عنوان مقاله [English]
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.