Systematic Assessment of Urban Seismic Risk by Microtremors

城市防震减灾计划的出发点是准确评估地震输入运动，为未来的地震提前覆盖大范围的区域。该项目是一个两年计划，目的是开发一种简单、有能力但系统的方法，利用长周期微震测量来评估地震输入运动。洛杉矶和札幌地区被选为实现这一项目的样板地。在项目的第一年，来自日本的三名成员在洛杉矶进行了实地测量。进行了以下研究：(1)背景数据的采集：积累了各种从浅到深的地下数据，组成了洛杉矶市区的初步三维模型。收集了1971年圣费尔南多和1987年惠特纳罗斯地震的强震记录。(2)微震的现场测量：在整个地区进行了150多个点的微动测量。在该地区北端的基岩场地上设置一个基准点，在整个野外测量期间进行连续测量。(3)小震记录分析：计算各场地与基岩场地的频谱比，绘制各周期范围的二维放大图。第二年在札幌地区进行补充观测，为城市微震危险性系统评价提供一种新的方法。为此，一名来自美国的成员应邀访问日本。总结了本项目所使用的方法，并提出了一种新的方法，以扩大微震测量在世界范围内的适用范围。

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.