Document Type : Research Article
Authors
1
MSc. in Department of Geophysics, Faculty of Nano and Bio Science and Technology, Persian Gulf University, Bushehr, Iran
2
Assistant Professor in Department of Geophysics, Faculty of Nano and Bio Science and Technology, Persian Gulf University, Bushehr, Iran
Abstract
The western Herat Fault system is one of Afghanistan's most seismotectonically active regions, having experienced four Mw >6 earthquakes in 2023. Given the significance of this region and the occurrence of successive seismic events, this study investigated the seismotectonic dynamics of the area through seismicity parameters and fractal analysis. Here, changes in the seismicity rate were analyzed based on variations in the seismic parameter (b-value) and the fractal dimension of fracturing and seismicity (D-value) using available seismic data.
The study area was divided into nine primary zones, with each zone's parameters calculated. The largest fault fractal dimensions were identified in the southeastern, central, and northwestern zones, while the highest seismic fractal dimension was associated with the central and northwestern regions. The calculated b-values in the center of the study area, coinciding with the Herat Fault, indicate elevated stress accumulation, potentially signaling future large earthquakes.
The seismic moment rate, representing the rate of energy change released through seismic activity, was estimated at 1.18 × 10¹⁸ Nm/yr for the entire study area. Combined fractal analysis and seismic moment calculations suggest that the Zendeh-Jan, Injil, Ghurian, and Kushan districts are potential candidates for future earthquakes. These results reinforce the interpretation that western Afghanistan, as an evolving deformation zone, remains highly susceptible to seismic events.
Extended Abstract
Introduction
The Alpine-Himalayan seismic belt passes through Afghanistan, making the country prone to frequent earthquakes that cause significant human and financial losses. The northeastern regions experience the highest rate of seismic activity. Recently, several earthquakes have occurred along the Herat Fault system, highlighting this region's seismic potential.
Fractal analysis is a statistical approach extensively used to analyze spatial variability (Turcotte, 1997; Dimiri, 2000). This research builds upon recent developments in seismic studies, including fractal analysis of earthquake distributions conducted in Turkey, Japan, and India (Nanjo & Nagahama, 2004; Singh et al., 2012), applying these methods to the Herat Fault system. The study estimates the b-value as an indicator of seismicity and the D-value as an indicator of earthquake and fault fractal dimensions using square-counting and least squares methods. The seismic moment rate is further estimated to study energy dissipation and tectonic mechanisms (Bridges & Gao, 2006; Pal, 2008). This represents one of the first comprehensive studies of seismic activity along the Herat Fault in Afghanistan.
Material and Methods
Seismic data from 2011 to 2024 were extracted from regional and global earthquake catalogs, including ISC, USGS, and historical earthquake reports. The data were refined using the Reasenberg spatial-temporal window in Zmap software to remove foreshocks and aftershocks. Earthquakes were plotted on fault and topographic layers in ArcGIS. Main fractures and faults were identified from USGS 1:500,000 geological maps and enhanced with remote sensing techniques using ETM+ and ASTER imagery in ENVI 5.6 and ER Mapper.
The b-value was calculated using the least squares method, where lower values indicate higher shear stress. Fractal dimensions were computed using the box-counting method and mapped using kriging in ArcGIS (D-value). Seismic moment rates in 36 subregions (30×30 km each) were estimated using instrumental and historical data, revealing stress concentrations on the Siyah Bobak and Herat faults.
Results and Discussion
The fractal dimension of fault zones reveals their geometry and complexity. Higher values indicate denser, more complex fault networks (Charchi et al., 2001). In this study, section 9 faults showed the highest fractal dimension, suggesting greater fault density and potentially higher seismic risk, particularly in southern and southeastern areas due to stress concentration.
Fractal analysis of earthquake distribution showed high fractal dimensions in central, northwest, and western subregions, indicating greater seismic clustering. However, these findings depend on catalog completeness, and Afghanistan's limited monitoring infrastructure necessitates reliance on corrected global databases.
The b-value (relative frequency of large vs. small earthquakes) is inversely proportional to stress. Values <1, as found along the Herat and Siyah Bobak faults, indicate higher shear stress and greater likelihood of large earthquakes (Scholz, 1968; Wyss, 1973). The D-value/b-value ratio suggests areas with low b-values and high D-values are more prone to significant seismic activity. High seismic moment rates around the Herat Fault correlate with 2023 seismic events, indicating blind faults and ongoing seismic potential (Johnston, 1996; Hanks, 1979; Kanamori, 1977).
Conclusions
The study reveals active deformation in Herat resulting from Indian-Eurasian plate movement, releasing substantial seismic energy. Zendeh-Jan, Injil, Ghurian, and Kushan represent high-risk areas, with the northwestern region showing particularly high seismicity. These findings underscore the need for improved fault mapping and infrastructure development.
Acknowledgment
The authors thank Persian Gulf University for their support and research environment. Special gratitude to Dr. Shabir Ashkpoor Motlagh and Dr. Sayyed Reza Mansouri for their valuable suggestions and article review.
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