بررسی اثرات حوزه نزدیک و جهت‌پذیری در مطالعات تحلیل‌خطر زلزله – مطالعه موردی گسل دورود

در این مطالعه، با توجه به واقع شدن شهر دورود در حوزه نزدیک گسل دورود، اثرات حوزه نزدیک و جهت‌پذیری ناشی از آن بررسی شده است. اثرات حوزه نزدیک و جهت‌پذیری، می‌تواند منجر به ایجاد پالس‌های پریودبلندی در جنبش زمین شده و برای ساختارهایی با پریودبلند از قبیل پل‌ها که در نزدیکی گسل‌هایی با نرخ فعالیت بالا هستند، گنجاندن اثرات جهت‌پذیری در روابط کاهندگی می‌تواند تاثیر زیادی در نتایج تحلیل‌خطر لرزه‌ای واقع‌گرایانه داشته باشد. گسل دورود در هنگام رخداد زمین‌‌لرزه برای پریود‌های بلند در محدوده شهر دورود، می‌تواند موجب تشدید پارامترهای جنبش زمین شده و باعث خسارات جدی در این محدوده ‌شود. در این مطالعه، پارامتر‌های جنبش نیرومند زمین در تحلیل‌خطر احتمالاتی زمین‌‌لرزه با اعمال جهت‌پذیری برای محدوده گسل دورود برآورد شده است. همچنین با بررسی واهمافزایی خطر زمین‌‌لرزه، تاثیر جهت‌پذیری برای میزان مشارکت فاصله و بزرگا در برآورد پارامتر شتاب جنبش زمین ارزیابی شده است. در دوره بازگشت‌های بلند‌مدت و کوتاه‌مدت، تاثیر جهت‌پذیری برای پریود‌های مختلف برای پارامتر شتاب جنبش نیرومند زمین با روش سامرویل و آبراهامسون برآورد و موردارزیابی قرار گرفته است. مقدار شتاب برآوردشده برای سه دوره بازگشت، 50، 475 و 2475 سال و در پریود‌های 0.75، 1، 2، 3 و 4 محاسبه و ارزیابی شده است. مقدار پارامتر شتاب جنبش زمین با افزایش دوره بازگشت و پریود رابطه مستقیم داشته به‌طوری‌که بیشترین مقدار افزایش شتاب با تاثیر جهت‌پذیری، در دوره بازگشت 2475 سال و در پریود 4 ثانیه، 17.16 درصد محاسبه شد.

Investigation of the near-field and directivity effects in earthquake hazard analysis studies - a case study of Doroud fault

In this study, considering the location of Dorud city in the area near the strikeslip and seismic fault of Doroud, the effects of the near site and the directivity due to rupture have been investigated in seismic hazard analysis studies. Doroud fault is located near the cities of Doroud and Boroujerd, in the western part of Iran. Dorud and Boroujerd are among the important cities of Iran in the agricultural industry and also due to the pristine nature in these areas has always been of interest to tourists. The microearthquakes recorded in this area indicate the activity of the Doroud fault system. In order to prevent possible earthquake damage in this area, seismicity studies can be useful to study the acceleration of the ground by considering the effects of the site in order to strengthen the construction of civil structures. Abrahamson (2000) and Somerville et al., (1997) were among the first researchers to establish studies based on this, and the relationships and methods proposed by them are more acceptable today in applying the directional effect. These researchers considered two parameters of angle and ratio of fault length as a direct factor in the effect of orientation and examined the results for the acceleration spectrum created. The effect of orientation can lead to the formation of longperiod pulses in the earth’s motion, which some proposed models (eg Somerville et al., 1997) can measure the quantity of this effect in estimating earthquake risk analysis with a deterministic and probabilistic approaches. (Abrahamson, 2000). In this study, seismic hazard has been investigated, compared and evaluated by considering the effects of Doroud fault in different periods and different return periods by considering the effect of orientation and also without applying the effect of orientation. Nearfield and directional effects can lead to longperiod pulses in ground motion parameter, and for structures with long periods such as bridges and dams near faults with high activity rates. The inclusion of directional effects in attenuation relationships, to see whether for deterministic and probabilistic approach can have a great impact on the results of realistic seismic hazard analysis. Doroud fault is one of the most important faults in Iran with a history of large earthquakes in the early instrumental period and its mechanism of strikeslip mechanism, It can intensify the strong motion parameters during earthquakes for long periods in the city of Dorud, and consequently cause serious damage to structures with long periods in this area. In this study, the parameters of strong ground motion in the analysis of probabilistic earthquake hazard by applying direction for the range of Doroud fault have been estimated. In addition, by examining the disaggregation of earthquake hazard, the effect of direction for the contribution of distance and magnitude in estimating the strong motion parameter has been evaluated. In the short and long return periods, the effect of directivity for different periods for the strong motion has been estimated and evaluated by the Somerville and Abrahamson method. The estimated acceleration is calculated and evaluated for three return periods, 50, 475 and 2475 years and in periods of 0.75, 1, 2, 3 and 4 sec. The value of the strong motion parameter was directly related to the increase of the return period and the period, so that the highest amount of acceleration increase (17.16 percentage) with the effect of directivity was calculated in the return period of 2475 years and in the 4second period.