تحلیل فضایی مخاطرات محیطی

تحلیل فضایی مخاطرات محیطی

بررسی مخاطرات لرزه ای با مطالعۀ تغییرات آهنگ کرنش در البرز مرکزی با استفاده از داده های GPS

نویسنده
شاهرخ پوربیرانوند
پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله
10.52547/jsaeh.8.2.71
چکیده
البرز مرکزی به علت استقرار کلان شهر تهران از اهمیت خاصی در مطالعات مرتبط با سوانح طبیعی از جمله خطر لرزه ای برخوردار است. این اهمیت باعث توسعۀ شبکه ای نسبتاً متراکم از ایستگاههای GPS در این منطقه و مناطق همجوار شده است. در این مطالعه از درون‌یابی بردارهای سرعت GPS بر روی یک شبکه مستطیل شکل و محاسبۀ کرنش در مرکز هر سلول برای مطالعۀ تغییرات آهنگ کرنش در گستره البرز مرکزی استفاده شد. نتایج دلالت بر تغییرات مکانی در محورهای اصلی آهنگ کرنش ژئودتیکی در منطقه و همچنین تغییر در آهنگ کرنش مساحتی (اتساع) در قسمتهای مختلف البرز مرکزی دارد که در تلفیق با جهت گیری گسلهای اصلی منطقه، واقعیت‌های مهمی را دربارۀ سازوکار حرکت گسلها آشکار نمود. بی‌هنجاری‌های مشاهده شده در این کمیتها، با لرزه‌خیزی و حرکتهای راندگی و راستالغز بر روی گسلهای مهم منطقه تطابق معنی داری را نشان داده و افراز دگرشکلی را در البرز مرکزی و نواحی پیرامون که به دلیل اعمال تنش های تکتونیکی مایل بر گسلها و صفحات شکستگی های ضعیف از پیش موجود رخ می دهد، مورد تایید قرار دادند. بی هنجاری های مثبت مشاهده شده در نقشۀ آهنگ کرنش مساحتی یا اتساع بیانگر مکان های مستعد مخاطرات لرزه ای می باشند.
کلیدواژه‌ها
آهنگ کرنش مساحتی
افراز دگرشکلی
البرز مرکزی
سازوکار گسلش
مخاطرات لرزه ای
GPS

عنوان مقاله English

Seismic risk investigation by Strain rate variation study in central Alborz by using GPS data

نویسنده English

Shahrokh Pourbeyranvand
International Institute of Earthquake Engineering and Seismology
چکیده English

Seismic risk investigation by Strain rate variation study in central

Alborz by using GPS data

Abestract

The Alborz Mountains, South of Caspian Basin and separates Central Iran from Eurasia. Talesh and Kopeh Dagh bound the Alborz as major thrust belts in the west and east respectively. The tectonic evolution of this important region is still unsolved and there are many questions to answer, such as the origin of the Alborz Mountains as well as its crustal structure. The Alborz is of great important in natural and most particular, seismic hazard investigations, because of the existence of Tehran megacity. This importance resulted in development of a relatively dense network of GPS stations in this regions and adjacent areas. The Alborz Mountains formed successively during the collision of Central Iran with Eurasia in the Late Triassic (Cimmerian Orogeny) and the collision of Arabia with Eurasia. Tectonic activity in this belt is currently thought to be controlled by two motions with different velocities, the 5 mm/yr northward convergence of central Iran to Eurasia causing a compression from 7 Ma and the 4 mm/yr left‐lateral shear northwestward motion of the South Caspian Basin resulting in a left lateral transpressive tectonic environment in the Alborz . Since middle Pleistocene transtensional motion is also observed in the region because of acceleration of SCB motion toward North West.

GPS studies in the Zagros started in 2000 and are continuing by gradual expansion of the permanent GPS network and several GPS campaigns and temporary stations. These studies have significantly improved our understanding of the surface deformation in the Alborz. In this study the interpolation of GPS velocity vectors in a rectangular grid and calculation of the strain at the center of each grid cell, were used for the study of the strain rate variations in the central Alborz. We used velocity vectors from Djamur et al. (2010) to estimate the strain rate field in the Alborz. To avoid edge-effects in the strain calculation, we only showed the results for the central part of the dataset. The GPS velocities are interpolated onto a rectangular north-south grid of 0.2 by 0.2 degrees and strains are calculated at the center of each grid cell, following the methodology of Haines et al. (1998) and Beavan & Haines (2001). The study of the strain rate variations can help in understanding the tectonic settings of the region and the obtained results, combined with other geodetic, geological and seismological studies, already performed in the region, can provide a comprehensive insight into the geodynamic evolution of the range.

The results showed spatial variations in principle strain rate axes directions and areal strain rate or dilation, which in combination with seismicity data, reveals important information about the fault movement mechanisms in the area. Observed anomalies in dilation, showed important correlations with seismicity, subsidence and uplift, dip slip and strike slip movements on the faults in the region and confirmed deformation partitioning which takes place due to tectonic forces, acting on pre-existing faults and weak fracture planes. The partitioning of the deformation causes dominant strike slip motion in some parts of the Central Alborz, while shortening occurs dominantly on other parts of the mountain range. These different parts are spatially separated in the region and correlate with the seismicity with regard to the faulting mechanisms expected from the orientation of the major faults and the directions of strain rate axes.



Key words: areal strain rate Central Alborz, deformation partitioning, dilation, faulting mechanisms, GPS, seismic ris

کلیدواژه‌ها English

areal strain rate Central Alborz
deformation partitioning
dilation
faulting mechanisms
GPS
Seismic Risk
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