Developing a Physical Model of The Cities of Mazandaran Province Using Morphometric Indices of Active Tectonics: A Case Study of Shirgah-Galugah

Introduction   
Active tectonic hazards pose a significant threat for urban resilience, particularly in seismically active regions such as the Alborz belt in northern Iran. A comprehensive understanding of these hazards is essential for sustainable urban planning, as resilience frameworks rely on the accurate identification of earthquake-vulnerable zones to facilitate proactive risk mitigation. Morphometric indices serve as practical and quantifiable tools for assessing active faulting. Previous investigations in Iran have successfully employed these tools; for instance, Alizadeh and Khalaj (2021) evaluated tectonic activity in Pardis, while other studies have assessed urban resilience in Zanjan and Marzanabad. Despite these contributions, a critical research gap persists in Mazandaran Province. Notably, no specific morphotectonic study has yet targeted the urban corridor between Shirgah and Galugah.
This corridor is situated within a critically active tectonic zone, lying directly along major faults, including the North Alborz and Khatirkuh faults, both of which possess a high potential for generating destructive earthquakes. Consequently, the seismic risk to the local population remains largely unassessed. This study is specifically designed to fill this lacuna by developing a physical model of active tectonics for the Shirgah-Galugah settlements. The central hypothesis posits that morphometric indices will reveal high tectonic activity in the region due to the persistent influence of these regional faults. To test this, the methodological framework employs five standard indices: Hierarchical Anomaly, Bifurcation Ratio, Drainage Density, Form Factor, and Relative Relief. Utilizing ArcGIS and a 30-meter Digital Elevation Model (DEM), these metrics are systematically calculated for 15 distinct sub-basins and integrated into a Relative Active Tectonics Index (I~at~). This integrated assessment aims to produce a detailed hazard zoning map, thereby offering a practical framework for urban planners to mitigate seismic risks and enhance public safety within this high-risk tectonic environment.
 
Mothodology
The methodological framework is founded on a combined quantitative and spatial analysis using geographic information systems. A 30-meter resolution DEM served as the primary topographic data source, supplemented by 1:25,000 topographic maps and 1:100,000 geological maps. All spatial data processing, drainage network extraction, and watershed delineation were conducted within the ArcGIS environment. Based on the extracted hydrographic features, the entire study area was systematically divided into fifteen distinct sub-basins, each serving as an independent analytical unit for subsequent evaluations.
To assess relative tectonic activity, five standard morphometric indices were selected based on their well-established effectiveness in active tectonic studies. These include the Hierarchical Anomaly Index (Ha), which quantifies perturbations in stream ordering; the Bifurcation Ratio (R), reflecting structural controls and lithological variations; Drainage Density (Dd), representing total stream length per unit area; the Form Factor (Ff), measuring basin elongation via the formula Ff = A/L², where A is basin area and L is its length; and Relative Relief (Bh), calculated as the elevation difference between the highest and lowest points in each basin.
Following the calculation of each index, the resulting numerical values were classified into five discrete tectonic activity ranks, ranging from very high to very low. To achieve comprehensive integration, the Relative Active Tectonics Index (Iat) was derived by averaging the assigned ranks of the five indices for each basin. This integrated approach facilitates a holistic assessment of cumulative tectonic deformation. Ultimately, the calculated Iat values formed the basis for generating a detailed spatial zoning map, which illustrates the physical model of active fault hazards and serves as a definitive output for seismic risk evaluation and urban planning.
 
Findings
The quantitative analysis of the five selected morphometric indices across the fifteen delineated sub-basins reveals distinct spatial patterns of tectonic activity that strongly correlate with the region's major structural faults. Each index was systematically classified into five categories, from very high to very low activity, providing a robust foundation for comparative assessment.
For the Hierarchical Anomaly Index (Ha), sub-basins 2, 3, 4, 8, 9, 10, and 14 exhibited the highest values, indicating significant perturbations in their drainage networks, while sub-basins 5, 10, 14, and 15 recorded the lowest values. Regarding the Bifurcation Ratio (R), the highest values were concentrated in sub-basins 2, 4, 11, and 13, spatially aligned with the North Alborz Fault, demonstrating the structural control of active faulting on stream ordering. Similarly, the highest Drainage Density (Dd) was recorded in sub-basins 11, 13, and 14, closely corresponding with the Dozdban and Marzanabad fault zones, where dense hydrographic networks indicate active surface deformation. The Form Factor (Ff) analysis confirmed the elongation of drainage basins in sub-basins 1, 2, 4, 5, 9, and 10, a morphological characteristic commonly observed in regions undergoing active tectonic uplift. Furthermore, Relative Relief (Bh) was most pronounced in sub-basins 11 and 12, where high elevation differences reflect rugged topography driven by recent vertical crustal movements.
Upon integrating these individual classifications into the composite Iat, the results demonstrate that sub-basins 1, 8, 9, and 14 possess the highest levels of tectonic activity, largely attributed to the persistent influence of the North Alborz and Khatirkuh (Orim) faults. According to the final zoning model, approximately 33.3% of the total study area falls into the high to very high tectonic activity categories. Notably, in several sub-basins, the presence of secondary faults—formed in response to the primary structural systems—amplified the morphometric values, contributing to elevated tectonic ranks. Collectively, these findings provide a clear, data-driven physical model of active fault hazards across the urban corridor, confirming the region's dynamic tectonic state and the substantial seismic risk to the settlements between Shirgah and Galugah.
Discussion and Conclusion
This research successfully addresses the significant knowledge gap concerning active tectonics in the Shirgah-Galugah urban corridor. The findings align well with previous regional studies, validating the effectiveness of the selected morphometric indices as reliable indicators of tectonic deformation. However, due to the specific geological complexities of the area, some commonly used quantitative indices—such as mountain front sinuosity—proved inapplicable, necessitating a careful methodological adjustment to ensure the integrity of the results.
The integrated Iat reveals that sub-basins 1, 8, 9, and 14 exhibit the highest tectonic activity, a phenomenon directly attributable to the persistent influence of the North Alborz and Khatirkuh faults. Furthermore, the presence of secondary faults, developed in response to these primary structural systems, has significantly amplified morphometric values in adjacent sub-basins, further elevating their tectonic classifications. The final zoning model indicates that approximately 33.3% of the study area is subject to high to very high tectonic activity, a reality that substantially elevates seismic risk for local settlements. The geographic overlap between active faults and densely populated residential zones creates critical vulnerability, as a strong earthquake could result in catastrophic human and economic losses.
In conclusion, the physical model developed in this research effectively highlights active tectonic hazards across the urban corridor, confirming that most settlements are situated directly upon or in close proximity to major active faults. These findings provide a concrete, evidence-based framework bridging geological assessment and practical urban safety planning. To mitigate these substantial risks, it is strongly recommended that urban planners and policymakers prioritize detailed seismotectonic and microzonation studies to accurately determine fault relationships and seismic potential. Moreover, strict enforcement of seismic building codes and systematic retrofitting of critical infrastructure—including bridges, hospitals, and older residential fabrics—must be urgently implemented. Finally, the active fault hazard map generated in this study should be explicitly incorporated into local master plans to prohibit future urban expansion within the highest-risk zones, thereby safeguarding lives and assets within this inherently vulnerable tectonic setting