After a large earthquake, it is often required to estimate strong ground motions during the earthquake at a site where an engineering structure was damaged. In this article, a new practical method of strong-motion estimation is proposed, based on aftershock records both at the site of interest and at a nearby strong-motion station and on the record of the large event at the strong-motion station. The method is simply composed of three steps. First, the Fourier amplitude at the site of interest for the large event is evaluated by correcting the Fourier amplitude at the strong-motion station for the difference of the path effect and the site amplification factors at the two sites. Then, the Fourier phase at the site of interest during the large event is approximated by the Fourier phase for a small event that occurred close to the main rupture area of the large event. Finally, an inverse Fourier transform is conducted to obtain the time history of strong ground motions at the site of interest during the large event. The method is applied to strong-motion records during the 11 August 2009 Suruga Bay, Japan, earthquake (Mw 6.2) to confirm its applicability. An obvious advantage of the proposed method is its simplicity. Unlike full strong-motion simulations, it does not require fault models. Therefore, the method will contribute to a prompt estimation of strong ground motions at a site of engineering interest after a large earthquake. At the same time, unlike conventional simple techniques, the present method is focusing not only on the spectral amplitude but also on the Fourier phase. Therefore, it leads to high-quality estimation of strong ground motions.

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