Vol.9 No.2 2016

Research paper : Constructing a system to explore shallow velocity structures using a miniature microtremor array (I. CHO et al.)−88−Synthesiology - English edition Vol.9 No.2 (2016) this series of maneuvers. The array used in the new system is much smaller than the one used in conventional microtremor array exploration, and hereafter we call this a miniature array in this article.The new system is an advanced version of an existing system called “i-bidou” that was created by a research group of Senna, one of the authors of this article.[1] The i-bidou is composed of a series of maneuvers including execution of single-point measurementTerm 4 using a microtremor measurement kit,[2] data transfer to a server, calculation of the spectral ratios of horizontal-to-vertical components of microtremors (H/V spectrumTerm 4), forecasting seismic intensity during an earthquake, and delivery of these analysis results. An H/V spectrum of microtremors is said to represent the oscillating characteristics of the ground. The new system is positioned as a system that allows high precision analysis of the wave propagation velocity as well as the oscillating characteristics of the ground, by replacing the single-point measurement of i-bidou with the miniature array.The goal of the authors is to provide information of the subsurface S-wave velocity that is of high density, high resolution, and quantitative as much as possible, in response to the various social demands related to surface geology such as earthquake disaster mitigation. To do so, it is necessary to collect, analyze, and accumulate massive amount of microtremor data, and we are attempting this using the new system. While not so much as the single-point measurement, measurement using the miniature array is still extremely simple. Therefore, by repeatedly performing measurement many times at different measurement points, the spatial changes of the S-wave velocity structure can be imaged easily. As mentioned at the beginning of this chapter, S-wave velocity is a material property that is directly linked to the shaking and stiffness of the ground, and may contribute to achieving high precision of seismic zoningTerm 5 to mitigate earthquake damage. That is, the ground surface shaking characteristics, which are currently estimated from the geomorphological classificationTerm 5 from the perspective of resolution, will be evaluated from the actual data such as S-wave velocities and ground oscillation characteristics, and the forecasting precision for earthquake shaking may dramatically improve. It also contributes to the evaluation of ground disasters such as liquefaction and helps determine the locational conditions of construction and civil engineering structures. Wide-ranging social value and ripple effect are expected of the results of this research.First, conventional microtremor array exploration and the positioning of this research will be explained (Chapter 2). Next, the core concept for the new system will be explained (Chapter 3), and the development process is discussed from a synthesiological perspective (Chapter 4). Subchapters 4.1 and 4.2 correspond to the charges of Cho and Senna, respectively. Finally, the future issues will be discussed (Chapter 5). 2 Conventional microtremor array exploration and the positioning of this researchMicrotremors are minute oscillations that cannot be detected by the human body, and are caused by wind, waves, industrial activities, and other various sources of oscillations. They are superposition of P-waves and S-waves (body waves) as well as Rayleigh waves and Love waves (surface waves).Term 1 Since the body waves are attenuated in high degree, they are dominant only near the oscill1ation source. As a result, the wave field of microtremors is composed of multiple surface waves that generally arrive from various directions. Based on the theory of elasticity, the body of such microtremors can be easily assumed, but the verification by measurement data was done only after 1950s–1960s.[3]–[5]After the nature of microtremors was clarified, research to establish a method for measuring the vertical component of microtremors by arrays was conducted[6][7] by extracting the propagation velocities of Rayleigh waves (phase velocity), and estimating the subsurface S-wave structure from the dispersion characteristics (the property where propagation velocity differs by frequency). This is the research for “microtremor array exploration.”In the latter half of the 1990s, the microtremor exploration was put to practical use to estimate the deep velocity structure as part of the subsurface structure survey by the Government and local governments.[8][9] Roughly explained, the estimation of the deep velocity structure is the evaluation of the foundation of S-wave velocity of 3 km/s called the seismic basement that appears at a depth of several kilometers in the cases Fig. 2 Measurement using a miniature array with a radius of 0.6 m [Fig. 1(b)]


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