Systematic Assessment of Urban Seismic Risk by Microtremors

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The starting point of an urban seismic disaster mitigation program is to assess seismic input motions in precisely covering a wide area in advance for future earthquakes. This project was two-year plan for developing an easy capable but systematic method for assessment of seismic input motions utilizing long period microtremors measurements. Los Angeles and Sapporo areas are chosen as a model place to realize this project.In the first year of the project, field measurements in the Los Angeles were carried out attending three members from Japan. The following studies were done ;(1)Gathering background data : Accumulating various subsurface data of shallow to deep, an initial 3-dimensional model of the Los Angeles Urban Area is composed. Strong motion records of 1971 San Fernando and 1987 Whitter Narrows earthquakes are collected.(2)Field measurements of microtremors : Microtremors measurements were carried out on more than 150 points covering the whole area. A reference point is set on the baserock site in the north end of the area and continuous measurements are done during the whole period of field measurements. Small earthquake records were also obtained during the period.(3)Analyses of obtained records : Spectral ratio at each site to the baserock site was calculated and two dimensional amplification maps for various period ranges were drawn.In the second year supplemental measurements were done in the Sapporo area in order to propose a new method for systematic assessment of urban seismic risk by microtremors. For this, one member from US was invited to Japan. The procedure using in this project was generalized and a new method was proposed in order to expand the applicability of microtremors measurements to the worldwide.