7 0 0 0 OA 詩朗読:ためいき、故園晩秋の歌、反歌
6 0 0 0 OA 日章旗の下に{下=もと}
4 0 0 0 OA 陣中の竪琴 : 森林太郎が日露戦争従軍記念詩歌集うた日記に関する箚記
2 0 0 0 OA むささびの冊子 : 佐藤春夫随筆集
2 0 0 0 OA ランダムな不均質による地震波の散乱
- 著者
- 佐藤 春夫
- 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.44, no.Supplement, pp.85-97, 1991-07-24 (Released:2010-03-11)
- 参考文献数
- 94
- 被引用文献数
- 3
Seismic waves are refracted, diffracted and/or reflected by random inhomogeneities which exixt in the real earth medium. Coherent phases have been well studied especially for the structure study of the lithosphere, where discrete impedance contrasts are supposed. Array obsevations made it possible to measure the precise velocity structure by tomography. Complementary to such a deterministic approach, the random inhomogeneous structure of the earth medium has been statistically studied, where a focus is put on the systematic change in amplitudes of incoherent phases of seismic waves. A latter part of S coda first attracted seismologists to its systematic decay in amplitudes, which offered the way to evaluate quantitatively the short wavelength components of randomness since coda waves are excited due to their large angle scattering. Several models have been proposed to simulate the excitation of coda waves: a single scattering model; a diffusion model; and an energy flux model. The scattering coefficient g which characterizes the coda excitation and coda Q-1 which characterizes the decay in coda amplitude have been often measured in various areas in the world as tectonic parameters. The energy conservation law suggests that the excitation of coda waves mean the reduction in amplitude of direct waves: the seismic energy of direct waves flows into a coda portion with the increase in propagation distance. Scattering due to random structure has been considered as a dominant mechanism of attenuation. Frequency dependence of attenuation and coda excitation has been studied in relation with the spectral structure of randomness. Recent observations report that Qs-1 decreases with frequency for frequency higher than 1Hz. A theoretical model predicts that Qs-1 is proportional to the reciprocal of frequency when the randomness is characterized by the exponential autocorrelation function. The separation of intrinsic loss and scattering loss from total attenuation is one of the most important subject. Seismic waves which are pulse-like in time when radiated from the hypocenter collapse with travel distance. Such a characteristic has been studied in relation with the diffraction and multiple forward scattering due to long wavelength components of velocity inhomogeneities. The parabolic approximation well explains the broadening of seismogram envelope and the delay in the maximum amplitude arrival from the onset of direct wave. We have to examine seismograms of not only a short time window around the direct wave but a long time window starting from the direct wave to the end of coda. That is, it is very necessary to make more sophisticated models for full seismogram envelopes adopting the difference in randomness related to seismo-tectonics.