كليدواژه :
توابع زماني , طيف چشمه زلزله , مدل حائل ويژه , مدل سينماتيكي زلزله , گسلش
چكيده فارسي :
ارتباط تنگاتنگ طراحي سازه‌ها در برابر زلزله با شناخت خصوصيات لرزه‌اي محل ساختگاه و برآورد زلزله طراحي در ساختگاه مورد نظر، دانش مهندسي را بر آن داشته تا گام در عرصه شناخت زلزله و عوامل ايجاد كننده آن گذارد و دانش لرزه‌شناسي را با ديدگاه‌ها و نيازهاي مهندسي همگام سازد. بي‌شك رسيدن به پيش‌بيني قابل‌اطمينان از حركات زمين، ناشي از وقوع زلزله در يك ساختگاه مشخص، بدون داشتن شناخت صحيح از سازوكار توليد امواج لرزه‌اي، عوامل ساختاري اثرگذار بر اين امواج در مسير انتشار و شناخت شرايط فيزيكي و ويژگي‌هاي ساختاري محل ساخته‌شدن سازه‌ها ميسر نخواهد بود. مدل حائل‌هاي ويژه كه از مشهورترين مدل‌هاي شبيه سازي زلزله است، گسل را به‌عنوان مجموعه‌اي از ترك‌هاي دايره‌اي در نظر مي گيرد. گسيختگي به‌صورت افت تنش‌هاي موضعي در اين ترك‌ها فرض مي‌شود و توقف‌ها و شروع‌هاي مكرر گسيختگي، عامل اصلي توليد امواج فركانس بالا در اين مدل است. استفاده از دواير يكسان و نيز استفاده از توابع چگالي احتمال با توزيع يكنواخت كه به‌منظور ساده سازيهاي اوليه در مدل اوليه استفاده شده است، با خاصيت ذاتي زلزله مبني بر تصادفي بودن اين رخداد مغايرت دارد. از اين‌رو، در اين مطالعه سعي شده با پيشنهاد روش جديد استخراج توابع چگالي احتمال رسيد امواج لرزه‌اي، توابع زماني متناسب با موقعيت هر ايستگاه توليد شود. روند به دست آمدن توابع چگالي احتمال ياد شده مبتني بر هندسه گسل و نيز موقعيت مكاني ايستگاه‌هاي گيرنده امواج مي‌باشد كه باعث مي‌شود طيف‌هاي چشمه به‌دست‌آمده از شبيه‌سازي به واقعيت نزديك‌تر شوند.
چكيده لاتين :
A reliable physical modeling of strong ground motion is required to examine the three crucial seismic parameters: seismic source specifications, wave propagation path and seismic site effects, which are all very important in seismic source simulation. Among various seismic source specifications, a more physically based realistic source model is the specific barrier model (SBM). The SBM is specifically more suitable for regions with poor seismological data bank and/or ground motions from large earthquakes with large recurrence intervals. In order to simulate seismic ground motions from a specific earthquake source model in an efficient way, the stochastic modeling method has been widely used. An essential part of the seismological model used in this method is the quantitative description of the far-field spectrum of seismic waves emitted from the seismic source. Since shear wave is one of the main factors of earthquake damages, the application of stochastic approach of the SBM is focused for on the far-field shearwave spectrum, in which two corner frequencies of the observed earthquake are represented. The ‘two-corner-frequency’ shows two considerable length-scales of an earthquake source: a length-scale that quantifies the overall size of the fault that ruptures (e.g., the length of a strike-slip fault) and another length-scale that measures the size of the subevents. Associated with these length-scales are two corresponding time scales: (1) the overall duration of rupture, and (2) the rise time. The SBM has a few main source parameters which have been calibrated by the parameters of earthquakes of different tectonic regions. The SBM may be considered as a general idealization of the faulting process of an earthquake. For example, a uniform probability density function (PDF) of ‘arrival times’ is assumed in the SBM. In this paper, the effects of various PDFs of arrival times on the far-field source spectrum of the SBM are studied. For this purpose, direct simulations of ground motion records for an earthquake source, which have fractally-distributed subevent sizes, is used. So, in this research, a new non-uniform more realistic PDF of arrival times for seismic waves corresponding to the fault’s geometry is derived to reach desirable time functions. To this end, the appropriate PDF of arrival time is simply computed by making various zones on the fault, based on their distance from a given receiver on the ground surface. Therefore, a large number of points on the ground surface is chosen as receiver positions, for which the PDFs of arrival time are obtained. To divide the fault to various zones with the same distance from a given receiver, several spheres of the various radius are drawn, whose centers are located on the mentioned receivers. Consequently, a group of different curves is produced by the intersection of different spheres and the fault plane. All points in the region between the semi-parallel adjacent curves are considered to have the same distance from the receiver (the center of various spheres). This means that all points surrounded by two adjacent curves have equal chance to get to the receiver position. As a result, for creating the PDF of the arrival time of seismic waves, the chance of arriving seismic wave in specific time window should be determined, based on the above mentioned simple assumption. By changing distance parameter to time parameter, the PDF of the arrival time of seismic waves may be easily obtained. Afterwards, by using the proposed time functions, the effect of site position to the fault on source spectra, as well as the effect of distance of site to the fault on time functions, are investigated.
