ارزيابي ضرايب تبديل شتاب زلزله‌ي طرح به شتاب‌هاي سطوح مختلف زلزله براي شهرستان‌هاي ساري و فريدون‌شهر

Assessment of Conversion Coefficients of Design Based Earthquake Acceleration to Accelerations of Different Seismic Risk Levels for the Cities of Sari and Fereidounshahr

براي نيل به اهداف طراحي لرزه‌يي، شناخت سطوح مختلف زلزله ضروري است. با وجود اين، در آيين‌نامه‌هاي طراحي لرزه‌يي، عمدتاً شتاب مبناي زلزله‌ي طرح ارايه مي‌شود و در مورد سطوح ديگر زلزله، صراحتاً اظهارنظر نمي‌شود. هدف اين پژوهش، تعيين ضرايب تبديل شتاب زلزله‌ي طرح (زلزله‌ي 10%) به شتاب‌هاي سطوح ديگر زلزله (2%، 5%،20%،50% و 64%) است. براي رسيدن به اين هدف، دو منطقه‌ي ساري و فريدون‌شهر انتخاب و نسبت شتاب‌ها در اين دو منطقه تعيين شده است. براي تعيين شتاب‌ها، از روابط كاهندگي مختلف استفاده شده است. اين نسبت‌ها براي ساري به ترتيب برابر با 41/2، 51/1، 64/0، 28/0 و 2/0 و براي فريدون‌شهر به ترتيب برابر با 88/1، 33/1، 74/0، 43/0 و 36/0 به‌دست آمده است. نتايج نشان مي‌دهند كه ضرايب حاصل از اين پژوهش با ضرايب پيشنهادي در آيين‌نامه‌ها مطابقت دارد.

The aims of considering structural seismic design regarding various seismic risk levels are different. For example, the structure should be serviceable after weak earthquakes and should not collapse after strong motion. Seismic design goals depend on earthquake motion levels and expected performance levels. Therefore, in order to approach certain design objectives, the recognition of a range of seismic levels is highly essential. Based on seismic design codes, design based earthquake ground motion is usually recommended, though, for other earthquake levels, it has not been greatly considered. This research attempts to compare the conversion coefficients of design based earthquake ground motion (10%) to other earthquake acceleration levels (2%, 5%, 20%, 50% and 64%). For this purpose, two susceptible areas of Sari (Elburz zone) and Fereidoonshahr (Zagros zone) were chosen, and their design accelerations relevant to the various seismic levels were determined. Afterwards, the ratios of obtained ground-motion compared to design based ground-motion were calculated. For each of the above mentioned areas, available seismic data were collected and considered in a radius of 200 km for calculating seismic factors, based on the Kijko method. Major fault systems of the sites were reviewed and modeled using suitable attenuation parameters. SeisriskIII was employed as technical software for calculating the ground-motion peaks of different seismic risk levels by applying logic tree coefficients. Isoseismal maps were prepared for both areas, yielding the ratios of acceleration levels for various seismic designs. These ratios are (0.2, 0.28, 0.64, 1.51 & 2.41) and (0.36, 0.43, 0.74, 1.33 & 1.88) for Sari and Fereidoonshahr, respectively. Finally, the results of this study show good correlation with the proposed coefficients of the standard code. The results indicate that the Iranian Code of Practice for Seismic Design (Standard no. 2800) estimates well the design based acceleration for Sari and Fereidoonshahr.