ダイラタンシー領域を伝わる地震波の速度変化

抄録

We have reexamined whether the velocity decrease of the seismic waves due to dilatancy can be detected or not. A numerical investigation was made to see the change of travel times by using synthetic seismograms of P-waves traveling through a dilatant region. In synthesizing the seismograms, the Brune's source model was used, and the P-wave velocity was assumed to increase with increasing frequency and to approach asymptotically an intrinsic velocity. This relation between P-wave velocity and frequency is based on the experimental result obtained by uniaxial compression test of a granitic rock sample; the velocity decrease is larger at low frequencies than at high frequencies. The dispersion curve is characterized by crack density ε, crack width 2a and intrinsic velocity α. A characteristic frequency f_M=5α/πa, above which no velocity decrease is found, is defined. We investigated the change of the P-arrival times in the synthetic seismograms, changing both crack density and crack width. When the P-wave forms of earthquakes with the same magnitudes are recorded without off scale, the travel time anomaly can be detected in the case of f_c<<f_M, where f_c is the corner frequency of seismic source spectrum. Therefore, for detecting the travel time anomaly, it is necessary to observe earthquakes in low frequency range of f<<f_M, where the significant velocity decrease is expected. For this purpose, group arrival times of the P-wave at low frequencies may also be useful.