ملاحظاتی درباره وضعیت ژئوتکنیک لرزه‌ای شهر اصفهان

شهر اصفهان یکی از بزرگ‌ترین شهرهای ایران و جهان از لحاظ جمعیتی و صنعتی بوده و دارای میراث تاریخی و فرهنگی می‌باشد. متاسفانه در این منطقه از حیث شهرت به ایمن بودن در برابر زلزله، مطالعات بررسی اثر ساختگاه در برابر امواج لرزه‌ای بسیار اندک انجام شده است. در این پژوهش با استفاده از به‌کارگیری روش‌های تحلیل عددی یک‌بعدی و دو بعدی و استفاده از نتایج میکروترمور، مطالعات ژئوتکنیک و لرزه‌زمین‌ساخت، بررسی منطقه اصفهان در دو راستای شمال جنوب (هم‌راستا با تغییرات توپوگرافی شهر) و شرق غرب هم‌راستا با مسیر رودخانه زاینده‌رود انجام شده است. نتایج نشان می‌دهد رفتار اثر ساختگاه اصفهان را در راستای شمال جنوب که همراه با تغییرات توپوگرافی می‌باشد به‌صورت دو بعدی و راستای شرق – غرب را با ملاحظاتی می‌توان به‌صورت یک‌بعدی در نظر گرفت. همچنین نتایج نشان می‌دهد برای یک نقطه مشخص رفتار دو راستا نتایج متفاوتی را ارائه می‌کنند، بنابراین لازم است در تحلیل‌ها این مورد در نظر گرفته شود. همچنین نتایج نشان می‌دهد فرکانس طبیعی شهر اصفهان از 0/8 تا 11 هرتز به ترتیب از سمت نواحی جنوبی شهر به سمت شمال دشت اصفهان تغییر می‌کنند.

Considerations about Seismic Geotechnical Characteristics of Esfahan

Conducting seismic geotechnical microzonation studies from the point of view of site effects requires an assessment of dynamic properties and especially the natural frequency of site in the study area.Common methods for estimating these parameters include using microtremors measurements and real earthquakes. In addition, in order to study the exact behavior of the site and determine the exact seismic geotechnical model of the region, it is necessary to perform numerical analysis and use the results of microtremor experiments and use geological information in the region. Without using numerical methods, solving problems of the frequency domain is usually impossible. For this purpose, numerical methods obtain the relations and equations governing the domain by examining the differential behavior of the problem (the model under study). In this regard, finite element methods and modeling of energy absorbing boundaries to reduce the reflection data of seismic waves have been used in numerical analysis. Also, in microzonation studies, the scale of work must be selected from valid instruction like TC4. For this purpose, the method of grade 3 in TC4 has been used.In this research, the study area of Esfahan has been selected, which is one of the important cities in terms of the existence of numerous historical monuments, relatively high population and strategic industries. Unfortunately, in this region, due to its reputation for safety against earthquakes, very few seismic studies and site effect studies have been conducted. Considering seismic characteristics of site in urban planning and design of structures can provide the basis for sustainable and safe development in cities. Carrying out studies in this regard can prevent the construct of structures in any area, which can be in the resonance period or take the necessary measures to minimize damage.The main geology texture of Esfahan is influenced by two elements. Firstly, the Zayandehrood River, which flows eastwest direction in the city of Esfahan, and next the mountains around Esfahan, especially Sofeh Mount, which the main part of the city are affected by its slope.In this study, using onedimensional and twodimensional numerical analysis and microtremor results, geotechnical and seismic studies, the seismic geotechnical behavior of Esfahan region has been studied in the northsouth direction (parallel to the topographic changes of the city) and eastwest along the river.One of the significant points identified in this study is the different seismic geotechnical behavior of the northsouth and eastwest directions with each other and the fact that the behavior of the northsouth profile of Esfahan should be considered due to changes of the surrounding environment.The results show that the behavior of Esfahan site in the northsouth direction, which is affected by topographic changes, should be considered as twodimensional and the eastwest direction onedimensionally can be considered. The results also show that the natural frequency values ​​of Esfahan increase from southern area to northern area respectively. In this study, after related analyzes and comparison of seismic geotechnical results and dynamic analysis, the study area was classified into three categories: The southern areas of the city with a natural period less than 0.4, the central areas with a period range of 0.4 to 0.8 and the northern areas. The city is in the natural period above 0.8 seconds.