بررسي آماري پارامترهاي لرزه اي در منطقه زاگرس

Statistical Evaluation of Seismic Parameters in Zagros Region

نوار زاگرس از مناطق فعال لرزه خيز ايران است كه عمده اين لرزه خيزي حاصل فعاليت گسلهاي معكوس و امتدادلغز مي باشد. در اين مطالعه تقسيم بندي نوار زاگرس بر اساس مرزهاي تفكيك- كننده مورفوتكتونيكي لحاظ گرديده كه شامل گسل معكوس اصلي زاگرس، گسل اصلي اخير، كمربند تراست زاگرس مرتفع، گسل زاگرس مرتفع، گسل جبهه كوهستان مي باشد. در اين تحقيق نوار لرزه زمينساخت زاگرس با به كارگيري روش فركتال، ويژگيهاي ساختاري و لـرزه اي آن مورد مطالعه قـرار گرفته شده است. به همين منظـور داده هـاي پايه مرتبط با گسلش و لرزه خيزي با بزرگـاي زميـن لرزه؛ توزيع رومركز زميـن لرزه-ها و گسلها مـورد بررسي قرار گرفته شده است. پايه و اساس آناليز فركتالي محاسبه پارامتر بعد فركتال (D c) و پارامتر لرزه اي (b) بوده كه به ترتيب با استفاده از روشهاي انتگرال سرشتي و بيشينه احتمال محاسبه شده است. متفاوت از ديگر روشهاي در اين روش توزيع زماني و فضايي پارامترها، ويـژگي تجمعي داده ها (بعد سـرشتي) و سطح تنش تكتونيكي منطقه (پارامتر b) مرتبط با فرآيندهاي مختلف لرزه زمينساختي همراه مي باشد. به منظور محاسبه اين پارامترها، از داده هاي لرزه اي مختلف (پايگاههاي داخلي و خارجي) استفاده شده است. با در نظر گرفتن توزيع رومركز زمين لرزه ها، لرزه خيزي منطقه و با در نظر گرفتن توزيع بزرگاي زمين لرزه ها، بعد سرشتي (47/2 > D > 15/2) و ميزان لرزه خيزي (6/1 > b > 3/0) محاسبـه گـرديـده و تغييـرات عمـده اين پارامترها در هر حـوزه متناسب با شرايط لرزه زمينساختي مشاهده مي-گردد.

Abstract The Zagros Fold-Thrust Belt (ZFTB) is a collision zone, which is formed due to the northeast motion of the Arabian plate toward the central plateau of Iran. At the surface, the Zagros consists of long, linear anticlinal ridges that form a series of ranges from eastern Turkey in the NW to the Strait of Hormuz in the SE, a distance of about 1200 km. Structural and seismological features of Zagros belt were studied by a new approach (Fractal). For this aim, fundamental data of faulting and seismicity, distribution of magnitudes, epicenters, and faults, were reanalyzed. The deep-seated master thrust faults bordering the morphotectonic units of the Zagros are: (1) the Main Zagros reverse fault (MZRF); (2) the Main Recent Fault (MRF); (3) the High Zagros thrust Belt (HZTB); (4) the High Zagros fault (HZF); and (5) the Mountain Front Fault (MFF). The scale-invariant geometry of many natural fracture systems and earthquake populations suggests that fault structure and seismicity can often be described in terms of fractal sets. In principle, the various fractal dimensions may be used as a quantitative measure of the degree of heterogeneity of seismic activity in fault systems. These in turn are controlled by the heterogeneity of the stress field and the pre-existing geological. However, fractal dimensions obtained by different methods generally reflect different aspects of the scale invariance, and need not be equal to or even positively corre-lated with one another. Parameters of fractal analysis are fractal dimension (correlation dimension DC and capacity dimension D0) and seismic b value. We investigate the spatial variations of seismicity along these zones in an attempt to investigate fault complexity along strike, quanti-fied by the Gutenberg-Richter b-value and the fractal (correlation) dimension of earthquake epicenters, by using the correlation integral method, from magnitude distribution (b value) through maximum likelihood method. Temporal and spatial variations of these parameters computed from different techniques are related with clustering properties of seismicity (correlation dimension), density and complexity of faulting and tectonic stress rate (b value). The investi-gation covers instrumentally recorded earthquakes of magnitude M > 4.5 occurring between 1900 and 2011. We find systematic spatial variations which may be related to structural or mechanical variability along strike. The correlation dimension (2.15 < D < 2.47) and b (0.3 < b < 1.6) value, respectively computed from the earthquake epicenter and magnitude distribution show significant temporal and spatial variation related with seismotectonic feature of fault zones. Besides, some parameters of fault zones were determined to have bifractal properties. The results show that the fractal dimension D in the Zagros thrust belt is changed between 2.1 to 2.47. The fractal dimension along the thrust zone shows a significant variation for the different fault segments. This maybe encouraging that the fractal analysis can be used to identify structural variations in a seismotectonically complex area. This statistical correlation is stronger when more objective seismic zoning is carried out (based on number of events) rather than more subjective seismotectonic zoning in common use in seismic hazard analysis.