شبيه‌سازي ركوردهاي زلزله حوزه نزديك با استفاده از توابع موجك

Simulation of Near-Field Records using Wavelet Functions

بررسي مشخصات اصلي جنبش‌هاي نيرومند زمين، سابقه طولاني دارد. مشاهدات و ارزيابي‌هاي انجام شده بر خسارات ساره‌اي ايجاد شده پس از زلزله‌هاي بزرگ نورتريج 1994 كاليفرنيا، كوبه 1995 ژاپن، بم 2003 و طبس 1978 ايران، بيان‌گر اثرات بسيار ويران‌گر ركوردهاي نيرومند حوزۀ نزديك است. مهم‌ترين ويژگي‌ اين ركوردها كه آن‌ها را از ركوردهاي حوزۀ دور متمايز مي‌كند، توانايي توليد اسپايك‌هاي پر انرژي و به نسبت كوتاه مدت شتاب همراه با پالس‌هايي با دامنه بزرگ و به نسبت طولاني مدت در تاريخچه زماني سرعت زمين است. هم‌چنين با توجه به ساختار آماري محدود و وجود كاستي‌هاي فراوان، پردازش داده‌هاي طيفي موجود ناتوان از ارائه كامل اطلاعات هستند. با توجه به اين‌كه زلزله‌هاي بزرگ داراي دورۀ بازگشت طولاني است و نيز بسياري از مناطق لرزه‌خيز ايران فاقد ثبت ركوردهاي نيرومند زلزله هستند، بدين لحاظ نياز به توليد و شبيه‌سازي رويدادهاي بزرگ محتمل با كاربرد مدل‌هاي فرم بسته و آناليز داده‌هاي موجود احساس مي‌شود. در اين تحقيق تلاش بر آن است تا با نگرش جامع تحليلي بر مدل فرم بسته ماوروديس و پاپوجورجيو به ارائۀ يك مدل رياضي تعميم يافته به‌منظور شبيه‌سازي پالس­هاي موجود در ركوردهاي حوزۀ نزديك بپردازيم.

Study on the main characteristics of strong ground motions, has relatively long history. The observations and investigations on the structural damages after strong earthquakes such as Northridge 1994 in California, Kobe 1995 in Japan, Tabas 1978 and Bam 2003 in Iran, are representatives of the destructive effects of strong near-field records. The most important specification of the near-field records which distinguish them from far-field records, is their ability to generate energized and relatively short-duration acceleration spikes as well as high amplitude and long-domain velocity pulses. Moreover, according to the lack of accurate statistical profiles as well as many deficiencies, processing the spectral existent data is not able enough to fully explain the seismic tremors. Based on the fact that the great earthquakes have long recurrence interval and also many high seismic zones of Iran do not possess strong tremors, hence generating and simulating feasible great events is required by applying closed form models and analysis of available data. In this study, in order to simulate the existent pulses in the time history of near-field records, the developed mathematical configuration is presented by analytical comprehensive attitude on the closed form model by Mavroeidis and Papageorgiou (2003). Material and methods Simulation of strong ground shakings, especially in areas where there is limited recorded data, plays a key role in assessing dynamic behavior of structures. Owing to unique characteristics of strong near-field ground motions, it is not possible to determine exact effects of these strong records on structures using simplified mathematical models. It is feasible to develop more complicated models which represent much more characteristics of near-field ground motions. Mavroeidis and Papageorgiou (2003) studied the parameters affecting near-fault ground motions. Their studies resulted in introducing a mathematical model capable of interpolating velocity pulses of near-field earthquake records (MP model). This closed-form MP model interpolates long duration pulses using a set of input spectral parameters. The pulse period, the pulse amplitude, the number and phase of half cycles are the key parameters that define the shape of velocity pulse. Thus, a four-parameter model has been developed to describe velocity pulses which contain forward directivity effects. In this research, it was observed that by using a combination of cubic and exponential terms, an enhanced model for interpolating the pulses presented in near-field earthquake records could be achieved (EMP model). Figure 1 shows the analytical interpolation of acceleration and velocity time histories using MP and EMP models.