پهنه‌بندي خطر زمين‌لرزه در ايران و برآورد مقادير بيشينه شتاب براي مراكز استان‌ها

Seismic hazard zoning in Iran and estimating peak ground acceleration in provincial capitals

به‌منظور ارزيابي سطوح جنبش نيرومند روي سنگ بستر براي بيشينه شتاب و مقادير طيفي در پهنه ايران، برآورد خطر زمين‌لرزه به روش احتمالاتي اصلاح شده صورت گرفته است. فهرست نامه‌اي (كاتالوگي) يكنواخت، شامل زمين‌لرزه‌هاي تاريخي و دستگاهي تا انتهاي 2012 در ناحيه‌اي محدود به 22 تا 42 درجه عرض شمالي و 42 تا 66 درجه طول شرقي، تهيه شده است. تعداد 238 چشمه بالقوه زمين‌لرزه به‌صورت پهنه‌اي تعيين و پارامترهاي لرزه‌خيزي هريك ارزيابي شده است. در برآورد آهنگ رويداد زمين‌لرزه براي بازه‌هاي بزرگي گوناگون در هر چشمه بالقوه زمين‌لرزه، از تابع توزيع مكاني استفاده شده است. با نرم‌افزار OpenQuake، برآورد خطر زمين‌لرزه در ايران براي شبكه‌اي شامل بيش از 40000 نقطه و سه رابطه تضعيف و دو مدل لرزه‌خيزي با استفاده از درخت منطقي، صورت گرفته است. نقشه‌هاي خطر زمين‌لرزه براي PGA و SA در دوره‌هاي تناوب 2/0 و 2 ثانيه روي سنگ بستر با 5% ميرايي براي 10% و 63% احتمال فزوني در 50 سال تهيه شده است. براي دوره بازگشت 475 سال، مقدار PGA در مناطق گوناگون كشور از g63/0 براي شمال شرق استان لرستان تا g1/0 براي مناطق مركزي ايران متغير است. در بين مراكز استان‌هاي كشور، بندرعباس و تبريز با g35/0 بيشترين و يزد و اصفهان با g11/0 كمترين مقدار PGA روي سنگ بستر را براي دوره بازگشت 475 سال به خود اختصاص داده‌اند. نتايج، با نقشه پهنه‌بندي خطر نسبي زمين‌لرزه در ايران (استاندارد2800، ويرايش 3) و تحقيقات صورت گرفته ساير محققان، مقايسه شده است.

Growing environmental and social concerns, both on the part of decision makers and public opinion, have brought a new perspective to the perception of hazard assessments a valid alternative in the long-term, and an effective complement in short and medium terms, to traditional design procedure for a resistant and safe environment. Results of the gradual development of research on the probabilistic seismic hazard assessment (PSHA) in the past 40 years make a framework that could be used for estimation of probability of occurrences of earthquakes, at certain return periods on each site. The primary advantage of the PSHA over alternative representations of the earthquake threat is that PSHA integrates over all possible earthquake occurrences and ground motions to calculate a combined probability of exceedance that incorporates the relative frequencies of occurrence of different earthquakes and ground-motion characteristics. Features of the PSHA allow the ground-motion hazard to be expressed at multiple sites consistently in terms of the earthquake sizes, frequencies of occurrence, attenuation, and associated ground motion. Potential seismic sources, seismicity models, ground motion prediction equations (GMPE) and site effects, are the most important factors in seismic hazard studies. In this research, a modified probabilistic seismic hazard assessment, developed by Chinese researchers, is used to estimate the level of the potential seismic ground motion in Iran. A unified catalog of de-clustered earthquakes containing both historical and recent seismicity until late 2012 in the area encompassed by 22-42?N and 42-66?E is used. An area source model which contains 238 potential seismic sources within 5 major seismotectonic provinces in the study region has been delineated. Considering magnitude uncertainty and incompleteness of the earthquake data, the seismicity parameters of the seismotectonic provinces are determined. Spatial distribution function is used to determine occurrence rates of potential seismic sources for different magnitude intervals. Also, the background seismicity has been determined for each province. Seismic hazard assessment of Iran for a grid of over 40,000 points with 10 km interval is carried out using OpenQuake software by three different GMPEs and two models of seismicity for potential seismic sources in a logic tree. The peak ground horizontal acceleration (PGA) and spectral accelerations (SA) for 5% damping ratio at 0.2 and 2 seconds corresponding to 10% and 63% probability of exceedances within 50 years (475- and 50-years mean return periods, respectively) are calculated. The resultant seismic hazard maps display a probabilistic estimate of PGA and 0.2 and 2 sec SA for different mean return periods of 50 and 475 years. Resultant peak ground horizontal acceleration for 475-years return period varies from 0.63g in North-East of Lorestan to 0.1g in central Iran. The resultant PGAs for the 475-year return period in provincial capitals indicate the maximum value (0.35g) in Bandar Abbas and Tabriz, and the minimum one (0.11g) in Esfahan and Yazd. Comparison of the results of this study with the last map of seismic hazard in the Iranian code of practice for seismic resistance design of buildings, seismic macrozonation hazard map of Iran, Standard 2800, shows significant differences. Seismic hazard levels estimated in this study in southern Iran, Sistan-Baluchestan, Hormozgan and Fars provinces, show significantly higher values.