On the 2D Characteristics of Small-Scale Trapezoidal Sedimentary Basins

Document Type : Seismology and Engineering Seismology


1 Ph.D. Student, Disaster Risk Management Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran

2 Professor, Disaster Risk Management Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran


Recent researches clearly revealed that the 1D lateral model fail to reproduce actual site response characteristics in complex wave propagation field. In spite of many available researches on 2D or 3D effects of large-scale basins, different 2D or 3D behavior of small-scale sedimentary basins, are not well understood and explained. In the previous study, the authors studied the effects of small-scale basins and found it very important, depending on the shape of the lateral irregularities. Among different shapes of lateral irregularities, the small trapezoidal basin shows special site amplification characteristics, which need to be examined further. In this paper, different aspects of small-scale trapezoidal basins such as slope angle, basin length, infill soil properties and basin thickness are parametrically investigated to clarify those effects on strong ground motion characteristics. For this purpose and in the absence of recorded earthquake data on such lateral irregularities, extensive parametrical studies are carried out by using finite difference numerical analysis. Then, huge numbers of trapezoidal small basin models are constructed and are simultaneously subjected to the earthquake motions in both horizontal and vertical directions. The site response at the points along the basin are analyzed in the frequency domain using Fourier spectral ratio, and in the time domain using the ratio of 2D horizontal and vertical peak ground accelerations with respect to 1D ones are defined as the horizontal and vertical aggravation factors (AGH and AGV). The AGH and AGV factors show large sensitivity to infill soil properties and thickness as well as slope angle. The AGH shows large variation in the middle of small trapezoidal basin in the range of 1.5 to 2. Meanwhile, the AGV shows large variation around the two basin edges with the values of 1.5 to 2.5. Finally, the outcomes provide some recommendations in design, and emphasize on the importance of 2D analysis in site effect estimation of small-scale trapezoidal basin.


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