Estimation of Conditional Mean Spectrum (CMS) in Zagros, Iran

Document Type : Research Article

Authors

1 Ph.D student, International Institute of Earthquake Engineering and Seismology, Tehran, Ira

2 Assistant Professor, International Institute of Earthquake Engineering and Seismology, Tehran, Iran

Abstract

Selection of ground motion time series provides a critical link between site-specific probabilistic seismic hazard analysis (PSHAs) and dynamic seismic response analysis to quantify the seismic performance of a system of interest. The conditional mean spectrum (CMS) can be used for selecting the target earthquake records and dynamic analysis of structures. The conditional mean spectrum gives the mean spectral form associated with the spectral acceleration of the target Sa(T*), so that the seismic records that match the target spectral shape can be considered as the representative of the seismic target records of the spectrum Sa(T*). In this paper, in order to extract the combinations of magnitude, distance and epsilon of the earth’s movements, seismic hazard analysis and deaggregation in the selected site of Bakhtiari Dam has been done based on the natural period of the structure and the return period of 475 years. After preparing the seismic data bank and near-fault data using Iran’s data (Zagros region) and regarding the seismic characteristics of the selected site, appropriate classification of the database based on the magnitude and distance has been done. The ultimate goal of this article is to prepare and present the conditional mean spectrum of the area near the fault for the selected structure of Bakhtiari Dam. For structures with short rotation times, the correlation relationship can have a significant effect on the final response spectrum.
   The results indicate that by selecting high periods as the target period, the difference between the uniform risk spectrum and the conditional mean spectrum increases, so the shape of the conditional mean spectrum is more sensitive to tall structures. In fact, the shape of these spectra is highly dependent on the target period. This point is more important in the dynamic analysis of structures with several degrees of freedom because in these structures, considering only one period of target rotation gives rise to downstream results. In the conditional mean spectrum, the closer the two cycles to each other, the higher the correlation of the epsilon values and the less scatter. This means that the farther apart the two rotations are, the less similar the spectral acceleration values are to each other.
   We know three methods by which ground motions can be selected for dynamic seismic response analyses of engineered systems when the underlying seismic hazard is quantified via ground motion simulation rather than empirical ground motion prediction equations. Even with simulation-based seismic hazard, a ground motion selection process is still required in order to extract a small number of time series from the much larger set developed as part of the hazard calculation.

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References

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Iranian Journal of Geophysics
Volume 16, Issue 1
May 2022
Pages 21-31

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