1991 Fiscal Year Final Research Report Summary
Investigation of shallow to deep subsoil structures in urban area by means of microtremors and its application to an assessment for regional seismic input motions.
| Project/Area Number |
02452207
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
Building structures/materials
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| Research Institution | Hokkaido University |
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Principal Investigator |
KAGAMI Hiroshi Hokkaido Univ., Fac. of Eng., Professor., 工学部, 教授 (70016476)
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| Co-Investigator(Kenkyū-buntansha) |
MURAKAMI Hitomi Hokkaido Univ., Fac. of Eng., Research Associate, 工学部, 助手 (10201807)
OKADA Shigeyuki Hokkaido Univ., Fac. of Eng., Associate professor, 工学部, 助教授 (50125291)
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| Project Period (FY) |
1990 – 1991
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| Keywords | Microtremors / Seismic baserock / Ishikari Plain / Spectral analysis / Site effect / Tree dimensional model |
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| Research Abstract |
The purpose of this study is to develop a feasible method for evaluating seismic input motions in an urban area by means of experimental way and Sapporo urban area, northern Japan, was selected as a test field.
Microtremors measurement was adapted for this study because of its easy applicability for detecting subsoil conditions. At the first, the observation system was upgrading in both terms of hardware and software. An A/D converter was installed in a laptop computer and a real time monitoring was introduced by computer program. With this system daytime measurements in busy business district can be performed in spite of low S/N ratio.
Extensive measurements of long-period microtremors were conducted in Sapporo urban area in 1990 and 1991. Seven observation lines parallel to the coastal line and two lines perpendicular to these were set up, covering the area of about 40 x 40 kilometers, obtaining records at totally 183 sites. One reference site was set up at Hokkaido University to monitor the time variation of microtremors during the period of measurements.
Spectral ratios were obtained for each site by dividing the Fourier spectra by the corresponding spectra at the reference site. These ratios were plotted on a map for each period of 0.5, 1, 2 and 4 second and four contour maps were drawn as seismic microzonig maps. These maps were verified by various of small number of deep drilling data, three strong motion records and seismic intensity distribution data of the 1982 Off-Urakawa earthquake.
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