تحليل و تجزيه‌ي ‌خطر لرزه‌يي شهر تبريز با توجه به اثر پديده‌ي حوزه‌ي ‌نزديك

Near-Field Effect in Probabilistic Seismic Hazard Analysis and Disaggregation of Tabriz City

زمينلرزه‌هاي ثبت‌شده در نزديكي گسل گسيخته‌شده، اغلب حاوي پالس‌ در ركورد سرعت- زمان هستند، كه مي‌توانند تقاضاي قابل توجهي را در سازه‌هاي اين مناطق به‌وجود آورند. روش‌هاي تحليل‌ خطر موجود به‌خوبي نمي‌توانند اثر اين پديده را در طيف پاسخ به‌درستي پيش‌بيني كنند. شهر تبريز، يكي از شهرهاي صنعتي و تاريخي ايران است، كه گسل شمالي آن به عنوان فعال‌ترين گسل منطقه، در نزديكي آن واقع شده است. لذا در اين مطالعه، با استفاده از روابط پيشنهادي، نتايج حاصل از دو روش تحليل ‌خطر عادي و اصلاحشده‌ي شهر تبريز به صورت نقشه‌هاي هم‌تراز شتاب طيفي به‌دست آمده است. براي تعيين اثر پديده‌ي حوزه‌ي نزديك در ميزان مشاركت پارامترهاي مختلف دخيل در تحليل ‌خطر احتمالاتي، تجزيه‌ي خطر عادي و اصلاح‌شده‌ي اين پارامترها براي نقاط مختلف شهر براي 2 دوره‌ي بازگشت 475 و 2475 ساله انجام و نقش پارامترهاي منحصر به پديده‌ي حوزه‌‌ي نزديك مانند پالس و دوره‌ي تناوب آن مشخص شده است.

Ground motion close to a ruptured fault, resulting from forward directivity, contains strong coherent dynamic long periods that cause it to be significantly different from other ground motion. Current PSHA (Probabilistic Seismic Hazard Analysis) methods are not able to capture such effects well and, therefore, are not able to predict these effects on spectral shape. Tabriz is an ancient and industrial city with many industrial factories and financial centers in North-Western Iran. In this region, the North-Tabriz Fault passes through at a close distance to urban areas and controls the seismic hazard scenario in the city. To ignore the directivity effect in PSHA may possibly lead to an underestimation of, in particular, the nonlinear demand. Herein, a PSHA analysis is carried out using appropriate relationships to consider near field directivity effects. Uniform seismic hazard curves of several points around Tabriz city are calculated using ordinary and modified PSHA, and the pulse effects on these curves are discussed. To depict the pulse effect in this city, iso acceleration maps of spectral periods of 2 and 4 seconds, for return periods of 475 and 2475 years, are calculated in accordance with ordinary and modified PSHA. Moreover, iso amplification maps of these two periods for the return period of 2475 years are calculated for better depiction of pulse effects in the Tabriz city region. Furthermore, the results disaggregated for better assessment of the effect of seismic sources, in terms of distance and magnitude. These calculations are carried out for 1 and 3 second spectral periods and for two return periods of 475 and 2475 years. The results show that accounting for pulse like effects in PSHA increases the relative contribution of close distances and low values of epsilons (?) to spectral acceleration exceedance. Moreover, the contributions of each earthquake scenario to long period spectral acceleration shift to larger magnitudes by including the effects of near fault directivity. The results confirm that due to the contribution of close distances at higher values of ground acceleration, consideration of near-field directivity effects is inevitable.