تحلیل خطر احتمالی ریسک محور زمین‌لرزه بندر سیراف

با استفاده از روش‌های آماری و احتمالی تحت عنوان تحلیل خطر لرزه‌ای، می‌توان از ایمن بودن سازه‌ها در برابر زمین‌لرزه ها اطمینان حاصل نمود، ازاین‌رو هرساله تحقیقات فراوانی حاوی روش‌های جدید پهنه‌بندی خطر زلزله در سراسر جهان ارائه می‌شود؛ بنابراین ضرورت استفاده از روش‌های جدید و به‌روزی که بتوان بر اساس آن نقشه‌های خطر لرزه‌ای را در کشور به‌روز کرد قابل احساس است؛ ازاین‌رو در این پژوهش با استفاده از رویکرد تحلیل خطر ریسک محور بر طبق استاندارد asce 07-10 به بررسی وضعیت لرزه‌خیزی ساختگاه بندر سیراف در استان بوشهر با استفاده رهیافت احتمالی پرداخته که بر اساس پارامترهای مربوط در این منبع نتایج حاصل از تحلیل خطر موردبررسی قرارگرفته است. در مطالعه حاضر، به‌منظور بررسی وضعیت لرزه‌ای ساختگاه مجموعه‌ای از داده‌های لرزه‌خیزی تاریخی و دستگاهی با پوشش زمانی تا سال 2019 تا شعاع 150 کیلومتری بکار گرفته شده و منابع لرزه زا مدل شده‌اند. بدین منظور منابع لرزه زا در گستره طرح با استفاده از نقشه‌های موجود، تعیین و پس از آن مدل مناسب از چشمه‌های لرزه زا به‌صورت خطی در منطقه ارائه شده است. فهرست زمین‌لرزه‌های رخ‌داده در گستره طرح از طریق اسناد و کتب تاریخی و ثبت دستگاهی جمع‌آوری شده و با استفاده از روش کیکوسلول، نواقص موجود در کاتالوگ برطرف شده است. جهت دستیابی به توزیع پوآسونی رویدادها با استفاده از دو روش پنجره زمانی مکانی گاردنر و نوپوف و روش نظامند گرانتسال، حذف پیش‌لرزه‌ها و پس‌لرزه‌ها انجام شده است. در نهایت با ترکیب منابع لرزه زا و استفاده از روابط کاهندگی مناسب، پارامترهای لرزه‌خیزی با استفاده از روش گوتنبرگ ریشتر و کیکوسلول، محاسبه و نتایج به‌صورت طیف خطر ریسک محور در سطح طراحی برای دوره بازگشت 2475 سال با استفاده از روش احتمالی تحلیل خطر لرزه‌ای برای بندر سیراف ارائه شده است. نتایج پژوهش حاکی است که مقادیر لرزه‌ای طیف به‌دست‌آمده بر طبق آیین‌نامه asce 07-10 از مقدار پیشنهادی برای این ناحیه در استاندارد 2800 متفاوت است، همچنین پیشنهاد شده است به دلیل لرزه‌خیزی بالای منطقه از روش‌های جدید و به‌روزی که در آن‌ها ثبت و برآورد زلزله به‌صورت مناسبی انجام شده است استفاده شود.

Risk-Targeted Probabilistic Seismic Hazard Analysis for Siraf Port

1. Introduction One of the major problems facing most of the world’s major metropolises and cities is natural hazards. In seismic countries, one of the most catastrophic is the earthquake event. Using statistical and probabilistic methods called seismic hazard analysis it is possible to ensure the safety of structures against earthquakes. Every year, a lot of researches have been done to determine the hazard of earthquakes around the world; Therefore, it is necessary to use new and uptodate methods based on which seismic hazard maps can be updated in Iran. In this study, using a probabilistic approach and risktargeted hazard analysis approach according to ASCE 0710, the seismicity of Siraf port in Bushehr province was investigated. 2. Materials and Methods In the present study, in order to investigate the seismic status of the site, a set of historical and instrumental seismic data with a time coverage up to 2019 up to a radius of 150 km was used and seismic sources were modeled. For this purpose, seismic sources in the desired area, using geological maps and satellite images, were determined and a suitable model of seismic sources in the region was presented.  The list of earthquakes that occurred in the project area was made through documents, books, and Accelerometer. The defect in the catalog were eliminated by using the Kijko Sellevoll method. In order to achieve the Poisson distribution of events, foreshocks and aftershocks were eliminated using Gardner and Knopo methods and Grünthal method. Finally, seismicity parameters were calculated based on data analysis in EZFRISK 7.43 (2010). The results of seismic hazard analysis using the probabilistic method for Siraf port are presented as a riskbased at the design level for a return period of 2475 years. 3. Study Area Siraf port is located in Bushehr province near Kangan city in the south of Iran, between the south and southwest of the country (27.6667 °N, 52.3425 °E). It is one of the oldest ports in Iran, which is located between Kangan port and Assaluyeh port. The geological structure of the site follows the general trend of the Zagros with a northwestsoutheast trend. The location of the site from a geomorphological point of view has a flat and plain topography and from the point of view of structural zones of Iran, it is located in the folded Zagros (Darvishzadeh, 1991). 4. Results and Discussion 4.1. Determining the maximum horizontal acceleration of the ground motion Using the results of the probabilistic method, the parameters of the maximum acceleration values of the horizontal component of the site at the seismic levels of operation basis level (OBL), design basis level (DBL) and maximum consider Earthquake (MCE) are 0.11g, 0.39g and 0.61g, respectively. 4.2. RiskTargeted response spectrum for design level The design response spectrum based on risk concept (ASCE 0710) for this site has higher values in the entire period interval (approximately 12%) than the design spectrum with a probability of exceedance 10% and 2% in 50 years. The increase in the values of the riskbased spectrum in the site compared to the uniform hazard spectrum is due to uncertainty in the collapse capacity of the designed structures. Therefore, the probability of collapse and failure of structures designed according to this spectrum by changing from one place to another and by changing the shape of the seismic hazard curve, leads to the probability of nonuniform collapse. According to the Hazard Zoning Map of Iran in Regulation 2800, the location of the site confirms the level of high seismic hazard and the amount of acceleration of the design is 0.3gacceleration. While the earthquake hazard analysis of this region with a probability of exceedance 2% and 10% in 50 years has determined the parameters of maximum horizontal acceleration 0.61g and 0.39g, respectively. The spectrum estimated in Standard 2800 is based on 10 percent probability of exceedance within a 50year period with a Return period of 475 years. In seismically active areas where earthquakes occur most frequently, such as the west, southwest, and south coasts of the country, this method may be a logical one. But in areas where earthquakes are less common or the sensitivity of the area or site is important, the prediction of an earthquake with a return period of 475 years is underpredicted; Therefore, the definition of a maximum considered earthquake with a 2 percent probability of exceedance within a 50year period with a Return period of 2475 years should be reconsidered. Finally it is worth mentioning that the estimated probability of collapse in 50 years for a structure designed for the probability of exceedance 2% in 50years, with the 2/3 factor, is indeed more geographically uniform than that designed for the probability of exceedance 10% in 50years ground motions, without any factor. 5. Conclusion Due to the high seismicity of Iran and especially the high importance of the southern regions of the country, it is recommended that the spectrum of regulations of the country (including standard 2800) be extensively studied and updated. Therefore, it is recommended to modify the spectrum in these regulations by updating the design spectrums of these regulations, using the methods available in valid standards such as ASCE7, in which earthquake estimation has been done properly. And for very important areas (including Tehran, Bushehr and Tabriz) due to the hazard of earthquake and irreparable damage, it is recommended to use the design spectrum based on the concept of risktargeted according to ASCE 710. The results of the study indicate that the seismic values of the spectrum obtained according to Regulation ASCE 0710 are different from the proposed values for this area in the 2800 standard. The reason for this is the uncertainty in the seismic design of the structure that the risktargeted approach is able to take into account and leads to achieving a uniform level of geographical distribution to prevent the collapse of the structure.