English

Special Seminar[2010 - ]

2011

  • Lecturer:木下 繁夫 教授 (横浜市立大学)
    Title:地震記録の補償方法 - 地殻変動の簡易推定
    Date:Mar. 2 (Fri.) 10:00-12:00, 2012
    Place:物理A棟7階412号 地球物理第2講義室
    2nd lecture room (room 412) in the Butsuri-A building
    Contents: 木下教授は,防災科研在籍時に,現在のK-net建設とデータ公開とを主導的に率いてこられた.特に,強震動の震源過程とボアホール観測に基づく地盤の増幅特性に関する独創的な研究は高く評価されている.
    今回のセミナーは,強震動用の加速度記録から長周期成分を高い精度で抽出する方法について紹介される予定である. 昨年3月の東北地方太平洋地震の強震記録の解析に基づく事例研究を含みます.
    Materials: (Materials are accessible only from Tohoku University)
  • Lecturer:Prof. Kees Wapenaar (Delft University of Technology)
    Title:Green's function retrieval from irregular noise: repairing the broken mirror
    Date:Feb. 24 (Fri.) 14:00-15:00, 2012
    Place:物理A棟7階412号 地球物理第2講義室
    2nd lecture room (room 412) in the Butsuri-A building
    Contents: The correlation of noise at two receivers is approximately proportional to the Green's function between these receivers. Hence, the correlation process turns one of the receivers into a virtual source, of which the response is observed by the other receiver. The virtual-source response is accurate when the medium is lossless and the noise field is equipartitioned. In practice these assumptions are often violated: the medium of interest is often illuminated from one side only, the sources may be irregularly distributed and losses may be significant. For those cases it is as if the virtual source is viewed in a broken (time-reversal) mirror, which causes a blurring of the source. This blurring is quantified by the so-called point-spread function which, like the correlation function, can be derived from the observed data (that is, without the need to know the actual sources and the medium). The broken mirror can be repaired by deconvolving the correlation function for the point-spread function. As a result the virtual sourceis refocused and hence the virtual-source response becomes more reliable. In the presentation I will illustrate the correlation and deconvolution methods with several examples and discuss the advantages and limitations of both methods.
    Materials: (Materials are accessible only from Tohoku University)

2010

  • Lecturer:木下 繁夫 教授 (横浜市立大学)
    Title:地盤の非線形応答 -やさしい現象論的解釈-
    Date:Mar. 7 (Mon.) 15:00-16:00, 2011
    Place:物理A棟7階719号 地球物理第1講義室
    1st lecture room (room 719) in the Butsuri-A building
    Contents: ---
    Materials: (Materials are accessible only from Tohoku University)
  • Lecturer:Prof. Francisco J. Sánchez-Sesma (Instituto de Ingeniería, UNAM)
    Title:A theory for microtremor H/V spectral ratio: Application for a layered medium
    Date:Jun. 4 (Fri.) 15:00-16:00, 2010
    Place:物理A棟4階412号 地球物理第2講義室
    2nd lecture room (room 412) in the Butsuri-A building
    Contents: Microtremors are produced by multiple random sources, typically close to the surface of the Earth. They include the effects of multiple scattering and their intensities, which depend upon the strength of random sources, are governed by diffusive-like equations. The involved energy in seismic noise may be relatively small but under appropriate circumstances, it is possible to use seismic noise to assess site structure. It has been established that within an elastic, inhomogeneous, and anisotropic medium subjected an uncorrelated set of random forces (equivalently, one can say that the seismic illumination is equipartitioned), the average cross correlation of the motions at given receivers, in the frequency domain, is proportional to the imaginary part of Green function. For a single receiver, average autocorrelation measures energy density, which is proportional to the imaginary part of Green function at the source. Assuming the seismic field is diffuse and such that equipartition holds also for the energy components, we compute the H/V ratio in terms of the imaginary part the Green function tensor components at the surface. Broad band noise records at Texcoco, a soft soil site near Mexico City, are studied and they are interpreted using the theoretical results herein presented for a horizontally layered medium overlaying a half space.Therefore, our approach naturally allows for the inversion of H/V, the well known Nakamura's ratio without using Rayleigh waves'ellipticity.
    Materials:---
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