مطالعه اثر ساختگاه مبتنی بر HVSR مایکروترمور در شهر زنجان (ایران)

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی علوم پایه زنجان، زنجان، ایران

چکیده

با بررسی زلزله­های پیشین رخ داده در جهان و ایران، مشخص شد یکی از عوامل مهم در برآورد آسیب مناطق در برابر زلزله، ویژگی­های خاک سطحی حین زمین‌لرزه است. این موضوع به اثر ساختگاه معروف است. شهر زنجان در شمال‌غرب کشور در منطقه‌ای با خطر زلزله‌خیزی زیاد و اثر ساختگاه متنوع قرار دارد؛ از این‌رو، در مطالعات مربوط به پهنه­بندی خطر زلزله، بررسی اثر ساختگاه در این منطقه اهمیت ویژه‌ای دارد. در این تحقیق، از روش ناکامورا برای بررسی رفتار خاک استفاده شده است. داده­ها در چهل و یک نقطه از شهر و روی مقاطع از­پیش­طراحی­شده برداشت شدند. نتایج نشان‌دهنده آن است که بیشترین فرکانس مشاهده­شده مربوط به مناطق شمالی شهر است که روی رسوبات کم‌ضخامت واقع است. در بخش جنوبی، در نزدیکی رودخانه زنجان­رود، فرکانس تشدید به کمترین مقدار خود یعنی کمتر از یک هرتز می­رسد. در شمال­غرب و نیمه جنوبی شهر، در بعضی از مناطق دو بیشینه فرکانس مشاهده می­شود که بیانگر وجود لایه کم­سرعت زیرسطحی است. با استفاده از درون­یابی فرکانس‌های به‌دست‌آمده با روش کریجینگ، نقشه­های پهنه­بندی اثر ساختگاه، ترسیم و با نقشه­های به‌دست‌آمده از اطلاعات گمانه­های ژئوتکنیکی برای سرعت موج برشی در شهر مقایسه شد. نتایج مقایسه، نشان‌دهنده تطبیق قابل­قبول روش ناکامورا با اطلاعات گمانه­هاست. در شمال شهر، فرکانس تشدید در محدوده چهار هرتز است؛ بنابراین باید از ساخت ساختمان­های یک تا سه طبقه جلوگیری کرد یا تمهیدات لازم را در سازه­های آنها لحاظ کرد تا تشدید صورت نگیرد. به‌طور مشابه، باید در محدوده گسترده‌ای از مرکز و غرب شهر که فرکانس طبیعی کمتر از یک هرتز است، از ساخت ساختمان­های ده تا دوازده طبقه و در شمال­شرق شهر که فرکانس تشدید خاک بین 7/0 تا 5/1 هرتز است، از ساخت ساختمان­های شش تا پانزده طبقه اجتناب شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Microtremor HVSR Study of Site Effects in Zanjan City (Iran)

نویسندگان [English]

  • Beheshteh Hakimi
  • Zohreh Masoumi
  • Abdoreza Ghods
  • Najmeh Etemad-Saeed
Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
چکیده [English]

Natural frequency of soils is one of the important factors in the study of vulnerability to earthquakes. In areas characterized by soft sediments, the maximum amplitude of ground motion is common that leads to enhanced seismic hazard and risk. Zanjan city is located in northwest of Iran with high risk of earthquake hazard according to Building and Housing Research Center (BHRC). So, investigation of site effect, beside other parameters, in earthquake vulnerability is considerable.
To map natural frequency of soil in Zanjan, microtremor horizontal-to-vertical spectral ratio (HVSR) method has been conducted. Specially, we used Nakamura’s method on ambient noise records. We employed 3-component medium band Guralp seismometer. Ambient noise was recorded in 41 sites in a pre-designed profile. At each site, noise was recorded for at least one hour long. Geopsy software was used for data processing. Evaluation of the gathered data was examined following the recommended guidelines of SESAME (Site EffectS assessment using AMbient Excitations) project.
The results of this study represent that there is one amplification peak in most stations. Considering the first peaks, the natural frequency of soil decreases from north to south in the city. The decrease of natural frequency represents an increase in the thickness of soil layer. In some sites, in west and center of the city, parallel to the Zanjanrud river, there are two peaks in spectral ratio. The second peak was always more than 3 Hz. The second peak is related to a shallow thin and low velocity sedimentary layer.
To evaluation of results, the data for standard penetration test of boreholes was collected and shear wave velocity was estimated. Using the shear wave velocity obtained from the boreholes, we estimated the thickness of soil from the measured natural frequency of soils employing quarter wavelength law. The estimated thickness of soil shows the presence of a thin sedimentary layer with high velocity in the north of Zanjan. The bedrock slope becomes steeper by moving towards south and flattens within the basin. Inside the basin, the natural frequency is less than 1 Hz and the thickness of the sediments increases to about hundreds of meters.
In general, the results of this study investigate one step for seismic hazard assessment and risk qualification of this urban area where great damages can be attained in case of strong earthquakes. Hence, these results should be taken into consideration before establishing the new urban constructions in the area of study.

کلیدواژه‌ها [English]

  • Site Effect
  • microtermorsite effect
  • Earthquake vulnerability zoning maps
  • microtermor

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