بررسی ناهمسانی لرزه‌ای پهنه‌ی خوزستان در جنوب باختری زاگرس به کمک تحلیل فرکتالی

با استفاده از هندسه فرکتالی می‌توان توزیع نامنظم عوارض زمین‌شناسی را به صورت کمی بررسی نمود. اهمیت تحلیل فرکتالی در تعیین پویایی زمین‌ساختی، ناهمگونی لرزه‌ای و بلوغ زمین‌ساختی می‌باشد. در این پژوهش از روش مربع‌شمار برای اندازه‌گیری بعد فرکتالی گسل‌های فعال، پارامتر لرزه خیزی bو زمین‌لرزه‌های رخ داده در پهنه خوزستان به عنوان یکی از مهم‌ترین مراکز انرژی های هیدروکربوری خاورمیانه، طی سال‌های 2019-1900 استفاده شده است. برای این منظور کل منطقه به 16 زیرپهنه تقسیم شده میزان بعد فرکتال در هر پهنه محاسبه شده است. محاسبه بعد فرکتال گسل‌های فعال، پارامتر لرزه‌خیزیb و نسبت آنها و فرکتال زلزله‌های رخ داده در پهنه‌ی خوزستان، نشان‌دهنده‌ی کاهش پویایی زمین‌ساختی از خاور و شمال خاور به سمت مرکز و جنوب می‌باشد. ضریب همبستگی حدود 0.86 – نشان می‌دهد که بین دو پارامتر bvalue و dvalue همبستگی مناسبی برقرار می‌باشد که بیانگر افزایش میزان احتمال رخداد زمین‌لرزه‌های بزرگ بر روی گسل‌هایی با مساحت زیاد می‌باشد. نتایج نشان می‌دهد که در مناطقی که تعادلی بین فرکتال لرزه‌ای و گسل‌ها وجود ندارد می‌تواند به عنوان کاندید زلزله‌های آتی یا وجود گسل‌های پنهان معرفی گردد. با توجه به نتایج این پژوهش مناطق اطراف گسل‌های ایذه، گسل mff و گسل لهبری از مناطق کاندید زمین‌لرزه‌های آتی در پهنه ی مورد مطالعه معرفی می‌گردند.

The Seismicity Dissimilarity Investigation of Khuzestan in the Southwest of Zagros using by Fractal Analysis

The earthquake phenomenon has been explained by powerlaw relations with respect to magnitude, time andspace. Fractal is one such powerlaw relation, which is a twopoint spatial correlation function for earthquakeepicenters [12]. It reflects the heterogeneity of seismic activity in a fault system. Another powerlaw relation is bvalue,which is a frequencymagnitude relation defined by GutenbergRichter [3]. The bvalue of a region reflectsthe frequencymagnitude characteristics of seismogenic structures, stress distribution in space and depth [48].In this paper, the fractal dimension (D) obtained by the boxcounting method as the most general approach forcalculating D (Turcott, 1989). According to this method, the study area was initially superimposed on a square gridsize 1 r . The unit square (r) of the area was divided into small squares of linear size 1 1 r / 2, r / 4 and 1 r / 8,sequentially. The geometry of fractals is calculated by powerlaw distribution and the potential D, is represented bythe fractal dimension:Ni=〖C⁄r〗_i^Dwhere, Ni is the number of objects, characterized by the linear dimension r, C; proportionality constant and Df fractal dimension, which is calculated by (Turcotte, 1992):D=(⁡Log((Ni+1)/Ni))/(Log(ri/(ri+1)))At simplest form of Equation (2), the fractal dimension was determined from the slope of the log N (ri)versus log (1/ri) plot.Log(N)=C+K log⁡(1/S)In this paper, bvalue, spatial fractal dimension of seismicity and faults D (s and f) are used toevaluate the seismicity of the Khuzestan zone in Southwestern part of Zagros zone in time interval 1900 to 2018.The seismicity data of the Zagros zone are extracted from unified seismic catalog of the Iranian Plateau. Spatialvariations of bvalue, D(s) and D(f) demonstrate large variations in seismicity behavior along the study area.The most vulnerable regions for the occurrence of the large earthquakes in the study area considering thecomputed lowest bvalues and the highest Dvalues. The relationships among DC b are used to classify the level ofearthquake hazards for individual seismic source zones, in which the calibration curves illustrate a negativecorrelation among the DC and b values. It is observed that the relationship among b and D may be used forevaluation of seismicity and earthquake hazard assessment because of the high value for correlation coefficients andlimited scattering of the calculated parameters.The results indicate low bvalues and high moderate D(s) and D(f) in the North study area while the Central and southwest is accompanied by low bvalues and high D(s) in time interval 19002018, which indicates different stress release regimes in northeast and southwest parts of the study area. The Index Regime Stress )R’( in study area is 2.24±0.44, that shows convergent in southwest of Zagros.Fractal analyses of the active faults and earthquake show increasing values from southwest to northeast. Kriging zoning maps of fractal variations show this content. Fractal dimensions of faults and earthquake in the Khuzestan area show different values, which varied in the NESW direction. Based on these variations, the northwestern parts of the study area have more tectonic activity than the southeastern parts.The lack of faults outcrop in Zagros range, nonequality in faults mechanism, lack of conformity in distribution of earthquake epicenters by faults exist. Consume of many states rate of energy duration of folding process and difference in tectonic and structural style of Sundries part of the study area causes difference in the amount of fractal dimension in southeast Zagros.According to the zoning maps and the identification of high stress zone in the study area, the cities of Izeh, Baghmalek, Haftkel and in the later stages Masjed Soleiman and Ramhormoz along with the surrounding settlements will be introduced as the main candidate areas for future earthquakes.References1. Kagan, Y.Y. and Knopoff, L. (1980) Spatial distribution of earthquakes: the twopoint correlation function. Geophysical Journal International, 62(2), 303320.2. Mandelbrot, B.B. (1982) The Fractal Geometry of Nature. W.H. Freeman, New York. 468p.3. Gutenberg, R. and Richter, C.F. (1944) Frequency of earthquakes in California. Bull. Seism. Soc. Am., 34, 185188.4. Mogi, K. (1967) Earthquakes and fractures. Tectonophysics, 5(1), 3555.5. Mori, J. andAbercrombie, R.E. (1997) Depth dependence of earthquake frequencymagnitude distribution in California: implications for the rupture initiation. Journal of Geophysical Research, 102, 1508115090.6. Wiemer, S. and Wyss, M. (1997) Mapping the frequencymagnitude distribution in asperities: an improved technique to calculate recurrence times. Journal of Geophysical Research, 102, 1511515128.7. Wiemer, S., Mcnutt, S.R., and Wyss, M. (1998) Temporal and threedimensional spatial analysis of the frequency magnitude distribution near Long Valley Caldera, California. Geophysical Journal International, 134, 409421.8. Wyss, M., Klein, F., Nagamine, K., and Weimer, S. (2001) Anomalously high bvalues in the South Flank of Kilauea Hawaii: evidence for the distribution of magma below Kilauea’s East Rift Zone. Geophysical Journal International, 134(2), 409421.