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

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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.