Short-Period Seismic Waves Generated in Asperity Models by Dynamic Simulation of Fault Rupturing

Abstract eng:
In Japan, asperity models, consisting of asperities of high stress drop and backgrounds of no stress drop, are often adopted as kinematic source models for predicting strong ground motions from scenario earthquakes. The asperities are assumed to generate long-period seismic motions more and short-period seismic motions far more than the backgrounds. In order to specify the asperity models, two theoretical equations and one empirical equation are applied to evaluation of the three parameters out of the following six parameters: the area of the entire fault, the seismic moment, the averaged stress drop, the area of the asperity, the stress drop of the asperity, and the amplitude of the acceleration source spectrum in the short-period range. Since the empirical equation describing that the amplitude of the acceleration source spectrum in the short-period range is proportional to the product of the stress drop on the asperity and the size of the asperity had not been studied systematically, we carried out the dynamic simulation of fault rupturing of the asperity models to verify this empirical equation. The faults were assumed to be 15 km long and 10 km wide with the asperity of 5.8 km by 5.8 km located in various positions, and the dynamic simulation was carried out by the three dimensional finite difference method. Most of the results showed that the asperities generated the short-period seismic waves more than the backgrounds did as granted in the current researches. But, some of the results showed that the backgrounds generated the short-period seismic waves as much as the asperities did. All of the results showed that the peak of the velocity slip time-function increased on the fault far from the hypocenter, because more strain energy released on the ruptured area could be applied to the fracture energy on the fault far from the hypocenter. This phenomenon indicated that we should check the location of the area that generates the short-period seismic waves based on the relative location of the hypocenter, when we specify the asperity models for predicting the strong ground motions from scenario earthquakes.

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