ارزیابی توان لرزه خیزی گسل هرات در غرب افغانستان بر پایه فراسنج های لرزه زمین ساختی و نرخ گشتاور لرزه ای (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
سیستم گسلی هرات غربی یکی از پهنه های فعال لرزه زمین ساختی افغانستان می باشد که در سال ۲۰۲۳ چهار زمین لرزه را تجربه نموده است. نظر به اهمیت این منطقه و رخداد لرزه ای متوالی، در این پژوهش پویایی لرزه زمین ساختی منطقه بر اساس فراسنج های لرزه خیزی و تحلیل فرکتالی مورد بررسی قرار گرفته است. در این پژوهش، تغییرات نرخ لرزه خیزی منطقه بر پایه تغییرات فراسنج لرزه خیزی (b-value) و بعد فرکتال شکستگی و لرزه خیزی (D-value) با استفاده از داده های لرزه ای موجود بررسی و تحلیل شده است. در یک تقسیم بندی منطقه مورد مطالعه به ۹ پهنه اولیه تقسیم شده و در هر پهنه فراسنج های لازم محاسبه شده است. بزرگترین ابعاد فرکتالی گسل محاسبه شده مربوط به پهنه های جنوب شرقی، مرکزی و شمال غربی است و بیشترین مقدار بعد فرکتالی لرزه خیزی مربوط به مناطق مرکزی و شمال غربی می باشد. مقادیر محاسبه شده برای پارامتر b در مرکز محدوده مطالعاتی و منطبق بر گسل هرات نشان دهنده افزایش تنش در این ناحیه بوده که ممکن است هشداری برای وقوع یک زمین لرزه بزرگ در آینده باشد. نرخ گشتاور لرزه ای نشان دهنده نرخ تغییر در انرژی سطح زمین است که به تغییرات لرزه ای تبدیل می شود. مقدار نرخ گشتاور لرزه ای برای کل منطقه مطالعاتی برآورد شده است. با توجه به مجموع تحلیل های فرکتالی و محاسبه گشتاورهای لرزه ای منطقه، مناطق زنده جان، انجیل، غوریان و کوشان می توانند به عنوان کاندیدای زلزله های بعدی معرفی شوند. مقایسه نتایج به دست آمده این دیدگاه را تقویت می کند که غرب افغانستان به عنوان پهنه ای دگرشکلی در حال تکامل، مستعد رخدادهای لرزه ای بیشتری است.Evaluation of the Seismic Potential of Herat Fault in the West of Afghanistan Based on Seismotectonic Parameters and Seismic Moment Rate
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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