Re-checking Spatial Auto Correlation method based on the theory of seismic interferometry

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  • Yokoi Toshiaki
    International Institute of Seismology and Earthquake Engineering
  • Margaryan Sos
    International Institute of Seismology and Earthquake Engineering

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  • 地震波干渉法理論に基づく SPAC 法の再検討
  • ジシンハ カンショウ ホウリロン ニ モトズク SPACホウ ノ サイケントウ

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Abstract

 We have re-checked the conventional SPatial Auto Correlation (SPAC) method for microtremor array measurement, especially on the necessity of the average over azimuth of the complex coherence function, based on the theory of the newly appeared research field Seismic Interferometry, on the formulas of Green's function tensor for surface waves without the far field approximation and also on the result of our field observation. Then, the followings are revealed. First, the theory of the conventional SPAC method and that of the retrieval of the elastodynamic Green's function are almost completely consistent with an exemption that is in-necessity of the average over azimuth for the cases with the wave length longer than or comparable with the station intervals and the dominance of surface waves. Next, the dependency of the power of microtremor on azimuth can be so moderate in urban areas and their environs that the average over azimuth can be skipped. This in-necessity, however, ought to be considered together with acceptable errors for the estimated phase velocity. Moreover, the following new aspects are suggested: the possibility of two or three dimensional underground velocity structure using SPAC method without average over azimuth, the way to estimate the phase velocity separately for two mode cases, a potential new informant about the velocity structure independent from the dispersion relation.<br>

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