric or elliptical pattern and is strongly controlled by local variations in fault dip, strike, depth, and the geometric linkage between adjacent segments. Along the eastern section of the Mosha–Fasham fault, the left-lateral strike-slip component reached a maximum rate of approximately 4 mm/yr at shallow depths and exceeded the reverse dip-slip component, which attained a maximum of 2.4 mm/yr. At depths of 13–14 km, these components decreased to approximately 2 and 1.1 mm/yr, respectively. In contrast, the reverse component was dominant along the central section of the Mosha–Fasham fault. Along the northwestern branch of the North Tehran Thrust, the maximum reverse dip-slip rate was 1.5 mm/yr, whereas the strike-slip component did not exceed 0.2 mm/yr. The maximum positive Coulomb stress change was +0.092 bar and occurred at a depth of 10 km in the northern Damavand area. In the Fasham–Mosha zone, a correlation coefficient of approximately 0.87 was observed between the spatial pattern of stress changes and the deformation field derived from interferometric synthetic aperture radar. Furthermore, the positive stress lobes exhibited substantial spatial overlap with areas characterized by high concentrations of microearthquakes and aftershocks recorded in the homogenized seismic catalog.
Conclusion: The integration of multitemporal interferometric synthetic aperture radar observations, Global Positioning System and Global Navigation Satellite System velocities, and three-dimensional boundary-element modeling demonstrates that subsurface fault geometry and segmentation are the principal controls on the heterogeneous partitioning of slip and the concentration of interseismic stress in the Central Alborz. Compared with the assumption of uniform slip, the one-kilometer fault discretization enables localized variations in fault-motion components and zones of stress concentration to be resolved more effectively. Because the model assumes a homogeneous elastic half-space and spatially invariant mechanical parameters, the results should be interpreted as relative patterns of fault loading and mechanical interaction rather than deterministic predictions of the timing or magnitude of future earthquakes. The proposed framework provides a quantitative basis for prioritizing geodetic monitoring and improving assessments of seismic potential in Tehran and the surrounding regions.

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