The traditionally used scheme of free field motions based on vertical incident seismic waves is expanded to the condition of oblique incident seismic waves in this paper. The assumption that free field motions in half plane space is contributed by oblique incident P and SV waves with arbitrary angles and their reflected waves is made firstly. Secondly the parameters of seismic quality factor, critical amplitude ratio and the influence distance of single wave are introduced to accomplish the model. Finally the free field motions based on surface earthquake records and considering the amplitude attenuation of seismic wave are obtained. The influence of critical amplitude ratio and seismic quality factor on the frequency characteristic and motions of specified point in half plane space is discussed. The result shows that a wide range of critical amplitude ratio is acceptable for the structure with low natural frequency such as hydraulic dams in the construction of free field motions. And in the condition of high quality rock mass and small spatial range, the amplitude attenuation effect could be ignored in the construction of free field motions. While in some extreme cases, the amplitude attenuation effect would change the free field motions in about 20 percent.
Key words
Free field motions /
Oblique incident /
Amplitude attenuation /
Seismic quality factor /
Critical amplitude ratio /
influence distance of single wave
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