伝播性破壊確率モデルと震源スペクトルの方位依存性

Abstract

Theoretical study has been made to investigate seismic directivity effects due to deterministic and stochastic rupture processes on a fault. An earthquake is modeled by a finitely propagating unilateral rupture on a fault plane where fault heterogeneities; fault patches, are randomly distributed. The parameters of the present dynamic stochastic source are (1) seismic moment M0, (2) fault dimension L and W, (3) fault patch intensity σ, (4) mean fracture time 2πγ-1 of fault patches, and (5) fluctuation angle α of rupture direction within fault patches. The seismic directivity effect due to the deterministic rupture process is represented by a linear and coherent superposition of fracturing fault segments, which is the Doppler effect. That due to the stochastic rupture process is described by an energy additive superposition of fracturing fault patches. The latter effect, patch multiplexing effect, is prevailling on the source spectra only in a frequency range from corner frequency to patch corner frequency. The fluctuation angle α is an important parameter to control the effect. It has been concluded that the stochastic directivity effect is maximum, when the rupture direction within fault patches is equal likely in every direction (α=π), and minimum when the rupture direction coincides with one for the deterministic rupture propagation (α=0).