Abstract

Extremely valuable strong-motion records during the 2008 Wenchuan earthquake were obtained at a number of stations close to the source region. We estimate the characterized source model for simulating ground motions using the empirical Green’s function (EGF) method and the hybrid method using strong-motion data from the earthquake. The characterized source model consists of several asperities with large slip in the entire rupture area of the earthquake. The locations of the asperities that generate strong motions were determined from the timing of pulse arrivals in observed records near the source fault. Because aftershock records of the earthquake had not been released yet, observed records from an aftershock (Mw 4.9) of the 2008 Iwate–Miyagi Nairiku earthquake (Mw 6.9), which occurred in the shallow inland crust of the northeastern region in Japan, are used as substitutes for the EGFs. The synthetic motions obtained by the EGF method agree well with the observed records with respect to acceleration and velocity waveforms in the period range of less than 2 s. Broader-band strong motions are simulated by the hybrid method, which combines the numerical method for periods longer than 2 s with the EGF method for periods shorter than 2 s. The best-fit source model was obtained with stress drop on each asperity of approximately 13 MPa. The ground motions numerically simulated at the Bajiao station (SFB) near an asperity and in the forward rupture direction show directivity effects that are too strong in comparison with the observed motions. We examined the influence of rupture-velocity fluctuation on the synthetic waveforms in the forward rupture direction near the asperity, finding that the forward directivity effects decrease with increase in velocity variances.

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