著者

小田 仁

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.42, no.3, pp.333-339, 1989-09-24 (Released:2010-03-11)

参考文献数

21

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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 fM=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 fc<<fM, where fc 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<<fM, where the significant velocity decrease is expected. For this purpose, group arrival times of the P-wave at low frequencies may also be useful.