محاسبه طیف طراحی برای انواع مختلف خاک در ایران، بر اساس داده های نزدیک گسل و مقایسه آن با آیین نامه 2800

زلزله های نزدیک گسل نسبت به زلزله های دور از گسل دارای خصوصیات متفاوتی می باشند. در سال های اخیر در آیین نامه های مختلف اثرات ناشی از زلزله های دور و نزدیک به صورت جداگانه بررسی شده است. مطالعه حاضر نتایج بررسی 214 شتاب‌نگاشت به‌دست‌آمده از زمین‌لرزه‌های کم‌عمق پوست ه ای رخ داده با فاصله کانونی کمتر از 80 کیلومتر از گسل مسبب، جهت تعیین طیف طرح در ایران و مقایسه آن با طیف طرح آیین‌نامه 2800 را نشان می دهد. این شتاب‌نگاشت‌ها از زمین لرزه های بزرگ ایران، آمریکا و اروپا جمع آوری و پردازش شده اند.  پس از محاسبه طیف پاسخ، طیف طراحی از میانگین طیف پاسخ شتاب‌نگاشت‌ها برای هر چهار نوع خاک (i، ii، iii، iv) ارائه شده در آیین‌نامه 2800 محاسبه شد. برای مقایسه نتایج این مطالعه با آیین‌نامه رکوردهایی با بزرگای بیشتر از 5/5 و فاصله رو مرکزی کمتر از 80 کیلومتر انتخاب شد و همچنین پارامتر میرایی 5 درصد مورد استفاده قرار گرفت. نتایج حاصل نشان می دهد که طیف طراحی هر دو مولفه خاک نوع یک تا سه در این مطالعه تا پریود 0/39 ثانیه با طیف طراحی آیین‌نامه همخوانی دارد درحالی‌که در پریودهای بالاتر از 0/39 ثانیه طیف طرح آیین‌نامه بالاتر از طیف طرح این مطالعه قرار می‌گیرد. از طرفی طیف طرح مولفه قائم خاک نوع iv بالاتر از طیف طرح آیین‌نامه قرار گرفته است که این می تواند به دلیل تعداد کم رکوردهای این نوع خاک باشد.

Calculation of Design Spectrum for Different Soil Types in Iran, Based on Near Fault Data and its Comparison with Iranian Building Code (Standard 2800)

Nearfault earthquakes have different characteristics comparing to farfield recorded events, they often contain strong coherent dynamic long period pulses and permanent ground displacements. In the recent years, the effects of nearfield and farfield earthquakes have been studied separately in various building codes.Building codes have been revised and updated depending on the improvements in the representation of ground motions, soils and structures. These revisions have been more frequently seen in recent years. One of the key changes in earthquake codes has been performed on the design spectra. Design spectra are used in seismic analysis methods such as equivalent static lateral force analysis, dynamic spectral analysis and time history dynamic analysis. Various seismological and geophysical parameters affect the shape of design spectra. Ambraseys et al. (1996) and Bommer and Acevedo (2004) presented and discussed the effects of earthquake magnitude, sourcetosite distance, site classification, and styleoffaulting on the strongmotion accelerograms and consequently design spectra.The present study shows the results of 214 accelerograms, obtained from shallow crustal earthquakes with epicenral distance less than 80 km from causative fault, to determine the design spectrum in Iran. A comparison is performed with the design spectrum of Iranian building code (Standard No. 2800).So far, due to the lack of information and registered accelerograms in Iran, as well as the unclear ground conditions at the place of record registration, no effective action has been taken to determine the design spectrum in Iran. The site conditions have been classified into different categories in earthquake codes. These categories are named ground types, soil profile types, local site classes or subsoil classes.In Iranian Seismic Building Code, four types of soils I, II, III, IV have been determined, Site Class  I: Rock,  or hard Rock, or thin alluvium on bed rock with Vs30 more than 750 m/s; Site Class  II, very dense soil or soft rock with high thickness on rock bed with Vs30 between 375 and 750 m/s; Site Class III stiff soil or soils with medium density or hardness with Vs30 of 175 to 375 meters per second; and Site Class  IV, soft soils with a Vs30 of less than 175 meters per second. The accelerograms, obtained from major earthquakes in Iran, United States and Europe have been collected and then processed. After calculating the response spectra, the design spectra was plotted from the average response spectra values for all soil types, (I, II, III, IV) presented in the Standard No. 2800.To compare the results of this study with spectra presented in Iranian Seismic Building Code, records obtained from event having magnitudes greater than 5.5 and recorded in the epicentral distances less than 80 km were selected and 5% damping was used for calculations. The results show that design spectrum for soil classes I, II, and III, were consistent with Standard No. 2800 for the spectral values of Periods 0.0 to 0.39 seconds, while in the periods longer than 0.39 seconds, the design spectrum of the Standard No. 2800 is more conservative  than the present design spectra. Meanwhile, the spectral ordinates for design spectra of vertical component for soil class IV were representative for higher values for the present spectra, comparing to that presented in Standard No. 2800. This might be related to epistemic uncertainty imposed by still few available records for soft soil site (class IV).