بررسی تغییرات تنش لرزه‌ای در زاگرس جنوبی

نوع مقاله : مقاله پژوهشی‌

نویسندگان

1 استادیار پژوهشکده زلزله شناسی، پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران

2 دانشجوی دکتری، پژوهشکده زلزله شناسی، پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران

چکیده

کوهزاد زاگرس با راستای شمال باختری- جنوب خاوری که طی همگرایی صفحه عربی و خردقاره ایران در کرتاسه پسین شروع به تشکیل کرده است، نتیجه برخورد قاره- قاره بین خردقاره ایران و صفحه آفریقا- عربی است که همچنان تداوم دارد. این برخورد در زمان میوسن- پلیوسن توسعه یافته است. لرزه­خیزی در منطقه زاگرس جنوبی به منطقه بین دشت ساحلی خلیج فارس و سامانه راندگی زاگرس محدود است. فراوانی زمین­لرزه­ها در زاگرس نشانه تغییر شکل فعال خارج ازتوالی (active out-of-sequence deformation) در این بخش است که در کوهزاد­های برخوردی مشابه نیز آن را می­توان مشاهده کرد. در این مطالعه پارامترهای لرزه­خیزی کمربند زاگرس جنوبی با توجه به رابطه گوتنبرگ- ریشتر ارزیابی و ویژگی آماری زمین‌‌لرزه­ها با درنظر­گرفتن توزیع مکانی و زمانی (یا چگالی توزیع زمان و مکان) آنها بررسی می­شود. به همین منظور داده­های پایه مرتبط با گسلش و لرزه­خیزی شامل بزرگای زمین­لرزه، توزیع رومرکز زمین­لرزه­ها و گسل­ها بررسی شد. مبنای این تحلیل، محاسبه پارامتر لرزه­خیزی (b) است که بر اساس بیشینه احتمال محاسبه شد. در این روش توزیع زمانی و مکانی پارامترها، ویژگی تجمعی داده‌ها (بعد همبستگی) و سطح تنش زمین­ساختی منطقه مرتبط با فرایندهای مختلف لرزه­زمین‌ساختی متفاوت از دیگر روش­ها لحاظ شده است. با درنظر­گرفتن توزیع رومرکز و بزرگای زمین‌لرزه­ها، میزان تغییرات پارامتر لرزه­خیزی مناطق کمربند پهنه گسل­های فلسی، مزوپتامین- خلیج فارس و کمربند پیش‌بوم زاگرس  به‌ترتیب 64/0>b>61/0، 05/1>b>98/0 و 7/1>b>1 برآورد شده است. بر اساس نتایج تغییرات پارامتر لرزه­خیزی، تغییرات اختلاف تنش بیشینه و کمینه (3σ-1σ) در پهنه زاگرس جنوبی در امتداد سه برش عرضی انتخابی محاسبه و مقادیر (MPa 470-230)، (MPa 296-205) و (MPa 550-60) به‌دست­آمد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Investigating seismic stress changes in the South Zagros

نویسندگان [English]

  • Mehrdad Mostafazadeh 1
  • Leila Mahshadnia 2
1 Assistant Professor, seismology research center, Tehran, Iran
2 Ph.D. student, seismology research center, Tehran, Iran, Tehran, Iran
چکیده [English]

Being a collision zone, the Zagros Fold-Thrust Belt was formed because of 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. In this study, the south of the Zagros zone was evaluated by considering the concepts of seismicity parameters considered in the Gutenberg-Richter relationship. In other words, the statistical characteristics of earthquakes are considered in accordance with their spatial and temporal distribution. For this purpose, the basic data of fault and seismicity associated with the magnitude of earthquakes are investigated by the distribution of earthquakes and faults. The basis of the calculation of the b-value parameter is estimated using a frequency–magnitude distribution and correlation integral methods.
    Contrary to other methods, the time and space distribution of parameters, the cumulative properties of the data and the tectonic stress level of the region associated with different seismotectonic processes are considered in this method. In order to calculate these parameters, different seismic catalogs (internal and external bases) have been used. Considering the distribution of earthquakes and magnitude data, the calculated seismicity parameter values were 0.61<b<0.64, 0.98<b<1.05 and 1.0<b<1.7 in IMBZ, MMF and SFB, respectively. By using the b-value quantity at any point, the map of differential stress (σ1-σ3) was calculated as (230-470MPa), (205-296MPa) and (60-550MPa) along the three selected profiles across the south Zagros region. Based on the study of changes in seismicity parameters in the south Zagros zone and clustering of large earthquakes that form areas with high-stress concentration, the heterogeneity of seismicity parameters covers a significant part of the study area. Large b-values indicate the random occurrence of small earthquakes and consequently, the existance of low-stress structures in parts of the region.
    The trend of changes in fractal dimensions in each of the seismic zones shows the changes of desired parameter in each of the fault systems. It has decreased to the lowest extent in the south of Zagros. In the north west of Zagros (west of the Kazerun fault), the shortening is mainly limited to the mountain edge fault, where large earthquakes are concentrated. To the east of the Kazerun fault, the fastest rate of shortening seems to occur along the zone of active thrust faults (Lar, Breez and Qir) and also along the main Zagros fault (regions where seismicity is concentrated). In contrast, the Persian Gulf coast has a relatively low seismicity rate.

کلیدواژه‌ها [English]

  • South Zagros
  • seismicity
  • b-value
  • spatial distribution of stress differences

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مجله ژئوفیزیک ایران
دوره 17، شماره 5
بهمن و اسفند 1402
صفحه 21-46

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