STRONG-MOTION PREDICTION BASED ON THE APPRAISAL OF SUBSURFACE STRUCTURES USING NEW ARRAY METHODS OF MICROTREMOR EXPLORATION

为了在城市地区实现高分辨率的强震预报，需要建立简单、经济的技术来探测局部尺度的地下土壤结构。SPAC方法是近年来被认为是最有前途的技术之一。这种方法包括用圆形地震传感器阵列测量微动(环境振动)，并通过分析记录来评估面波的相速度频散曲线。一旦知道了频散曲线，就有可能间接评价地下土壤结构和土壤的振动特性。我们发现，通过对SPAC方法的基本理论的新认识，有可能设计出一些新的圆阵地震动探测方法，在2004财年和2005财年，我们集中精力阐述了它们的理论，使它们成为可行的实用方法。其中，两种新技术值得特别关注，我们分别将其命名为“双半径法”和“无中心圆阵(CCA)法”，前者是因为其方法学上的独创性，即消除了所有关于瑞利波的信息，而只提取了关于Love波的信息，而后者是因为它在极大的波长范围内有效。在2006财政年度，我们使用半径为1米的“微型”地震台阵获取了现场地震动记录，并研究了不同的分析方法是否可以有效地应用于它们。结果表明，用CCA方法可以高精度地推断瑞雷波的相速度，其波长约为地震台阵半径的100倍。这一发现表明，即使在人口密集的人工障碍物密集的城市化地区，推断深层地下土壤结构也有可能相对容易。

In order to realize strong-motion prediction of fine resolution in urban areas, it is desirable to establish simple, economical techniques to explore subsurface soil structures on local scales. The SPAC method is one of such techniques that are considered most promising in recent years. This method consists in measuring microtremors (ambient vibrations) with a circular array of seismic sensors, and in evaluating phase velocity dispersion curves of surface waves by analyzing the records. Once the dispersion curves are known, it becomes possible to indirectly evaluate subsurface soil structures and vibration characteristics of soil.We have found out that, by shedding a new light on the basic theory of the SPAC method, it was possible to devise a number of new circular-array methods of microtremor exploration, and in fiscal 2004 and 2005, we concentrated our efforts on elaborating their theory and making them viable methods for practical use. Among others, two new techniques, which we have named the "Two-Radius method" and the "Centerless Circular Array (CCA) method," respectively, deserve special attention, the former because of its methodological originality that consists in cancelling out all information on Rayleigh waves and extracting information on Love waves alone, and the latter because of its validity over an extremely wide range of wavelengths.In fiscal 2006, we acquired field microtremor records using "miniature" seismic arrays, with radii on the order of 1 meter, and examined whether the different analysis methods could be validly applied to them. It was shown that, by using the CCA method, phase velocities of Rayleigh waves could be inferred with sufficient accuracy up to wavelengths worth about 100 times the seismic array radius. This finding points to the new possibility of inferring deep subsurface soil structures with relative ease, even in urbanized areas with a dense population of artificial obstacles.