Analysis of Seismicity Parameters b-value and Fractal Dimension (Dc-value) in the Zagros Collision Zone

The Zagros Fold-and-Thrust Belt (ZFTB), as one of Iran's most active seismic zones, is situated at the convergent boundary between the Arabian Plate and the Central Iranian Block. Characterized by a high convergence rate, complex tectonic structures, and a rich seismic history, it has consistently been a focal point for tectonic and seismological research. This study aims to provide a comprehensive spatiotemporal analysis of the seismic indices b-value and Dc-value within this region, utilizing the earthquake catalog compiled by the Iranian Seismological Center (IRSC) from January 2006 to November 2024. The catalog, containing over 40,000 events with magnitudes greater than 1.5, served as the basis for analysis after refinement and removal of duplicate data. The b-value was estimated using the Gutenberg-Richter relationship coupled with the maximum likelihood method. To investigate structural changes in faults, the Dc-value parameter was also calculated. Event clustering was performed using an enhanced version of the Uhrhammer algorithm to optimally distinguish seismic clusters and remove aftershocks and foreshocks from the dataset. Spatial results revealed significant heterogeneity in the b-value across the Zagros, with values in the southern Zagros being substantially lower (0.25–0.55) on average compared to the central and northern sections. This pattern indicates higher stress concentration and a greater potential for larger earthquakes in the southern Zagros. In contrast, the higher b-values in the northern parts may reflect smaller-scale fracturing and more distributed stress. Temporal analysis of the b-value demonstrated a significant decrease during specific periods, particularly preceding large earthquakes (Mw > 6). A prominent example of this behavior was recorded before the Mw 7.3 Sarpol-e Zahab earthquake (2017), where the b-value decreased from approximately 0.85–1.0 to about 0.55–0.7 during the foreshock stage. This drop could be attributed to increased stress concentration, changes in fracture density, or crustal fluid migration, suggesting its potential as a possible precursor signal. During the co-seismic stage of this event, the lowest b-values (0.6) coincided with the main rupture, reflecting the sudden and intense stress release along the High Zagros Fault zone. In the post-seismic phase following the mainshock, a trend of relative b-value (1.2) recovery was observed, likely resulting from tectonic relaxation processes, stress redistribution, and aftershock activity. Analysis of the Dc-value, which expresses the correlation length of fractures and fault structure, revealed a complementary pattern to the b-value. Regions with low b-values exhibited higher Dc-values (1.6-2). This inverse correlation suggests that as a fault approaches its ultimate rupture stage, the fracture structure becomes more coherent and correlated, with energy release concentrating on larger scales. The combined spatiotemporal results for the b-value and Dc-value provide a comprehensive picture of the stress state and seismic dynamics of the Zagros. These findings indicate that a decrease in b-value accompanied by an increase in Dc-value can be considered a warning pattern for elevated seismic hazard. If confirmed by independent data over longer timeframes, such a pattern could be integrated into seismic monitoring systems and earthquake hazard assessment frameworks for Iran. From a tectonic perspective, the spatial heterogeneity of the b-value and Dc-value reflects differences in convergence rates, fault geometry, crustal rheological properties, and the role of hydrothermal processes in different segments of the Zagros. The southern Zagros, with its higher convergence rate and presence of long, active faults such as the MFF (Mountain Front Fault) and MZF (Main Zagros Fault), is prone to stress concentration and larger events, whereas the northern and central parts, with more complex structures and distributed fracturing, exhibit a different pattern of seismicity. In summary, this study, employing advanced statistical analyses and seismic clustering, has established a significant correlation between variations in b-value and Dc-value and the active tectonic processes in the Zagros. The obtained results not only contribute to a better understanding of stress distribution and seismic dynamics in this region but also provide a basis for developing predictive seismic hazard models and designing risk mitigation strategies for high-risk areas in Iran.