Vol.9 No.2 2016

Research paper−87−Synthesiology - English edition Vol.9 No.2 pp.87–98 (Sep. 2016) one seismometer in the center point of a circle and three seismometers evenly placed around the circumference (in total, four seismometers) is used [Fig. 1(b), Fig. 2]. This array is used to observe microtremors for about 15 min to estimate the S-wave velocities of shallow ground to the depths from several meters to several tens of meters. Assuming the observers to be general users (such as university labs or high school geology clubs) who are not experts on subsurface structure exploration, one-touch operated measurement equipment is adopted. After measurement, the microtremor data obtained are transmitted wirelessly to a server, and are automatically analyzed. The users are sent analysis results, and the measurement and analysis data are stored in a database in the server. Our aim is comprehensive provision of 1 IntroductionWhile devastating tsunami damage occurred by the 2011 off the Pacific coast of Tohoku Earthquake, there were also considerable damage by strong motion and liquefaction. The damage differed according to the soundness of the ground. The S-waveTerm 1 velocity of the ground is important information to learn about shakiness by earthquakes, and it is also useful in disaster prevention. As one of the methods to estimate the S-wave velocity, minute oscillations of the ground called microtremors can be used as data.The microtremor array exploration method is a method for estimating the S-wave velocity structure from several tens of meters to several kilometers underground. This involves array observation of microtremors [Fig. 1(a)] over several hours by laying out the seismographs on the ground surface at several tens of meters to several kilometers scale. This is a nondestructive exploration where it is not necessary to excavate the ground as in logging.Term 2 Since the collected data is for microtremors that are present everywhere and anytime, the advantages are that it is lower in cost than the seismic wave exploration that uses artificial seismic sourcesTerm 3 and it can be applied easily in urban areas.The authors are constructing a system for microtremor array exploration that specializes in shallow ground, and in which the observation and analysis are thoroughly simplified (hereinafter, this will be called the “new system”). In the new system, an array with a radius of 0.6 m composed of - Accumulating and utilizing large microtremor datasets-Our final goal is to provide quantitative information on subsurface S-wave velocity structures in response to a variety of social needs regarding geological and soil matters. Since S-wave velocity is a physical property directly related to site amplification and ground stiffness, it is expected to contribute to, for example, improving accuracy of seismic zoning for the mitigation of earthquake disasters. Currently, we are constructing a system for observation and analysis of microtremors to explore S-wave velocities within the depth range from several to tens of meters on the basis of 15-minute observations with a miniature seismic array having a radius of 0.6 m. The simplicity and objectivity of our system affords automization and quality control, with an expected capacity to acquire large amounts of microtremor data.Constructing a system to explore shallow velocity structures using a miniature microtremor array Keywords : Microtremor survey method, array observation, Rayleigh wave, S-wave velocity, soil-structure model, seismometer [Translation from Synthesiology, Vol.9, No.2, p.86–96 (2016)]Ikuo Cho1* and Shigeki Senna2**1. Research Institute of Geology and Geoinformation, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan * E-mail: , 2. Disaster Risk Research Unit, Department of Integrated Research on Disaster Prevention, NIED 3-1 Tennodai, Tsukuba 305-0006, Japan ** E-mail: Original manuscript received September 1, 2015, Revisions received November 30, 2015, Accepted December 2, 2015Fig. 1 Microtremor arrayThe “r” is the array radius, and black dots show the seismometers. In the conventional microtremor array exploration, the array radii range from several tens of meters to several kilometers [array (a)]. The new system will basically use 0.6 m radius [array (b)], and a radius of 5 m [array (c)] may be used supplementarily.321r(c)(b)(a)rrrr


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