Abstract
Theoretical study has been made to investigate the excitations of short-period P- and S-waves on the basis of a dynamic stochastic fault model. The long-wave description of the earthquake source is obtained through a linear superposition of double-couple point sources over a fault, and the seismic directivity is represented by the Doppler effect. Meanwhile, the short-wave description would be obtained by an energy additive superposition of the fracturing fault patches. The seismic directivity of short-waves would be totally under the controll of a patch multiplexing effect, which represents the degree of overlapping of random pulses due to the random fracturing of fault patches. It has been shown that the excitation of short-period S-waves is by far prominent than that of P-waves. This is confirmed through an analysis of energy density spectra of P- and S-waves from the dynamic stochastic source. This result would provide us with an answer to the question why the strong ground motion due to large earthquakes in short epicentral distances is always controlled by S-phases not by P-phases.
