QUALITY FACTOR IDENTIFIED USING KIK-NET IN JAPAN

Many seismographs have been installed in Japan since the 1995 Hyogoken-Nanbu earthquake, which brought severe damage to the city of Kobe and the surrounding area. The National Research Institute for Earth Science and Disaster Prevention (NIED) deploys digital strong-motion seismographs (KiK-net) all over the country in Japan, to provide vertical array records of ground motions. The total number of observation sites is nearly 700. Data observed by KiK-net can be downloaded on the website. They can be used to identify the dynamic properties, such as shear wave velocity and quality factor of the subsurface ground. In this study, a new approach is proposed to identify the quality factor. It is applied to some KiK-net sites to extract the frequency-dependent characteristics of the quality factor. Introduction Strong ground motions are largely affected by the amplification effect of subsurface layers of the ground. Therefore, it is very important to estimate the dynamic soil properties of the subsurface ground in order to predict the characteristics of strong ground motions that influence the behavior of structures based on the ground or lifeline facilities buried underground. Recently, several studies (Ohta 1975, Tsujihara 1996, 2004, Sato 1994, Annaka 1994, Yoshida 1995, Nakamura 2002, etc.) have been done on the identification of dynamic soil properties of subsurface ground using vertical array records of ground motions. Among the properties, the damping is known to be difficult to be identified in particular. Vertical array observations of ground motions are energetically carried out in Japan. Digital strong-motion seismographs, so called KiK-net, are deployed by the National Research Institute for Earth Science and Disaster Prevention (NIED) at nearly 700 sites. An enormous amount of data has been accumulated since 1997 and can be downloaded from the website, providing a great opportunity to identify the dynamic properties of subsurface ground. The shear wave velocity and quality factor are generally identified as the stiffness and damping parameter, respectively, supposing one-dimensional multiple reflection of shear wave in the horizontally laminated soil deposits. The accuracy of identifying the shear wave velocity has been improved. But, the improvement of accuracy is not very notable in identifying the quality factor. Several models were proposed for the quality factor. Some researchers identified the quality factor of each layer and others the average through the layers. The influence of the damping models and conditions of analysis on the accuracy has been 1 Associate Professor, Dept. of Civil Engineering, Wakayama National College of Technology, Gobo City, Japan 2 Professor, Dept. of Civil Engineering, University of Tokushima, Tokushima City, Japan 1918 investigated (Tsujihara and Sawada 2006). It was reported that when the damping parameter of every layer was identified, the accuracy of identification was remarkably deteriorated with the increase of the layers. In the numerical experiments, it was demonstrated that there was the case in which the accuracy of the estimated damping parameter differed by a few hundreds times, depending on estimating the value of the parameter in every layer or the average value through the layers. However, it was also reported that the average value of damping through the layers could be estimated almost to the same level of accuracy as the shear wave velocity of the layers, when these parameters were identified simultaneously. The detection of the frequency dependency of the quality factor has become of major interest lately. In this paper, a new method, called the Sweeping Method, is proposed to detect it. The quality factor is estimated by two stages in the process of identification. In the first stage, the quality factor is identified assuming that it is independent of frequencies together with shear wave velocity of laminated each layer. In the second stage, the quality factor at each frequency point is identified with shear wave velocity fixed. The quality factor in the frequency bands where it is not sensitive is swept out. As a result, the values of the quality factor at significant frequency points are highlighted. Theory and Method Multiple Reflection Theory Horizontally laminated soil deposits are assumed to be excited by vertical incident SH wave. Consider the identification of subsurface ground model as shown in Fig. 1, in which H,ρ, V and Q denote the thickness, density, shear wave velocity and quality factor, respectively. The displacement and shear stress at the points p in the p-th layer and q in the q-th layer are represented by multiple reflection theory as follows(Haskell 1960, Toki 1981). [ ]       =       0 0 u R u p p p τ (1) [ ]       =       0 0 u R u q q q τ (2)